Progression to 0.3 mm Pitched BGA SMT Assembly

Michael Kochanowski

Intel Corporation

5200 NE Elam Young Parkway, Hillsboro, Oregon 97124, USA

michael.kochanowski@intel.com

ABSTRACT

As consumer electronics devices increase in performance

and feature density and decrease in size, pitch reduction in

BGA packages will occur. The current state of our

industry uses 0.4 mm pitched packages on the smallest

package. We anticipate that 0.3 mm pitch is on the near

horizon.

Progression to smaller pitches creates new challenges

with Surface Mount Technology (SMT) assembly. We

will explore issues with the manufacturability that are

complicated by making the step to 0.3 mm pitch. We

have done equipment tolerance variability analysis as well

as printing and assembly experiments to evaluate the

implications of this anticipated pitch advancement.

Critical factors that were investigated include solder paste

selection (powder size), stencil design and manufacture,

SMT equipment performance (X-Y tolerances), printed

circuit board design (board topography and tolerances),

and warpage of area array packages at reflow

temperatures.

Results indicate that procedures that work at pitches of

0.4 mm and above will need to be revised which will take

time and effort to achieve.

Key words: 0.3 mm pitch, solder paste print, package-on-

package, stencil, stencil “gasketing”, warpage, coplanarity

INTRODUCTION

The current state of the art for commercially available

consumer electronic devices such as cell phones, tablets,

and laptop computers has the smallest pitches for BGAs

at 0.4 mm. Industry roadmaps project that by 2017, 0.3

mm pitched devices will be on the market and by 2019,

0.2 mm pitched devices will follow. Our industry has

progressed from 0.7 to 0.4 mm pitch over many years

with moderate effort. However, it looks like the step from

0.4 mm to 0.3 mm will require that several challenges be

addressed.

METHODOLOGY

This paper explores the 0.3 mm pitched assembly through

three steps. The first step was a theoretical and statistical

application of some generally accepted assembly rules.

The second step included preliminary 0.3 mm pitched

print and assembly experiments. The last step includes

analysis of experience gained through supporting 0.4 mm

pitched commercial assembly.

SECTION 1 - THEORETICAL ESTIMATE OF SMT

EQUIPMENT PERFORMANCE BASED ON

VARIANCE ANALYSIS

SMT assembly normally uses paste print, placement and

reflow. We will explore any rules associated with each of

these areas and describe equipment requirements to assemble

0.3 mm pitch.

Solder Paste Print Feature Alignment Capability

A rule accepted in our industry for paste print is that the area

printed in paste should cover at least 75% of the pad to

achieve an acceptably robust high yielding process.

Figure 1 – Drawing illustrating the maximum acceptable

misalignment of solder paste to pad where 75% of the pad is

covered by paste.

Based on this rule we derived an equation that describes the

overlap of printing,

Over-lap = 2((2 π r2/360) – (sin θ

. cos θ r

2))/(

π r

2)

where:

Over-lap = fraction of the pad overlapped by solder paste,

θ = angle between line connecting centers of the pad and paste

print and the point where paste edge and pad edge overlap in

degrees, and

r = radius of the pad and paste print (1:1 print assumed which

is a likely estimate for 0.3 mm pitch. Smaller apertures would

create less than optimal print volume and larger apertures

risk bridging defects.).

To meet the 75% coverage rule we solve and get an angle,

θ, of 78.6º. Our estimates are that for 0.3 mm pitch that 7

mil pads on a board with a 7 mil stencil aperture would be

a likely geometry. To achieve this overlap the maximum

allowable offset between center of the pad and the center

of the print is 2 . cos θ

. r (derived from geometry) which

equals 1.38 mil. Therefore, ± 1.38 mils is the maximum

offset in print alignment that we can tolerate and still have

an SMT process where we would expect to have good

yield. We will use this result later in our analysis.

The relationship that describes the location of print on the

board is a sum of the alignment of the printer to the board,

the alignment of the board features across the board, the

alignment of the stencil features on the stencil and

alignment due to stencil flex during printing.

First, we calculate the variation in the printing process.

