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P E R G A M O N

Microelectronics Reliability 38 (I 998) 1277-1286

MICROELECTRONICS

RELI BILITY

Electronic systems p ackag ing future re l iabi l ity chal lenges

J B a r r e t t

National Microelectronics Resear ch Centre University College Cork Ireland

A i m r a e t

P ackag ing has a dominan t e f f ec t on e l ec t ron ic sy s tem cost , pe r fo rmance , w e igh t , s i ze and long t e rm

rel iab il ity. Recen t yea r s ha ve the r e fo re s een r ap id deve lopmen ts in packag ing to me e t the cha l l enges o f

min ia tu r i sa t i en and cos t r educ t ion w h i l e de l ive r ing inc r eas ed e l ec t ri ca l pe~ o rma nce and r e l iab il ity. M ee t ing the

re l iab i l i ty dem and s posed by these deve lopm ents a l ready presents ma jor cha l lenges to th e re l iab i l i ty pract it ioner .

H ow ever , t he t echno logy roadm aps fo r in t eg ra ted c i r cu it s and e l ec t ron ic s y stems imp ly a con t inuous inc r eas e in

the s ca le o f ex i s t ing cha l l enges w h i l e emerg ing componen t and s ys tem techno log ies , s uch a s op t i ca l

in te rconnec t ions and mic ros ys tems , w i l l p r e s en t comple te ly new cha l l enges. Thes e emerg ing cha l l enges w i l l

me an tha t the r e l iab i l ity p r ac t i tione r w i l l i nc r easing ly be invo lved f rom the ve ry f i r s t phas es o f coml~ m en t o r

sys tem conce pt ion and des ign a l l the way throu gh to product ion an d green d isposal. Fur ther, to ensm'e that

re l iab i l i ty targets are real is t ica l ly se t and m et , the re l iab i l i ty pract i t ioner wi l l increa s ingly funct ion as p er t o f a

mu l t id i s c ip lina ry t eam, ma ny o f w hos e m emb er s m ay no t be long to the t r ad i tiona l e l ec tron ics d is c ip lines . By

exam in ing c tw rent r e li ab i li ty cha l lenges and by the u s e o f t echno logy readm aps , th i s pape r t r i e s to fo recast f u t~ e

rel iab i l i ty chal len ges in e lect ronic packaging . In the context of th is paper the term Packaging is used to

encompas s the va r ious a s s embly and in te rconnec t ion t echno log ies and t echn iques w h ich a r e u s ed to b u i ld an

e lec t ron ic com ponen t o r s y stem. ©

1998 Elsevier Science Ltd. All rights reserved.

1 . l a t r o d a e t i e a

The e lect ronic sys tem packaging re l iab i l i ty

chal len ge covers a very broad spectrum: a t one

extrem e there are short - time- to-prof it , shor t product

l i fe cycle , cos t dr iven volume products , such as

mob i l e phones and pe r s ona l compu te r s , w h ich

operate in re la t ively ben ign environments ; a t the

other ex trem e are cr i tica l sys tems , such as those for

av ion ics app li ca tions , w h ich m ay be m any yea r s in

ope ra t ion in ha r s h env i ronmen ts and a r e

cha rac te r i s ed by longe r deve lopmen t cyc le s ,

s ma l l e r p roduc t ion vo lumes and h ighe r cos ts . A t

each ex t r eme and ac ros s the s pec t rum as a w ho le

the p acka ging re l iab i l i ty practi tioner has a ro le in

ens u r ing a l eve l o f s y s tem r e l iab i l ity w h ich m ee t s

the r equ i r emen t s o f the end u s e r and the end u s e

environment . Given the d ivers i ty of mater ia ls and

componen t s w h ich e r e u s ed in e l ec t ron ic s y s tem

m a n u f a c t u r e a n d t h e c o m p l e x i t y o f m a n y o f t h e

indiv idual sys tem components used , par t icu lar ly

in tegrated c ircu its , there are po tent ia l ly m any w ays

in w h ich an e l ec t ron ic s y s tem may f a i l . The

packag ing r e l i ab i li ty p r ac ti t ione r has by neces s i ty

been , o r has had to become , an e l ec t ron ic j azk -o f -

a l l - l rades requir ing expert ise in mater ia ls , ICs ,

pass ive components , product engineer ing ,

manufac tu r ing and the env i ronmen t a s w e l l a s in

0026-2714/98/ - see fron t matter. © 1998 Elsevier Science Ltd. All rights reserved.

PII: S0026-2714(98)00129-2

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  278 J. Barrett/Microelectronics Reliability 38 1998) 1277-128 6

t he m ore obv i ous d i s c i p l i nes o f r e l i ab i l i t y phys i c s

and s ta t is ti cs. Increas ing ly , pack agin g re l iab i l i ty

has becom e a t eam , r a t he r t han an i nd i v i dua l ,

d i s c i p l i ne and i t is a d i s c i p l i ne wh i ch wi l l becom e

i nc reas i ng l y cha l l eng i ng , d r i ven by deve l opm en t s

s uch as :

• T h e g r o w t h i n I C c o m p l e x i t y

• Por tabi l i ty

• Har s h env i ron m e n t app l i ca t i ons

• Op t o -packag i ng and i n t e rconnec t i on

• Green e l ec t ron i cs

• M i c r o s y s te m s a n d m i c r o m e c h a n i c a l s y s te m s

5000

4 0 0 0

3000

2000

1000

0

ie a r e a m m 2 )

' 7 8 ' 8 8 ' 9 8 ' 0 8

T h e s e d e v e l o p m e n t s a n d t h e i r i m p l i c a ti o n s f o r

fu t u re s y s t em s packag i ng r e l i ab i l i t y a re d i s cus s ed

i n t he r em a i nder o f t h i s pape r .