To calculate the total variance of the printing process we

just add the variance of the individual components as

follows:

σ2

total printing = σ2

printer alignment + σ2

PCB features +

σ2

stencil features + σ2

stencil flex/movement during print

Note: For simplicity our analysis is done in only one

dimension. (Our analysis was done in X only versus X

and Y combined.) To be exact we would look at the

performance in two dimensions (Z = (X2 + Y

2)

½) but the

math gets very complicated. In another case, we did this

analysis in the combined X and Y dimensions but we

found that the conclusion with a one dimensional

approach is effectively the same.

Next, we will apply this tolerance to the variation in print

conditions that we would expect to find in normal good

equipment used in the assembly of smart phones, tablets

computers.

We used the expertise from various board, machine and

stencil experts to estimate the variance of the components

as follows:

1. σ2

printer alignment = (0.492 mil /6)2 = 0.006727

mil2 (based on the specs of a DEK Galaxy

machine)

2. σ2

PCB features = (0.430 mil /4)2 = 0.01156 mil

2

(based on board class 2 feature tolerances scaled

to a 5”x5” board size)

3. σ2

stencil features = (0.3 mil /4)2 = 0.005625 mil

2

(based on conversations with various stencil

manufacturers)

4. σ2

stencil flex/movement during print = (1.0 mil /4)2 =

0.0625 mil2 (based on conversations with a

printing machine manufacturer).

Adding all of the variances, we get a total variance of the

printing process as σ2

total printing = 0.0864 mil2 and σtotal

printing = 0.294 mil. When we apply a Cpk rule of 1.33, the

variability in the alignment of the printing process would be ±

4. σ total printing = ± 1.176 mils. This analysis confirms that the

cumulative variability of the solder paste printer, the printed

circuit board, the stencil features and the stencil movement

during printing is within the allowed calculated tolerance of ±

1.38 mil. However, we are right at the margin of being

acceptable.

Our conclusion is that we expect that the printing step with the

quality of assembly equipment generally used by reputable

ODMs, with normal class 2 board quality and with normal

stencil tolerances to be just sufficient for 0.3 mm pitch

assembly in X-Y alignment.

It should also be noted that of the total variance in the printing

process, about 72% of the variance comes from stencil

flex/movement. Printer alignment, PCB features, and stencil

features account for only 8, 13 and 7% of the variance,

respectively. Although these numbers come from estimates,

we believe that the order of magnitude their impact is realistic.

What this means is that the flex and movement of the stencil

in the printing machine is by far the largest component of

variation in this system. The dynamic movement of the stencil

in the printer during its operation is becoming more relevant at

this low pitch and is the limiter to the progression toward finer

pitches.

At 0.3 mm pitch, the X-Y location of the print location is

expected to move from board to board (with commonly

available equipment). An example of this variation is shown

in Figure 2. Notice that the first print completely covers the

pad and another print with the same printer set up but a few

prints later the print has shifted to the right so that the edges of

the copper pads are exposed. We found that this kind of drift

is routinely when working with some boards at this pitch.

Figure 2 – Example of Print Position Variation on 0.3 mm

Pitch BGA Site.

BGA Part Placement Alignment Capability

Similar to print, a rule in SMT is that when a BGA is placed,

the solder ball should cover at least 50% of the pad. (see

Figure 3)

Figure 3 – Drawing illustrating the maximum acceptable

misalignment of BGA solder ball to pad where 50% of the

pad is covered by the area projection of the solder ball.

We apply the same mathematical analysis to this system

as we did in the prior solder paste print situation. (A good

estimate for 0.3 mm pitched geometries is a 7 mil pad

diameter and a 7 mil solder ball diameter.) In this case

we find that ± 2.83 mil is the maximum placement offset

that we can tolerate and still have an SMT process where

we would expect to have good yield.

Similar to before, we apply this tolerance to the variation

in placement conditions that we would expect to find in

state of the art equipment in the assembly of smart

phones, tablets and laptop computers.