2 . G r o w t h i n I C c o m p l e x i t y

M o o r e ' s L a w , w h i c h p r e d i c t s a d o u b l i n g o f I C

p r o c e s s i n g p o w e r e v e r y 1 8 m o n t h s h a s p r o v e d t o b e

m ore o r l e s s va l i d s i nce i t was f i r s t p ropos ed by

G o r d o n M o o r e , o n e o f t h e f o u n d e r s o f I n t e l , in

1965 . Th i s has been ach i eved t h rough IC fea t u re

s i ze r educ t i ons and t h rough i nc reas ed f ab r i ca t i on

y i e l d s wh i ch a l l ow l a rge r d i e s i zes . Inev i t ab l y t h i s

h a s s e t e q u a ll y c o m p l e x c h a l le n g e s f o r p a c k a g i n g

eng i nee r s a s l ead i ng edge IC a rea , ope ra t i ng

f requenc y and powe r d i s s ipa t i on have i nc reas ed , a s

s hown i n F i gu res l . ( a ) - . ( c ) [1 ] . Thes e evo l u t i ons

have been pa r t i cu l a r l y r ap i d i n t he l a s t f i ve yea r s

a n d t h e p a c e s h o w s n o s i g n o f s lo w i n g i n t h e n e x t

t e n y e a r s. A t t h e s a m e t i m e , t h e p r e d i c te d c o s t p e r

I / O o f h i g h p in c o u n t p a c k a g e s r e q u i re s a 5 0 c o s t

r educ t i on i n t he nex t 10 yea r s [1 ], a t r end w h i ch i s

t yp i ca l o f cos t evo l u t i on i n t he pas t decade . Th e

p a c k a g i n g e n g i n e e r h a s h a d a n d w i l l h a v e t h e t a s k

o f p r o v i d i n g m o r e c o m p l e x p a c k a g i n g s o l u ti o n s a t

r educe d cos t . Th e t echn i ca l cha l l enges f ac i ng t he

re l i ab i l i t y p rac t i t i one r have g rown and wi l l

c o n t i n u e t o g r o w a c c o r d in g l y . A r e v i e w o f t h e s e

cha l l enges i n t he e l ec t r i ca l , t he rm a l and

t h e r m o m e c h a n i c a l d o m a i n s f o ll o w s .

6000

5 0 0 0

4000

3 0 0 0

2000

1000

0

m P ower supply (mV)

m F r e q u e n c y ( M H z )

- |

l m m

m m m

78 88 98 08

200

150

100

50

0

I P o w e r ( W ) |

i

|

_ I I

| I

78 88 98 08

F i gu res 1 .( a )- (c ) . Evo l u t i on o f IC a rea and I / 0

c o u n t ; o p e r a t i n g f r e q u e n c y a n d p o w e r s u p p l y

vo l t age ; and pow er d i s s ipa t i on v s . t i m e .

2 1 E l e c t r i c a l r e l i a b i li t y

El ec t r ica l r e l i ab il i ty i nvo l ves t he i n t eg r it y o f

s i gna l p ropaga t i on i n a s y s t em under a l l ope ra t i ng

cond i t i ons . F ac t o r s wh i ch a re m ak i n g t he r e l i ab il i ty

cha l l enge m ore d i f f icu l t inc l ude :

• h i ghe r ope ra t i ng f r equenc i es

• l o w e r p o w e r s u p p l y v o l ta g e s

• m i n i a t u r i s a t i on

• E M C / R F I r e g u l at o r y m e a s u r e s

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J. Barrett/Microelectronics Reliability 38 1998) 12 77-1286 279

H i g h e r o p e r a t i n g f r e q u e n c ie s a n d l o w e r s u p p l y

v o l t a g e s m a k e t h e t a s k o f e n s u r i n g s i g n a l

p ro p ag a t i o n i n t eg r i t y i n c reas i n g l y d i f f i cu l t . As

d i g i t a l f r e q u e n c i e s e n t e r t h e G H z r e g i o n , R F a n d

m i c r o w a v e p a r a m e t e r s , w h i c h c a n l a r g e l y b e

i g n o r e d a t l o w e r f r e q u e n c i e s , m u s t b e t a k e n i n t o

acco u n t an d each i n d i v i d u a l s i g n a l t r ace i n a

p a c k a g e o r c i r c u i t b o a r d m u s t b e d e s i g n e d w i t h

i m p e d a n c e a n d n o i s e c o n t r o l i n m i n d . I n t h i s

co n t ex t , a t r ad i t io n a l w i re -b o n d wi l l n o t b e s u i t ab l e

fo r ch i p - t o -b o a rd i n te rco n n ec t i o n a s i t s i m p ed an ce

can n o t b e t i g h t l y co n t ro l l ed . M i n i a t u r i s a t i o n

s i m p l i f i e s t h i s ch a l l en g e t o a ce r t a i n ex t en t , i n t h a t

s i g n a l t r aces a re o v e ra l l m ad e s h o r t e r , r ed u c i n g

t h e i r p a ras i ti c l o ad i n g an d co u p l i n g len g t h s .

Ho wev er , m i n i a t u r i s a t i o n d o es n o t i n g en e ra l

i m p l y a r e d u c t i o n i n p a c k a g e o r s y s t e m c o m p l e x it y :

t h e p res s u re , l a rg e l y p ro d u ced b y t h e d es i r e fo r

p o r t ab i li t y , co n t i n u o u s l y ex i s t s t o p ac k m e t e

fu n c t i o n a l i t y i n t o s m a l l e r o u t l i n es . Th i s i s u s u a l l y

ach i ev ed b y a co m b i n a t i o n o f g rea t e r i n t eg ra t io n a t

t h e I C l e v e l a n d b y i n c r e a s i n g t h e I C p a c k i n g

d en s i t y a t b o a rd l ev e l . P ack i n g d en s i t i e s ( r a t i o o f

t o t a l a r ea o f a l l IC s t o t h e a rea o f t h e c i r cu i t b o a rd )

o f 1 0 we re r e l a t i v e l y r ecen t l y r eg a rd e d as h i g h ; it

i s n o w n o t u n u s u a l t o s e e p a c k i n g d e n s i t ie s g r e a t e r

t h a n 5 0 i n h i g h d e n s it y m u l t i c h i p m o d u l e s ( s e e

F i g u re 2 ) .

5 :

/ / / / t l l i l i i i i i l I I I I I I I I I I I ~ , ~ \ ~ X \ \ \ \ \ °

sel f - inductm~ce and in e lect r ical coupl ing between

adja cen t l ines resu l t s .

F u r t h e r co m p l i ca t i n g t h e t a s k a re t h e t i g h t e r

r eg u l a t i o n s o n e l ec t ro m ag n e t i c co m p a t i b i l i t y an d

rad i o f r eq u en cy in t e r f e ren ce (EM C / R F I ) . D es i g n

t o o l s fo r E M C / R F I h a v e l a g g e d b e h i n d t h e n e e d s o f

i n c reas i n g l y co m p l ex , m i n i a t u r i s ed s y s t em s

o p e r a t i n g a t h i g h f r e q u e n c i e s . E M C / R F I

p recau t i o n s i n m a n y s y s t em d es i g n s a re s t il l r o o t ed

i n ru l e s -o f - t h u m b an d d es i g n e r ex p er i en ce r a t h e r

t h an i n an a l y s i s o f s o u rces an d e f f ec t s an d t h e i r

im pac t on e lect r ical re li ab il ity . D es ign ers s t i l l hav e

t o w a i t fo r te s t i n g r e s u l t s f ro m a co m p l i an ce

l a b o r a t o r y b e f o r e h a v i n g c o n f i d e n c e i n E M C / R F I

s y s t em p e r fo rm an ce . S o l v i n g t h i s i s s u e wi l l b e o n e

o f t h e m a j o r ch a l l en g es i n d es i g n fo r e l ec t r i ca l

reliability.

C u r ren t a p p ro ach es t o t h e e l ec t ri ca l r e li ab i li t y i s s u e

i n c l u d e :

M i n i m i s i n g ch i p - t o -b o a rd i n t e rcu rm ec t i o n

p aras i t i c s b y a l m o s t co m p l e t e e l i m i n a t i o n o f

i n te r m e d i a t e p a c k a g i n g t h r o u g h t h e u s e o f f li p -

ch i p a s s em b l y . Th i s a l s o eas es t h e t h e rm a l

re l i ab i l i t y t a s k b u t s i g n i f i can t l y i n c reas es t h e

ch a l l en g e o f t h e rm o m ech an i ca l r e l i ab i l i t y ( s ee

sect ion 2.2).

U s e o f n e w l o w e r d i e l e c tr i c c o n s t a n t o rg a n i c

m a t e r i a l s a n d c o n t r o l l e d i m p e d a n c e

i n t e rco rm ec t i o n s i n p ack ag es an d c i r cu i t

b o a rd s . H o we v er , l o n g t e rm i n t ~ Tac i a l s tab i l it y

o f t h es e d i e l ec t r i c m a t e r i a l s u n d er t h e rm a l ,

t h e r m o m e c h a n i c a l a n d e n v i r o n m e n t a l s ~ e s s

t h en en t e r s t h e r e l i ab i li t y ch a l l en g e .

F i g u re 2 . A P en t i u m s i l ico n -o n - s i l ico n m u l t i ch i p

m o d u l e w i t h h i g h p a c k i n g d e n s i ty pho t o

cour t es y Eur opr ac t i ce -MC M/ ET H) .

T h i s l e v el o f p a c k i n g i s a c h i e v e d b y t h e u s e o f

f i n e p i t c h w i r i n g b o a r d s w h e r e t r a c k d i m e n s i o n s

a n d s p a c e s a r e d r i v e n t o t h e m i n i m u m a l lo w e d b y

m a n u f a c t u r i n g te c h n o l o g y . C o n s e q u e n t i n c r e a se s i n

D e v e l o p m e n t o f m o r e s o p h i st ic a t e d i n te g r a te d

d es i g n t o o l s wh i ch can e f f i c i en t l y an a l y s e a l l

aspec t s of e lect r ical re li ab il ity . Th is i s an

i m p o r t an t r eq u i r em en t o n t h e e l ec t r i ca l

r e l iab i l it y ro a d m ap .

In t h e fu t u re , it i s l i k e l y th a t o p t i ca l i n t e rco n n ec t i o n

wi l l n eed t o b e u s ed a t b o a rd l ev e l fo r c r i t ica l s i g n a l

p a t h s ( s ee S ec t i o n 5 )

2 .2 . T her ma l and t her m omecha n i ca l r e l iab i l it y

T h e t r e n d t o w a r d s i n c r e a s i n g d e n s i t y i n

i n te r c o n n e c t io n s is m a t c h e d b y i n c r ea s e d p o w e r

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128 J. Barrett/Micro electronics Reliability 38 1998) 1277 -1286

dens i ty in sys tems as the forces of increased

operating ~eq uenc y and increased packing dens i ty

com bine to increase IC po wer dissipat ion and the

num ber of ICs in a given board area. M eet ing the

chal lenge of heat rem oval is on e the m ost important

aspects of package and sys tem des ign and

increas ingly requires the use of n ew mater ials (e .g.

thermal inter face pads a nd ph ase chan ge mater ials )

and techniques (e.g. integrated micro-channel heat

pipes, m icro-fans and heatsinks). Overall, ho wev er,

the pressure exists to al low higher junct ion

temperatures and consequent exposure of

packaging mater ials to higher cont inuous

temperatures. T his is com bined with the in creased

use of organic mater ials to reduce elect r ical

parasitics, c ost and weight. T he reliabili ty challen ge

therefore becom es that o f ensur ing stabi li ty of the

organ ic ma terials them selves, and the ir interfaces to

other organic and inorganic materials , under

cont inuous exposure to m ore elevated temperatures .

To quote t~om the SIA roadm ap for semiconductors

[1]:

Standard methods and acceptance cr i ter ia for

interfacial adhesion are lacking. Funda me ntal wo rk

is nee ded to establish adhe sion strength and

degrad ation rate versu s environmental factors, all as

a function of interfacial physical and chem ical

proper t ies . New mater ials are being added to an

already complex, poorly understood interfaeial

adh esion reliability issue.

Adding to the thermal s t ress on mater ials and

inter faces is the problem of thermom echanical

stress. Increases in die area, coupled with higher

temperature gradients and transients, and the

additional com plication o f the use of organic

mater ials whose coeff icients of thermal expans ion

(CTE) are usual ly higher than that of s il icon, m ean

that both absolute and cy cl ic thermom echanical

stresses on ICs, components and materials will

continue to increase. This is without even

consider ing the ef fect of cycles in the external

therm al environment. Compressive, tensile and

shear forces exis t at ev ery level of pack aging and

can lead to delaminat ion, fat igue and cracking of

ICs, plastics an d solder joints.

2. 2.1. Flip chip assembly

The problem of thermom echanical st ress is

bec om ing particularly acute in flip-chip assem bly

of ICs. In fl ip-chip, metal bumps are deposited

Figure 3. M icrograph of bump ed IC a nd cross-

section o f fl ip-chip assem bly to circuit board.

direct ly on the bond-pads of the IC and th e IC is

then d irectly reflo w soldered to i ts circuit board, a s

shown in Figure 3. This almost completely

elim inate s the electrical parasities associated w ith

the chip-to-boa rd interconnections. Flip-chip also

al lows an almost unl imited number of I /O

connections as it facilitates area-array distribution

of connections across the entire IC surface.