To calculate the total variance of the placement process

we just add the variance of the individual components as

follows:

σ2

total placement = σ2

placement positioning alignment +

σ2

PCB features + σ2

ball-to-ball

Again, using the expertise from various board and

machine vendors to estimate the variance of the

components as follows:

1. σ2

placement machine position alignment = (2.0 mil /4)2

= 0.25 mil2

(Universal GSM machine)

2. σ2

PCB features = (0.430 mil /4)2 = 0.01156 mil

2

(Based on board class 2 feature tolerances scaled

to a 5”x5” board size)

3. σ2

ball-to-ball ≈ 0 ≈ 0 mil2 (variation of balls

within the array respect to each other are

assumed to be negligible)

Adding all of the variances, we get a total variance of the

placement process as σ2

total placement = 0.26256 mil2 and

σtotal placement = 0.511 mil. When we apply a Cpk rule of

1.33, the variability in the alignment of the placement

process would be ± 4. σ total placement = ± 2.05 mils. Again, the

calculated accumulative variation of ± 2.05 mils is less than

the ± 2.83 mil allowable variation. The placement systems in

place at reputable quality ODMs are capable to assemble at

0.3 mm pitch.

It should also be noted that the placement step is more capable

statistically at 0.3 mm pitch than paste printing.

Ideal Paste Printing Mechanism

Before we continue we should discuss a mechanism for an

ideal solder paste print. Figure 4 animates this print event.

Figure 4 – Ideal Solder Paste Print Animation – View with

Stencil Engaged (above) followed by View with Stencil Lifted

(below)

In an ideal case, the stencil sits perfectly flat against the board

surface. After a pass of the squeegee, solder paste fills the

aperture and makes complete contact with the pad below.

Next, the printing machine lifts the stencil off of the board.

The adhesion force of the solder paste on the pad (represented

by the green line) is greater than the adhesion force of the

solder paste against the stencil wall (represented by the red

line) and the solder paste releases from the aperture.

“Gasketing” is defined as the ability of the stencil to form a

temporary seal with the board surface such that printed solder

paste has nowhere to flow other than on the intended pad

surface. Gasketing is key to proper print performance. So far

there is no way to quantify or measure the quality of gasketing

of a stencil/board/printer system.

All of the release-ratio rules around solder paste print are

predicated in a system that is dominated by these dynamics.

Unfortunately, as pitches get smaller, the scale of board

topography irregularities, board support and stencil flex

issues combine and challenge the ability to get proper

gasketing.

SECTION 2 - PRINTING EXPERIMENTS

TARGETING 0.30 MM PITCHED ARRAYS

We conducted several print screening studies to determine

what conditions may be more conducive to assembling

packages near 0.3mm pitch. A discussion of some of our

observation follows.

Stencil Design Screening Study

A screening study was done to see the repeatability of

solder prints with a variety of round aperture stencil

constructions targeted toward 0.30, 0.35 and 0.4 mm

pitched devices with some commonly use type 4 solder

pastes.(1)

A matrix of these stencils is shown in Table 1.

Table 1 – Stencil Construction for Screening Study

No Nomi

nal

Aperture

Diam

eter, mil

Mfg

Method

Thick,

mil

Treatments Area

Ratio

(from

measured

aperture

diameters)

1 10 laser 4 0.59

2 7 efab 3 0.58

3 7 efab 3 nano 0.55

4 7 laser 3 Ni plate,

nano 0.56

5 7 efab 4 nano 0.46

6 7 laser 4 Ni plate,

nano 0.39

7 8 efab 3 0.63

8 8 efab 3 nano 0.64

9 8 laser 3 Ni plate,

nano 0.65

10 8 efab 4 nano 0.53

11 8 laser 4 Ni plate,

nano 0.46

A summary of the volume of paste printed for multiple

prints with the fore stencils from Table 1 is shown in

Figure 5.

Figure 5 – Paste Volume Distribution for Various Stencil

Designs colored by Area Ratio (On the X-axis, print set up is

identified by stencil thickness in mils, stencil aperture

diameter in mils, and method of construction, respectively.)

We looked at the distributions of solder paste volume versus

the stencil types. Specifically we wanted stencils that both

delivered higher average solder paste volume and, more

importantly, minimal occurrences of prints below about 50

mil3. We know from prior work that with print volumes

below 50 mil3 we tend to get more small-solder-joint and open

defects as measured by a three dimensional X-ray and

electrically open joints. The stencil designs that most

consistently met this criterion were the 8 mil diameter

apertures with 3 mils of thickness. This was expected because

of the area ratio of these stencils compared to the others was

higher. (Colors closer to green have the most favorable area

ratios and those more white and red in the spectrum are less

desirable.)