Howe ver , the IC is now co nnected with less than a

100 lain gap to its substrate. In the ea rly application

o f fl ip-chip, eerarnic substrates we re used by IBM

[2] which had a reasonable CTE match to sil icon

and IC areas w ere cons iderably smal ler than current

and pro jected sizes. E ven tod ay, ce rarnie substrates

are used for the Motorola PowerPC chip [3] .

However , for reasons akeady explained above,

organic substrates are inoreasingly used for flip-

chip interconnection. The C TEs o f even the most

optimised organic substrates remain sufficiently

higher than that of sil icon to cause early fatigu.e

fai lure of the m icro-solder joints (< .001 mm in

volume) induced by cyc l i c thermumechanica l

stress. Th e use o f organic underfil l adhesives [4]

has e nabled this problem to be alleviated for current

generation flip-chip assemblies. However, given

projected IC size, power dissipation and I/O,

considerable work must be done on materials ,

interfaces [5], solder metallurgy and thermo-

mechanical simulation tools if future generation

flip-chip assemblies are to meet their reliability

targets.

2.2 .2 Plas tic packag ing

While fl ip-chip assembly is l ikely to be the

technique of choice for chip-to-board

interconnection o f leading ed ge ICs, conventional

transfer moulded plastic encapsulated packaging

wil l cont inue to dominate the market in terms of

volume. Plastic packaging has made considerable

progress since the early stages of dual-in-lines with

low reliabili ty caused its use to be restricted in

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J. Barrett/Microelectronics Reliability 38 1998) 12 77-1286

1281

many hi-tel applications, a restriction which has

only in recen t years begun to be l if ted. Intervening

year s have seen the emerSance of new body

formats, progression to ever higher pin¢ounts and

improvements in rel iabi l i ty which now al low

osmmercial -off - the-shelf COTS) plas t ic pachtged

ICs to be used even in hi-tel applications. This

rel iabi l ity improvem ent has been brought about

throug h a combination o f ma terials and process

engineer ing to redu ce s tress and m ois ture induced

failures, an d application o f finite eleme nt

simulation tools to the therm om ech anica l s | ress

problem. How ever, the reliabili ty challeng es can be

expected to cont inue growing in the coming years

in topics suc h as:

• Stress and m oisture issues for large die in very

th in packages where there m ay be on ly a f ew

htmdred micron s of plas tic over the IC.

• The dem and for increased power handling.

• The use of plas t ic packages in increas ingly

harsh application enviro nm ents (see Section 4).

• The abi li ty to handle higher FO counts in

sm alle r footprints.

Am ong the tools that wi ll be ne eded to succeed in

meeting these challenges, and those of f l ip-chip,

will be:

The abili ty to quickly develop application

specific encapsulant materials through the use

of t echniques such as molecu lar model l ing and

synthesis

Integrated design and simulation tools th at will

be able to perfo rm virtual prototyping o f

packages through integrat ion o f thermal and

therm om echa nical simulations with reliabili ty

models. Essential here also will be validated

compact models (and the means to obtain

them) for material and interface behaviour in

the intended use environment. While i t

unlik ely that absolute a ccu racy can be obtained

fi 'om such tools, they shou ld at least be a ble to

quic kly assess the comparative reliabili ty

performa nce of di f ferent designs and mater ial

sets

Por tab i l i t y

It is estima ted that in 2000, ov er 50% o f all

electron ic products m anufa ctured will be portable.

This will includ e increa singly sophisticated sma rt

cards as wel l as the com mun icat ion, oomputin8 and

entm ainm ent systems a lr eady in use a nd which a te

conv erging to one integrated portable system. Ne w

applications a re also eme rging in portable chem ical

and biological analysis systems (see Section 7

below). Portability's m ain sy stem c riteria are:

• Light weight and smal l s ize

• Low to modera te cos t

• M axim um functionality per cm 3

• Low power consumpt ion

• Resistance to shock, vibration and wa ter

These are dr iving man y of the issues al ready

discussed in section 3 above:

• Maximum integration on sil icon with

consequent growth in die s ize, I /O count and

individual die po wer consumption

• Reduce d overal l sys tem pow er consumption

• Use of organic mater ials

• Use o f low profile, thin pack aging techniques

• Use of high dens i ty ci rcui t boards

To reduce size and increase reliabili ty there is

active ong oing research in the integration o f passive

components onto and into both inorganic and

organic circuit boards. Doing this provides more

board surface area for ICs (thus increasing the

pack ing density and associated reliability p roblem s)

and also eliminates the so lder joints associated with

discrete components , o f which there m ay be m any

hun dreds even in a relatively sma ll system such as a

mob i le phone. As solder joints are viewed as w eak

links in electronic systems, this should,

theoretically, le ad to increased reliability. H ow eve r,

the ci rcui t board i tsel f wi l l becom e a m ore com plex

ma terials system, incorporating resistive, dielec tric

and/or magnet ic mater ials . New chal lenges in

electrical reliability and in materials/interfacial

reliability will the refore have to be addressed.

Smart cards , which are among the highes t

production volume consumer electronic products,

present unique problems in terms of packaging

reliability. Th ey are in ge neral constrained to credit

card thickness and the protection that can be

afforded to the IC is m inimal . Future smar t cards

will be expected to have increased functionality

while cont inuing to be able to wi ths tand the wear

and tear o f daily use. Considerable ingen uity will

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b e r e q u i r e d t o d e v e l o p p a c k a g i n g w i t h a d e q u a t e

re l iab i l it y an d y e t b e co m p a t i b l e w i t h h i g h v o l u m e ,

v e ry l o w co s t p ro d u c t i o n .

4 H a r s h e n v i r o n m e n t a p p l i c a ti o n s

I t h as g en e ra l l y p ro v en p o s s i b l e t o o b t a i n

ad eq u a t e e l ec t ro n i c p ack ag i n g r e l i ab i l i t y ev en i n

t h e h a r s h e s t a e r o sp a c e a n d m a r i t i m e e n v i r o n m e n t s .

Ho wev er , t h i s r e l i ab i l i t y h as t y p i ca l l y n o t co m e

c h e a p l y a n d p r o d u c t d e v e l o p m e n t t i m e s w e r e

u s u a l l y l o n g an d p ro d u c t i o n v o l u m es r e l a t i v e l y

s m a l l. A n u m b e r o f h a r s h a p p l ic a t io n e n v i r o n m e n t s

a re , h o wev er , i m p o s i n g a r ev e r s a l o f t h i s tr en d .