It should be noted that the area ratio of these stencils (per

measurements) was 0.63 to 0.65. For stencil apertures, area

ratio is calculated as the area of the aperture opening divided

by area of the vertical aperture wall. More aperture opening

area helps promote adhesion of the solder slug to the board

and is helpful. More aperture wall area causes the slug of

paste to stick to the stencil and interferes with full release.

Higher the values of this ratio promote better paste release.

0.3 mm Pitch Specific Print Study

After the prior screening study, we conducted a second print

study with a 0.3 mm pitched land pattern. We chose to use a 3

mil thick, nominal 8 mil circular aperture electroformed

stencil and a same sized laser cut nickel plated, nano-coated

stencil based on the good results achieved in our first

screening study. The stencils that we acquired were as follows

in Table 2.

Table 2 – Stencil Construction for Comparison Study No Target

Aperture

Diameter

(mil)

Measured

Aperture

Diameter

(mils)

Manufacture

Method

Thickness

(mils)

1 8 8.38 laser, Ni 3

plated, nano-

coated

2 8 7.99 electroform 3

We used these stencils on a board with two 0.3 mm

pitched arrays with a 12x12 mm footprint and 1600 metal

defined OSP pads on a 4-layer, 40 mil thick board.

Figure 6 shows the footprint of one of the arrays.

Figure 6 – 0.3 mm Pitched Array Test Pattern – Metal

Defined Pads

In our experiment we printed with solder pastes with

powder of type 4, 4.5, 5 and 6, with identical flux

chemistry with each of the two stencils. We printed 15

times with each solder paste and stencil combination.

Prior to running our print study, we spent some time

optimizing the solder paste print for each stencil.

Unfortunately, we found that we could not get either of

these stencils to hold a consistent print without bridging

unless we cleaned the stencils frequently. We needed to

clean the laser cut, nickel plated, nano-coated stencil after

every second print to maintain a bridge free print. With

the electroformed stencil, we needed to clean the stencil

after every print to maintain a bridge free operation. This

cleaning frequency would not be acceptable for high

volume manufacturing.

In retrospect, this frequent need for stencil cleaning is

probably linked to less than optimal gasketing of the

stencil to the board surface. A test to validate the stencil

with good gasketing conditions will be discussed later in

this paper.

Bridging Caused By Stencil Cleaning Solvent

During these experiments, we noticed that we often got a

high incidence of bridging prints resulting immediately

after automated cleaning of our stencil unless we allowed

time for the stencil to dry. (see Figure 7)

Figure 7 – Typical Bridging Resulting in 0.3 mm Pitched

Printing Graphed by a Koh Young Solder Paste Inspection

Machine.

We observed that the cleaning solvent that we were using,

isopropyl alcohol, did not evaporate quickly from the stencil

aperture area. This small amount of remaining liquid seemed

to be enough to wet with the solder paste and cause a bridge.

We need to reevaluate our choice of stencil cleaning solvent

for a more volatile solvent that may evaporate quickly for high

speed assembly.

Print Performance versus Solder Paste Type

Most of the consumer electronics industry running 0.4, 0.5 and

0.6 mm pitched devices use type 4 solder paste. We anticipate

a need to migrate to finer particle size paste for pitches below

0.4 mm. Particle size versus type is listed in Table 3. (Particle

size ranges were obtained and from two different solder paste

suppliers and were the same. However, with paste types

beyond 4, particle size ranges may vary from supplier to

supplier, particularly for the half sized ranges.)

Table 3 – Solder Paste Particle Size versus Type

Paste Type Powder Particle Size

in microns

3 25-45

4 20-38

4.5 20-32

5 15-25

6 5-15

A graph showing the volume distribution of solder paste

volume as a function of the stencil construction and the solder

paste type is shown in Figure 8.