D o m i n a n t a m o n g t h e s e i s a u t o m o t i v e e l e c t r o n i c s

b u t t h e d e m a n d f o r l o w e r c o s t c o m m u n i c a t i o n s

s a t e ll it e s , co m m erc i a l a i r c r s R c o n t ro l s y s t em s an d

d r i l l -h ead e l ec t ro n i c s y s t em s a re a l s o i n f l u m t i a l .

T h e u n d e r - h o o d a u t o m o t i v e e n v i ro n m e n t i s p e t h a p s

t h e h a r s h e s t c o n v e n t io n a l e n v i r o n m e n t i n w h i c h

an e l ec t ro n i c p ack ag e h as t o f i m c t i o n . Ho w ev er , t h e

a u t o m o t i v e i n d u s t r y d e m m d s h i g h v o l u m e

p ro d u c t i o n , l o w u n i t co s t an d r e l a t i v e l y s h o r t

s y s te m d e v e l o p m e n t t im e s . A s t h e s o p h is t ic a t io n o f

en g i n e m o n i t o r i n g an d co n t ro l s y s t em s i n c reas es ,

e l ec t ro n i cs i s c l a i m i n g a h i g h e r p ro p o r t i o n o f

o v era l l v eh i c l e co s t an d t h e t r en d t o ward s co s t

r e d u c t io n i n p a c k a g i n g c a n t h e r e fo r e b e p r e d i c t e d t o

i n c reas e . R e l i ab i l i t y s p ec i f i ca t i o n s a re h o wev er

l i k e l y t o b e c o m e m o r e s t r i n g e n t a s a u t o m o t i v e

e l ec t ro n i cs i n c reas i n g l y t ak e o v e r s a fe t y c r i t i ca l

fu n c t i o n s . Th e r e l i ab i l i t y ch a l l en g e s e t o u t i s

t h e re fo re t o p ro v i d e h i g h r e l iab i l it y a t l o w co s t i n a

v e r y h a r s h e n v i r o n m e n t . T o m e e t t h i s c h a l l en g e , a l l

l ev e l s o f s y s t em re l i ab i l i t y w i l l h av e t o b e

ad d res s ed b o t h o n an d o f f - ch i p an d s o p h i s t i ca t ed

d es i g n fo r r e l iab i l it y m e t h o d o l o g i es w i l l h av e t o b e

d e v e l o p e d a n d a p p l i e d w h i c h w i l l c o n c t a r e n t ly

a d d r e s s m a n y o f t h e i s su e s w h i c h a r e d i s c u s se d i n

t h e o t h e r s ec t i o n s o f t h i s p ap e r . S u cce s s fu l l y

m e e t i n g t h i s c h a l l e n g e w i l l h o w e v e r p r o v i d e

m at e r i a l s , p ro ces s es an d d es i g n m e t h o d o l o g i es

w h i c h w i l l s u p p o r t l o w e r c o s t a p p l i c a t i o n o f

e l ec t ro n i c s y s tem s i n o t h e r h a r s h en v i ro n m en t s .

5 . O p t o - p a d m O n g a n d I n t e r e o n n e e t i o n

D u e t o i n c r e a s i n g o p e r a t i n g

f r eq u en c i es ,

o p t i ca l i n . c o n n e c t i o n s a l r e a d y i n u s e a t

b a c k p l a n e l e v e l a r e i n c r e a s in g l y l i k e l y t o b e

ap p l i ed a t i n d i v i d u a l b o a rd l ev e l . On e o f t h e f i r s t

ap p l i ca t i o n s wh i ch wi l l d r i v e t h i s d ev e l o p m en t i s

l i k e l y t o b e c l o ck d i s t r i b u t io n b u t i t a l s o wi l l b e

increas ingly requi red for cr i t i ca l s ignal

i n te r c o n n e c ti o n s . T w o d e v e l o p m e n t s ar e n e e d e d t o

en ab l e b o a rd l ev e l o p t i ca l i n t e rco n n ec t i o n s : l o w

c o s t p o l y m e r i c o p t ic a l i n t e rc o n n e c t s a n d a m e a n s o f

a l l o wi n g co n v en t i o n a l s i l i co n IC s t o e f f i c i en t l y

em i t l i g h t .

R & D i s a l r e a d y u n d e r w a y o n o p t i c a l

i n t e rco n n ec t s t h a t can b e i n t eg ra t ed in t o

m a i n s t r eam b o ard m an u fac t u r i n g p ro ces s es [6 ]. I t is

t h e re fo re l i k e l y t h a t t h e i n t eg ra t ed o p t i ca l

i n t e rco n n ec t i o n s wi l l a t l eas t b e av a i l ab le a l t h o u g h ,

i n a s i m i l a r m an n er t o i n t eg ra t ed p as s i v es , n ew

re l i ab i l i t y ch a l l en g es i n m a t e r i a l s an d i n t e r f ac i a l

s t ab il it y w i l l h av e t o b e ad d res s ed . Th e re d o es n o t

s eem t o b e an eq u a l p o s s i b i l i t y t h a t co n v en t i o n a l

s i l i co n IC s wi l l b e ab l e t o e f f i c i en t l y em i t l i g h t .

H o w e v e r , l o w c o s t , s m a l l s i z e d c o m p o u n d

s e m i c o n d u c t o r l a se r s a n d L E D ' s d o e x i s t a n d

h y b ri d _i ~ ti on t ech n i q u e s can b e u s ed t o a t t ach t h es e

t o s i l i co n IC s t o a l l o w t h em t o em i t l i g h t [7 ] .

I m p o r t a n t a m o n g t h e r e li a b il it y i s s u e s w h i c h n e e d

t o b e a d d r e s s e d i n c lu d e t h e i m p a c t o f t h e p i c k a n d

p l ace o p e ra t i o n o n o p t i ca l d ev i ce re l i ab il it y an d t h e

e f f e c t o f c y c l i c t h m n o m e c h a n i c a l s t r e s s o n t h e

re l iab i l it y o f t h e V C S EL i t s e l f an d i t s a t t ach m en t .

T h e d e v i c e a t t a c h m e n t i s h o w e v e r o n l y t h e f i r s t

s t ep i n b u i l d i n g th e o p t i ca l h y b r i d s y s t em . Th e i s s u e

o f i n t e r f a c i n g t h e V C S E L t o t h e o p t i c a l

i n te r c o n n e c t io n a n d e n s u r i n g t h e l o n g t e r m

m ec h an i ca l , th e rm a l an d e n v i ro n m en t a l i n t o g ri t y o f

t h a t i n t e r f ace m u s t a l s o b e ad d res s ed . Ad d t o t h i s

t h a t p r o j e c t e d c o m p l e x s y s t e m s m a y e v e n t u a l l y

r e q u i re t h e m o u n t i n g o f h u n d r e d s o f s u c h d e v i c e s

an d t h e s ca l e o f t h e r e li ab i li t y ch a l l en g e b eco m es

a p p a r m t . P a c k a g e a n d b o a r d l e v e l o p t i c a l

i n t e rco t m ec t i o n wi l l r eq u ir e , an d w i l l b e a v eh i c l e

t o d e m o n s t r a t e , m a n y n e w d e v e l o p m e n t s i n

re l iab i l i ty sc ienc e.

6 . G r e e n e l e c t ro a i e s

A m a r k e t a n d c o n s u m e r b a s e d d r i v e r f o r

a d v a n c e s i n p a c k a g i n g i s t h a t o f e n v i r o n m e n t a l l y

f r i en d l y e l ec t ro n i cs wh i ch i n c l u d es b o t h

m a n u fac t u r i n g an d u l t i m a t e d i s p o s a i/ r ecy c ii n g . Th e

p r i n c i p l e o f ex t en d ed p ro d u ce r r e s p o n s i b i l i t y

w h e r e , f o r e x a m p l e , a n I C p a c k a g i n g c o m p a n y

m u s t t ak e u l t im a t e en d o f l if e r e s p o n s i b i li t y fo r i t s

p ro d u c t s , h as l o n g t e rm t ech n o l o g i ca l i m p l i ca t i o n s

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f o r t h e I C p a c k a g i n g p ro c e s s. A c o m m o n e x a m p l e

i s & a t o f t r a n sf e r m o u l d e d p l a s t ic p a c k a g e s ,

f a b r i c a t e d i n v o l u m e s o f b i l l i o n s p e r y e a r . T h e

p l as t i c m a t e r i a l u s ed i s l a rg e l y t h en n o s e t ep o x y

wi t h a h i g h s i l i ca f i l l e r co n t en t an d o t h e r m i n o r

co n s t i t u en t s ( i n c l u d i n g en v i ro n m en t a l l y u n f r i en d l y

a n t i m o n y a n d b r o m i u m b a s e d f l a m e r e t a r d a n t

c o m p o u n d s ) . O n c e t h e e p o x y i s c u r e d i t b e c o m e s

l a rg e l y i m p e rv i o u s t o ch em i ca l a tt ack , w i t h o n l y

b o i l i n g fu m i n g n i tr i c ac i d ( i t s e l f n o w a r e s t r i c t ed

m a t e r i a l) c a p a b l e o f c h e m i c a l l y s tr i p p in g t h e e p o x y

s o th a t t h e c h i p a n d l e a d f r a m e c a n b e r e c o v e r e d a n d

recy c l ed . A l t h o u g h o rg an i c d i g es te r s a r e u n d e r

d e v e l o p m e n t f o r th e e p o x y , t h e p r i nc i p a l te c h n i q u e

fo r ep o x y r em o v a l a t en d o f l i f e i s i n c i n e ra t i o n ,

wi t h r e l eas e o f u n d es i r ab l e b y -p ro d u c t s . To

faci l i t a te r e y l i n g f p la s t i c p ack ag es , a p res s u re i s

b e i n g ex e r t ed t o u s e t h e rm o p l as t i c en cap s u l an t s , a

m o v e t h a t w i l l r a i s e m a n y n e w r e l i a b i l i t y

ch a l l en g es .

T h e e n v i r o n m e n t a l i s s u e w h i c h h a s r e c e i v e d

t h e m o s t p u b l i c i t y i s h o w e v e r t h a t o f l e a d - b a s e d

(P b ) s o l d e r s . I t h as b een t h rea t en ed fo r a t l eas t t h e

l a s t f i v e y ea r s t h a t l ead wo u l d b e l eg i s l a t i v e l y

b a n n e d f r o m e l e c t r o n i c p r o d u c t s i n E u r o p e a n d

e l s ewh ere . Th a t t h rea t h as f a i l ed t o m a t e r i a li s e y e t

m a n y c o m p a n i e s ( e . g . N o r t e l a n d H i t a c h i ) h a v e

a n n o u n c e d t h e r e m o v a l o f l e a d f r o m t h e i r e l e ct r o n ic

p ro d u c t s . Th ere i s a l s o ac t i v e Eu ro p ean r e s ea rch

i n t o t h e d e v e l o p m e n t o f l e a d f r e e s o l d e r a l l o y s

co m p a t i b l e w i t h cu r ren t e l ec t ro n i cs a s s em b l y

p ro ces s es . Th e d r i v e r fo r th es e d ev e l o p m en t s i s

l a r g el y d r iv e n b y p e r c e i v e d m a r k e t d e m a n d , f u e l l e d

b y c o n s u m e r e n v ir o n m e n t a l c o n c e rn , f o r l e a d f r e e

p ro d u c t s . F ro m a r e l i ab i l i t y p o i n t o f v i ew, t h e re

ex i s t d ecad es o fa ccm n u l a t ed r e l iab i l i ty d a t a fo r t in -

l ead s o l d e r s an d r e l iab i f it y p red i c t io n s h av e b e co m e

i n c reas i n g l y accu ra t e . W i t h t h e i n t ro d u c t i o n o f l ead

f r e e al lo y s , u r g e n t w o r k h a s h a d t o b e c a r r ie d o u t

a n d w i l l c o n t in u e to b e r e q u i r e d to e v a l u a te t h e

re l i ab il i ty o f l ead - f r ee a l l o y s i n a l l t h e i r ap p l i ca ti o n s

f r o m l a r g e p o w e r d e v i c e a t t a c h d o w n t o f l i p - c h i p

s u k le r j o i n ~

P l as t i c IC p ack ag i n g an d s o l d e r a l l o y s a re

o n l y t w o e x a m p l e s - m o s t p r i n te d c i r c u it b o a r d s a r e

t h e r m m e t e p o x y b a s e d a n d c o n t a i n c o p p e r, n ic k e l ,

c a d m i u m , l e a d a n d g o l d w h i c h a r e u n d e s i r a b l e

p o l l u t an t s i f a l l o wed i n t o ch i n k i n g wa t e r . R ec y c l i n g