Figure 8 – Solder Paste Volume Distribution versus

Stencil Construction and Solder Paste Type

We observed that as the solder paste powder type goes

from 4 to 4.5 to 5 and then to 6 (from larger particles to

smaller particles), the average volume printed gets larger

and the distribution get tighter. This makes sense because

it is easier to push smaller particles through an aperture

with less interference. (Generally accepted industry rules

say that 5 to 7 particles would fit across the diameter of an

aperture opening to ensure a reasonable chance of paste

flow. Particles of a larger size tend to interfere

significantly with flow.)

If we look just at a laser cut stencil and look at solder

volume versus paste type we get the following

distribution in Figure 9.

Figure 9 – Solder Paste Volume Distribution versus

Solder Paste Type

The type 5 and 6 solder pastes give a significantly higher

volume of solder per print. More work is need to be done

to see if these paste types significantly increase SMT

yield for 0.3 mm pitched devices.

Data summarizing the solder paste volume data from our

experiment is listed in Table 4.

Table 4 – Summary Statistics on 0.3 mm Print Study by

Stencil and Paste

Stencil Paste

type

Mean

volume

mil3

Transfer

efficienc

y

Std. Dev.

in

volume

St Dev

as % of

mean

volume

efab 4 127.4 84.7 29.7 23.3 efab 4.5 138.9 92.3 31.5 22.7 efab 5 187.0 124.2 22.7 12.1 efab 6 170.7 113.4 25.0 14.6 laser 4 148.9 90.0 27.5 18.5 laser 4.5 156.6 94.6 27.7 17.7 laser 5 201.8 122.0 16.1 8.0 laser 6 199.8 120.7 15.3 7.6

Stencil Performance

In our experiment, one stencil out-performed the other. The 8

mil laser cut stencil with nickel plating and nano-coating beat

out the 8 mil electroformed stencil. (The laser cut stencil

delivered a tighter distribution of solder paste volume and

fewer prints below 50 mil3.)

For visual comparison we obtained some cross sections of

laser etched, nickel plated stencils versus electroformed (efab)

stencils. Images of cross sections of typical stencils of both

types are shown in Figures 10 and 11.

Figure 10 – Cross Section Photos of a Laser Cut Stencil with

Nickel Plating (photos courtesy of Photo Stencil)

Figure 11 – Cross Section Photos of an Electroformed

Stencil – Board Side is Down (photos courtesy of Photo

Stencil)

Both photos of the surfaces of the metal generated with

both the nickel plated laser and the efab look very good

(smooth). The surfaces of the efab stencil are generally

pitched in the right direction (the larger opening in the

aperture is down) to promote solder paste release. These

photos show much smoother stencil walls than some that

we have seen for some prior older technology laser cut

stencils and either type appear to be suitable for 0.3 mm

pitched apertures.

From this and other work, nano-coating may be

advantageous for paste release and should be evaluated

further. We have not done enough work with nano-coated

stencils yet to make a universal recommendation yet.

Printing on Boards with Silkscreen Ink is Not

Acceptable at 0.3 mm Pitch

At one time we assembled some 0.3 mm pitched packages

on test boards. Out of habit, we used silk screened

nomenclature markings in the vicinity of the 0.3 mm

pitched BGA pattern. We found that with 0.3 mm pitch,

having the nomenclature nearby drastically affected the

topography of the board, prevented proper gasketing and

caused unacceptable bridging defects. Figure 12 shows

the bridging defects caused by this poor board

topography.

Figure 12 – Bridging Caused by Silkscreen Ink Raised Board

Topography

When we saw the bridging, we looked at the topography of the

board nearby the outer edge pads using a three dimensional

microscope. We found that the silkscreen ink can add a

thickness of about 25μm (1 mil) above the solder mask. (see

Figure 13 and 14)

Figure 13 – Profile of Silkscreen Nomenclature – 25.3 μm

Thick

Figure 14 – Topography Dimensions in a Poorly Designed

PCB because of Silk Screen Ink near a Pad

We found that our solder mask can be about 1 mil above the

pad surface and the silkscreen can rise to 1 mil above the

solder mask. When this occurs near a pad the board structures

can lift the bottom of the stencil approaching 2 mils off of

the pad leading to poor print conditions. Figure 15 shows

the overflow of solder paste that we get near the edge of a

BGA package with silkscreen ink too close to the

package.