o f P C B s w i l l d e m a n d r e cl a m a t i o n o f th e s e m e t a l s

f r o m b o t h o u t e r a n d b u r i e d l a y er s a n d a m o r e e a s i l y

r e m o v e d m a t e r i a l t h a n t h e r m m e t e p o x y w i l l b e

n e e d e d . T a b l e 5 0 , w h i c h i s t o o l m g e t o r e p r o d u c e

h e r e , o f t h e S I A r o a d m a p f o r s e m i c o n d u c t o r s [1 ],

l i s t s fu r t h e r ex am p l es o f m a t e r i a l an d

m a n u f a c t u r i n g c h a n g e s t h a t w i l l b e n e e d e d t o m e e t

e n v i r o n m e n t a l r e q u ir e m e n t s . T h e c o n s m n e r c a n

h o p efu l l y l o o k fo rward t o a c l ean e r fu t u re ;

r e l i ab i l i t y p rac t i t i o n e r s can ce r t a i n l y l o o k fo rward

t o a c h a l l e n g i n g o n e w h e r e th e r e w i l l b e a d e m a n d

fo r r ap i d a s s es s m en t o f t h e r e l i ab i l i t y i m p l i ca t i o n s

o f u s i n g a l t e rn a t iv e m a t e r i a l s e ts . I t i s u n l i k e l y t h a t

t h e re wi l l b e t i m e av a i l ab l e t o d o ex t en s i v e

rel iab i l i ty t es t ing of many d i f ferent a l t ernat ives .

Th e t o o l s h o wev er a l r ead y d o ex i s t , an d wi l l

b e c o m e m o r e s o p h i s t i c a t e d , t o s i m u l a t e t h e

e l ec t ri c al , t h e r m a l a n d t h e r m o m e d u m i c a l e ff e c ts o f

u s i n g a l t e rn a t i v e m a t e r i a l s . W h a t i s n o t r ead i l y

av a i l ab le i s a f a s t an d accu ra t e m e t h o d o l o g y fo r

o b t a i n i n g m a t e r i a l an d i n t e r f ac i a l p ro p e r t i e s o f

m a t e r i a l s i n co n f i g u ra t i o n s wh i ch a re r ep res en t a t i v e

o f t h e i r en d u s e ap p l i ca ti o n . F o r ex am p l e , i f a

t h e rm o p l as t i c i s g o i n g t o b e u s ed a s a 2 5 t t m t h i ck

l a y e r , w e w i l l n e e d t o k n o w h o w i t b e h a v e s

e l ec tr i ca ll y , t h e rm a l l y an d m e ch an i ca l l y a t t h a t

t h i c k n e s s a n d n o t a s a b u l k s a m p l e . T h i s

i n f o rm a t i o n w i ll a l l o w t h e m o s t p r o m i s in g m a t e r i al

s e t s t o b e s e l ec t ed . Th es e can t h e n s u b j ec t ed t o t h e

re l iab i l it y t e s t in g w h i ch i s u n l i k e l y , a t l eas t i n t h e

n ea r t e rm , t o b e r ep l aced b y r e l i ab i l i t y s i m u l a t o r s

f o r c o m p l e x m u l t i - m a t e r i a l a n d m u l t i - c o m p o n e n t

s y s t em s wi t h m u l t i p l e p o t en t i a l f a i lu re m e ch an i s m s .

T h e d e v e l o p m e n t o f m e t h o d o l o g i e s f o r e f f i c i e n t

m e a s e r e m e n t o f m a t e r ia l p r o p e rt ie s , a n d h o w t h e s e

p ro p er t i e s ch an g e wi t h t cm p eraUae , h u m i d i t y e t c . ,

i s an i m p o r t an t ch a l len g e .

7 . M i c r m y a t e m s

M i c r o s y s t e m s t e c h n o l o g y (M S T ) , d e f i n e d a s t h e

i n t eg ra t i o n o f s en s o r s , ac t u a t o r s an d

m i c ro e l ec t ro n i cs i n a m o n o l i t h i c o r h y b r i d c i r cu i t ,

an d i t s co u n t e rp a r t m i e ro -e l ec t ro m ech an i ca l

s y s t em s ( IVlEM S ) a re r eg a rd ed as b e i n g a t a s i m i l a r

t e c h n o l o g y a n d m a r k e t s t a g e a s m i c r o e l e c tr o n i c s 3 0

y ea r s ag o [1 3 ] . R e l a t i v e l y s i m p l e d ev i ces , s u ch as

au t o m o t i v e a l rb ag s en s o r / ac t u a t o r IC s , a r e b e i n g

u s e d i n h i g h v o l u m e s w h i l e i n c r ea s i n g ly c o m p l e x

s y s t e m s a r e b e i n g d e m o n s t r a t e d i n m a n y

l ab o ra to r i e s an d a re b eg i n n i n g t o ap p ea r o n t h e

m ark e t . Ex am p l es i n c lu d e :

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J . B a r r e t t/ M i c r o e l e c tr o n i c s R e l i a b i li t y 3 8 { 1 9 9 8 1 2 7 ~ 1 2 8 6

Micro-total analysis systems OtTAS) for

chemical/biological analy sis wh ich contain

pumps, chemica l or biologica l sensors and

signal pro cessing on a single chip or hybrid.

• M icro-m etrology system s such as miniaturised

spec t rometers and gyroscopes .

Micro-optical systems containing integrated

lasers, adjustable lenses, mirrors and filters

such as the Texas Insmnnents micro-mirror

project ion IC [8].

M any other exam ples exi s t but tw o pr imary b arr ie rs

to vo lum e comm ercialisation are consistently

mentioned: cos t efficien t pac kag ing and reliability.

The reasons for thi s can be readi ly seen i f we

examine some of the new re l iabi l i ty cha l lenges

which M ST/M EM S present .

hundreds o f microns in s ize , the presence o f

particulates or contaminants can comp rom ise

system operation and reliability. Indeed, wear in

moving micro-components can i t se l f genera te

particulates even if they are absen t from the

package as fabr ica ted [13] . In components which

use biological layers for sensing, biological

contaminat ion from packaging materials and the

effec t of packaging thermal processes on the bio-

layers must be taken into account . A micro-sensor

which i s intended to measure he avy meta l (Pb, Cu,

Zn, C d) content in water to an accuracy of par t s per

bi l l ion cannot be contaminated by leaching of such

metals fro m pac kaging materials [ 10].