Figure 15 - High Solder Volume Caused by Raised

Stencil and Poor Gasketing

From time to time we get pads that have no solder paste

on them after a print. We speculate that this occurs if the

stencil is raise significantly from the surface of the board

that little or no contact between the solder paste and PCB

occurs. We see zero and very low levels of solder paste

print as shown in Figures 16 and 17.

Figure 16 - Zero Solder Deposit Caused by Raised

Stencil

Figure 17 - Low Solder Volume Caused by Raised Stencil

In later test boards we eliminated the silkscreen nomenclature

because of the bridging. Without silkscreen we looked at the

topography of the board again with a three dimensional

microscope. See Figure 18.

Figure 18 – Profile of a PCB without Silkscreen Ink Showing

7.7 μm Height Difference between Pads and Solder Mask.

The boards were much flatter. The difference in Z-height

between the solder mask and the copper pads was 7.7 μm or

about 1/3 of a mil. As this work was done using a 3 mil

stencil, the 1/3 mil difference is small but still in the same

scale as the stencil thickness. With some proper adjustment

we achieved consistently good prints on this board with good

gasketing and infrequent stencil cleaning (up to ten boards

between prints).

Print Machine Set Up when Printing Solder Paste at 0.3

mm Pitch

We conducted several print studies and product builds in the

0.3 - 0.4 mm pitch range. In some cases our printing has been

very successful and in other cases we struggled to maintain

consistent bridge free and low to no paste volume free

printing.

Conventional wisdom is to keep track of all print machine set

up conditions, board support and solder paste parameters to

aid in a reproducible print process. Print conditions that we

have monitored and optimized include:

1. Board support – (customized support fixture or pin or

plate supports)

2. Squeegee speed

3. Squeegee pressure

4. Squeegee size

5. Separation speed

6. Separation distance

7. Print gap

8. Cleaning frequency

9. Cleaning mode (wet and dry wipes, vacuum, clean

speed)

10. Clamp force

11. Board thickness

Board support is one of the most influential variables at

pitches of 0.4 mm and below. With very densely

populated bottom sides of boards as the norm for in high

performance consumer electronics, board support will

continue to be a challenge. While we can often get good

prints on a test board with minimal components, it is

much more difficult on a densely populated product.

Vacuum fixturing will most likely be required to keep

work pieces flat when printing a fine pitches (below 0.4

mm). With the increasing density of double sided boards

it will be a challenge to develop vacuum fixtures that have

enough area devoted to vacuum sites to hold boards flat

through printing.

From conversations with industry suppliers, we think that

board support in screen printers where the support

mechanism clamps the top surface of the PCB in the

machine can be difficult. We understand that some

equipment vendors are working on machines that do not

hold the boards on their top surface.

Most of our work has been done on simple test vehicle

boards with few parts on the top side other than those that

we are studying and no parts on the bottom side that could

interfere with board support. In real product designs,

complex and densely populated boards pose a much more

difficult print problem. Board support to print on a board

with a densely populated secondary side already built is

very difficult.

Printing Process Check When Performance is Poor

With one particular 0.3 mm pitched device board, we

were using a 3 mil thick stencil with 6.5 mil diameter

apertures we were having printing difficulties. As a check

to demonstrate that printing can work with this particular

stencil and paste, we printed repetitively on a heavy piece

of featureless copper laminate instead of the circuit board.

Printing on this flat rigid surface gave immediately good

results.

Without any special set up we achieved 15 perfect prints

in a row with no signs of bridging or low volume prints

without any stencil cleaning evaluated by visual

inspection under a microscope. Figure 19 shows some

selected photographs from this copper board print study.

Figure 19 – Examples of Sustained, 0.3 mm Printing

done on a Flat Copper Board with No Stencil Cleaning –

Print quality is maintained with no evidence of bridging or

low or insufficient solder deposits.

This tells us that the stencil and the printing process is capable

if we can get the stencil and the board to “gasket” better.

As a comparison, the area ratio of this stencil used in this

particular case was 0.54 which is considered to be aggressive

in terms of consistent solder paste release. Area ratios near

0.66 and above are considered to be safe. This shows that

printing can be pushed further than normal limits if conditions

are near optimal.

Densely populated and irregularly shaped double sided boards,

typical in devices like today’s smart phones, create havoc with

the printing process. With this type of board, we have moved

toward customize vacuum support jigs and printing remains

challenging.

SECTION 3 - BGA PACKAGE WARPAGE

Warpage Definition

Warpage for an electronic package can be defined as follows.

Warpage is difference in Z height between the tip of the

highest solder ball and the lowest solder ball with respect to a

reference plane.2 The reference plane is a least squares fit of

all of the tips of all of the solder ball measurement points on

the bottom of a package. Warpage of a BGA package is

generally measured in mm or sometimes in μm. A positive

number means that the center of the package is higher than the

corners (with the BGA balls facing downward). A negative

number means that the high point is somewhere on the outer

edge and the center is lower. This number can be measured

for both room temperature and for reflow temperature. All

packages tend to warp differently from room temperature to

reflow temperature. The relationship between room

temperature warpage and reflow temperature warpage is

unique to each package construction.

Warpage Risks in Surface Mount Assembly

We know that BGA package warpage plays a significant role

in the yield of the SMT assembly process.3 Warped packages

at room temperature is important for flux and paste dipping

processes. Warped packages at reflow temperatures can cause

open joints, head and pillow defects and/or bridges. An

example of where we see these defects in a package-on-

package construct where, at elevated temperature, the bottom

package warps into a convex shape while the top package

stays relatively flat is shown in Figure 20.

Figure 20 – SMT Risk Areas in Package-on-Package

Structure Due to Different Package Warpage Behaviors

During Reflow

In the case shown in Figure 20, the likelihood of defects

including open solder joints (hanging ball and head and

pillow solder joints) increases at the edges and the corners

of the top package and in the center and center edges of

the bottom package. (see Figure 21)

Figure 21 – Examples Hanging Ball and Head and Pillow

Defects on 0.3 mm Packages

Conversely, the likelihood of bridged solder joints

increases at the corners of the bottom package.

A shape versus profile animation for a bottom package is

shown in Figure 22.

Figure 22 – Package Flatness versus Temperature for a

representative Flip Chip BGA Package – One package

construction warps predictably with changes in

temperature.

If a flux or paste dipping process is being used, the shape

of the package at room temperature is also important

because it influences the amount and location of material

picked up in the dipping process. Figure 23 illustrates a

non-uniform flux pick up during a dipping process when

the package being dipped is not flat.

Figure 23 – Non-Uniform Flux Coverage Caused when a

Warped Package was Dipped (corner view)

Figure 24 illustrates a uniform paste pick up during a dipping

process when the package being dipped is flat.

Figure 24 –Uniform Paste Coverage Resulting when Dipping

a Flat Package (full view on left, close up of some solder balls

on right)

Within the industry, there are examples of packages that warp

in either a convex or a concave shape at reflow temperatures

depending on materials used, design features and assembly

process.4

The primary protection against open solder joints or bridging

is the compliance or buffering effect of the stack up of the

solder ball height and the printed paste thickness at reflow.5

We have successfully demonstrated 0.4 mm ball pitch PoP

package assembly in the 12x12 mm to 14x14 mm footprint

range by using a 4 mil stencil and 8 mil solder balls.

When migration to 0.3 mm ball pitch FCBGA packages

occurs, the stencil thickness will most likely be near 3 mils

and BGA solder balls will be near 7 mils. Solder ball height

and stencil thickness (paste deposit thickness) are two factors

that can be used to enable successful SMT yields. (Other

factors include paste formulation including low-temp solders,

board and package design/control for warpage behavior, etc.)

SECTION 4 – OTHER PROCESSES

Solder Paste Inspection

We conducted a metrology capability analysis of

commonly available solder paste inspection equipment

(about 6 year old technology). Our conclusion was that

these machines are capable of measuring solder paste

prints of 5 mils in diameter and 2 mils thick. This is

significantly smaller than the 7 mils diameter and 3 mils

thick prints that we anticipate are required for 0.3 mm

pitch. Solder paste inspection capability should not be an

issue with any kind of modern machine.