7 1 Lo w o r z e r o s tr e s s p a c k a g i n g

Ma ny s i l i c on ba se d MST/ MEMS c ompone nt s

conta in micro-machined mem branes or beams, used

for sensing and/or actuation, w hich are o f the order

of a f e w mi c rons t h ic k bu t w hi c h ma y b e hundre ds

or thou sands o f micro ns in length/width. These

structures are ver y sens itive to mechanical slress

which may cause incorrect operat ion or complete

fai lure. At the same t ime, these components must

undergo the normal packaging processes of die

attach, electrical connection and encapsulation, all

of which impose bending s t resses of var ious sor t s

on the component , especial ly as the preferred route

for low cost is to use organic packaging materials.

Figure 5.(a) shows a stress simulat ion o f on e com er

of a micro-machined m embrane subjec ted to plas t ic

encapsulation stresses. Critical stresses were

predicted to exist in the membrane (lower left )

which exceeded the memb rane s t rength and in a

micro-m achined channel (low er right). Fai lure

analysis confirm ed the simulat ion and the cracke d

me mb rane is show n, after decapsulation, in Figure

5. (b) [9] . Methodo logies mu st therefore be found

which mechanica l ly and the tmomechanica l ly

de c oupl e t he mi c ro-ma c hine d s ~c t u re s f rom the

pack age w ithout com prom ising reliabil ity.

Figu re 5.(a). Thermo mechanical stress simulation

o f a micro -mac hined si licon structure

7 2 P a r t i c u l a te a n d c o n t a m i n a n t f r e e p a c k a g i n g

In MST/MEMS components which conta in micro-

mechanica l moving par ts of the order of tens o r

Figu re 5.Co). Stress induced cracks in th e micro-

mac hined si licon structure of Fi~n'e 5.(a).

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285

7. 3 .

Mult imedia

i n p u t o u t p u t

The problem of deal ing with elect r ical input-

outputs w as discxmsed n Section 2.1 abov e and the

issue of optical input-ontput was re ferred to in

Sect ion 6. I f added to this is f luidic and/or gaseous

input-output, the challenge is considerably

complicated. At the simplest level is a sensor IC

which has to be immersed in ambient temperature

aqueous analytes and even this represents a

cons iderable chal lenge in def ining and proving the

reliability o f a low cost packa ging process w hich

exposes the sens ing element whi le protect ing the

microelectronics [10 ] . The extension of such a

packaging process to higher temperature or more

chem ically aggressive analy tes is a considerable

reliability challenge. H owe ver, m an y MST/M EMS

comp onents are being developed which wil l include

act ive f low-throogh sys tems dr iven by integrated

micro-pumps. Suc h com pone nts requ ire micro-

plum bing input-output and internal packaging to

isolate the fluid from micfo electron ics sections o f

the component. Som e of these com ponents wil l a lso

use m icro-spectrometers to p erform analysis of the

passing fluid and an optical interfac e will therefore

also have to be provided. Add the compfications

that such a system should be compact, portable,

low-cost and recyc lable and th e scale o f the

reliability challen ge becom es apparent.

MST/MEMS component s p resen t mos t o f the

reliability cha llen ges associated with conv entional

microelectronics and add m any new ones . A good

exam ple of the additional challenges to b e

considered, and discovered, in bringing a MEMS

com ponen t to ma rket is given in the IRPS'98 paper

on the rel iabi l i ty engineer ing of the Texas

Instruments mic ro-m irror projection system [8].

Rev i ew nd summ ry

This paper has at tempted to forecas t the major

reliabili ty challenges that will need to be faced in

the com ponen t and sys tem s packaging f ields in the

com ing years. I t is obviously d ifficult to discuss all

the possible challen ges as i t is s imilarly difficult to

be quantitatively specific on their nature or t iming;

how ever som e overal l t rends can be forecast :

• Reliability is mo ving and will con tinue to

move fur ther back in the sys tem development

c y c le w i t h d e s i g n f o r re l i a b i l it y b e c o m i n g an

in teg ra l pe r t o f t he cyc le f r om t he beg inn ing

Increasingly, pedatging and reliabili ty will be

appficafion driven and system focused. The

traditional sequential,

a n d

not necessar i ly

interaetive, W oc ~s o f chip design -~ chip-fab

--~ package -~ board --~ box --~ burn-

in/reliability test will not be adequate.

Con current multi-disciplinary effort will be

required to deliver a reliable system to tim e

and budget.

The re is a growing expectation o f increased

reliability a t reduce d cost and o f shortcr system

developme nt cycles . The t im e and budget for

prototyping and tes t ing wil l therefore

progressive ly shrink. Simulation and virtual

prototyping will therefo re increasing ly beco m e

important reliability activities. T he y will

how ever only be viable i f the inputs on

ma terial data an d beh aviour are accurate.

Reliability cha lleng es in conventional

mictoeleetronics and opto-electronies

packaging can be reasonably forecas t but the

pace o f progress wi ll be v ery fast over the next

ten years . Th e reliabi li ty com mu nity therefore

needs to begin work now on developing i ts

own roadmaps to meet these chal lenges . The

recent ly published European p ackaging

rcadm ap is an exam ple [11] al though i t i s not

specifically focused o n reliability issues.

New chal lenges are l ikely to cont inue

appearing from sources such as environmental

legislation and consumer concern.

Methodologies to q uick ly assess th e reliabili ty

impact o f these and to r ecommend op t imum

solutions w ill be required.

MST and M EMS technology of fe rs p robably

the greates t new chal lenge in packaging

reliabili ty. New materials will be used and

ma ny ex tra mater ial parameters and behaviours

will need to be cons idered in des igning for and

assessing reliability. The multidisciplinary

concurrent development teams that are

important for microe lectronic systems will be

essent ial for MST and MEMS packaging. As

we ll as consisting o f the traditional electronic,

physics, materials and mechanical disciplines

which, to at least some extent share a com mon

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technical language, the team s w ill also contain

disciplines such as micro-chem islry and m icro-

biology. The integration of these disciplines to

produce a reliable product will itself be an

interesting challenge. To quote the IEEE

Transactions on C PM T: '~lectronic Packaging:

Cross-Discipline is the Only Discipline [12].

A e i m e w l e d g e m e n t s

The content of this paper has been informed by

discussions with m any colleagues in N MR C, with

partners in collaborative projects and at

conferences. Too many to mention individually,

their collective enthusiasm to discuss these issoes is

gratefully acknow ledged.

R e f e r e n ~

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