Reflow

There are no anticipated factors where a drop in pitch to

0.3 mm would have any implications to the standard

reflow process.

Rework

Rework of BGAs at 0.3 mm pitch is anticipated to be

much more difficult than 0.4 mm. 0.3 mm pitched rework

is not explored in this paper.

CONCLUSIONS

1. Solder paste printing on complex boards at pitches

below 0.4 mm is not capable with current mainstream

equipment and operation. The impediment is the

inability to get the stencil bottom to mate tightly

against the board surface to enable a consistent and

crisp print. We refer to this as stencil “gasketing”.

Our industry may be able to achieve 0.3 mm pitched

printing with some improvements to the printing

process, particularly with engineered board support

and vacuum fixturing.

2. Assembly yield will become more sensitive to

warpage levels as packages decrease to below 0.4

mm pitch. SMT optimization using smaller solder

balls and thinner paste print at 0.3 mm pitch are

currently less capable to mitigating package warpage.

3. Using generally accepted rules and variance analysis,

solder paste printing machines typically used

throughout our industry should be capable of

achieving X-Y alignment requirements for 0.3 mm

pitched assembly once board support issues have

been fixed.

4. Using generally accepted rules and variance analysis,

placement machines typically used throughout our

industry should be comfortably capable of achieving

X-Y alignment requirements for 0.3 mm pitched

assembly.

5. 0.3 mm pitched assembly will likely require

approximately 3 mil thick stencils with aperture

diameters close to 7 mils.

6. Silkscreen nomenclature on circuit boards with

devices at 0.4 mm pitch is marginally acceptable. At

pitches below 0.4 mm silkscreen nomenclature is not

acceptable.

7. As we move below 0.4 mm pitch, the solder paste powder

size (solder paste type) requirement expected to get finer.

A migration from the typical type 4 paste to type 5 paste

or smaller would give higher average print volume and

tighter print distributions. Type 5 paste looks like the

likely candidate based on printing volume and

consistency at 0.3 mm pitch.

8. Small amounts of stencil cleaning solvents can be

detrimental and cause solder paste bridging at 0.3 mm

pitch. This pitch is much more sensitive to residual

solvent than 0.4 mm and above. Stencil cleaning solvents

should be chosen that evaporate quickly enough to be run

with fine pitches in HVM.

9. The industry standard area ratio rules that ensure good

paste release are still valid at pitches down to 0.3 mm.

Area ratios of about 0.65 and above are still considered to

be safe at 0.3 mm pitch and more aggressive ratios may

be usable if good stencil to board mating is achieved.

10. The relationship between room temperature package

flatness (convex or concave) and reflow temperature

package flatness (convex or concave) should be

understood. This is particularly applicable when the

pitches of devices are 0.4 mm or less. Package flatness

changes with temperature can be significant and vary

widely dependent on each packages unique construction.

AKNOWLEDGEMENTS

I created this paper with the consultation from many others.

Contributors include: Layannah Feller, Lilia May, Andy

Wijaya, Srini Aravamudhan, Prawin Paulraj, Tom Worley,

Dudi Amir, Abhay Watwe, Bill Alger, Gary Brist, Hank

Hsiao, Raiyo Aspandiar, Alan Donaldson, James Wade, J.

Lewis, Todd Harris, Karen Cheng, Dan Burkholder, Mukul

Sakalkale, Doin Davenport, John Schneider, Shawn Lloyd, Ian

Williams, Steve Vandervoort, Gaurang Choksi, Bill Coleman

(Photo Stencil), Sue Holmes (Photo Stencil), Julian Chen

(Shenmao), Tim Jensen (Indium Corporation), Anny Zhang

(Indium Corporation)

REFERENCES

(1) Print Screening Study conducted by Lilia May, Intel

Corporation.

(2) JEDEC standard JESC22-B112A

(3) IPC7095 for SMT defect mitigation techniques

(4) JEDEC standard JEP-95-SPP-024, Table 1 & 2

which includes HT flatness requirements for both

convex and concave packages.

(5) IPC7095 for SMT optimization approaches