ELIMINATION OF EPOXY BRIDGING IN DIEBOND PROCESS

Novin Yap, Caloy Hennosura, Freddie Pascual

ON Semiconductor Philippines Inc. Golden Mile Business Park, Special Economic Zone

Governor's Drive, Brgy. Maduya Carmona Cavite novin. yap@onsemi.com, caloy. hennosura@onsemi.com, freddie.pascual@onsemi.com

Abstract

Diebond process is one of the key assembly process step in terms of defining reliability and performance of an integrated circuit, IC product. The process requires epoxy dispense accuracy and consistency to ensure that IC is secured and within the required position in the lead frame pad ready for wire bonding. These requirements though in the realm of assembly manufacturing is easier said than done. In the actual manufacturing Diebond process environment, there are several machine technologies that process different devices of Ie. This complex condition coupled with increasing volume for processing was observed to be potentially related to the increase of Epoxy bridging defect. Normally a unit of IC with Epoxy bridging defect fails electrical testing as the conductive epoxy tails or spreads connecting the lead frame flag and the internal leads causing electrical short. However there are cases of very thin Epoxy bridging that can pass electrical test which has a risk of failing during application. ON Semiconductor Philippines Inc, OSPI formed a DMAIC team to immediately address opportunities in the trend of Epoxy bridging. Using the disciplined Six Sigma approach, the team identified Z level offset and Z level position as key input variable, KPIV using a resolution 4 fractional factorial design of experiment. These KPIV was found necessary as standard for all Diebond machine technologies. And even with the complexity of number of devices, with this KPIV's controlled and locked, Epoxy bridging was eliminated.

1. Introduction

Die bond process secures a singulated dice into the lead frame for assembly process. But before a die is secured, the process has first to dispense a conductive epoxy into the lead frame pad. Dispensing is dependent to the size of device or IC with respect to the qualified lead frame pad. Given this complexity, this step was suspected to be a contributor to the consistent trend of epoxy bridging. The complete dispense process steps shown in figurel.

Start dispense cycle

3 Air pressure applied to squeeze out the epoxy

Dispense head descent to top surface of a flag

4 Dispense head moved up preparing ror the

next cycle

� Figure 1. Dispense process step.

Dispense parameters if not properly controlled will cause a very thin epoxy connection between lead frame pads to leads with a given risk in escaping electrical test screening. This failure is also known as epoxy bridging shown in figure 2. A DMAIC team was formed to zero out this defect.

Figure 2. Photo of epoxy bridging that can escape electrical Test

35th International Electronics Manufacturing Technology Conference 2012

2. Methodology and Experimentation

DMAIC is a disciplined tool for Six Sigma improvement approach. It is composed of 5 phases in order - Define, Measure, Analyze, Improve and Control.

In the first two phases the team validated that there really is an opportunity in Epoxy bridging and that data from Reject Bin Analysis, RBA were accurate. This was performed in the Measurement System Analysis review to determine the accuracy of judgment and recording of every epoxy bridging occurrence. In each of the detected Epoxy Bridging at RBA, the units undergo Failure Analysis which involves Decapsulation and Scanning Electron Microscope, SEM analysis. There were no reports or record of any Epoxy Bridging false alarm call-out, hence the RBA trend as shown in figure 3 is valid.

DB PROCESS DATE Epoxy Bridging Internal RBA

1: E , z

Month Figure 3. Internal Epoxy Bridging trend in Reject Bin Analysis, RBA

With the baseline performance established, started the Analyze phase which listed 43 KPIV's of which 23

of them in 5 categories are of top priority based on cause and effect matrix prioritization. The KPIV's were determined using macro map or commonly known as SIPOC and the cause and effect analysis tools. These tools generated all potential root causes for Epoxy Bridging, refer to figure 4.

Macro Map (SIPOC) Supplier Input Process Output Customer

SUpplier A Epoxy

SupplierA1 Leadframe

Nozzle SupplierB

Pepper pot

Pick-up tool SupplierB1

Ejector Needle

Fob Wafer

PE Parameter

EE Set-up

EE Anvil

[:)

Oiebond

(a)

Die bonded units Internal

External

Epaxy8ridging

�i"O"Y

Pol lite

Epoxy Packaging

Uniform Color (silver) I Zero VoidslCompact I

Material \l\Jlth short side marking I V\IIthChamfer j

Nozzle Internal Diameier I Groovewictth I

Lead/rame I Die to flag clearance I Die Size I

\l'lJl'ong nozzle used

Man Expired Epoxy used I Nozzle cleaning I Nozzleinstelletion I

Nozzle installation I Dispense Pressure I Dispense Vacuum I

Method Dispense Time I Dispense Set-up

Nozzle Planarity I

Measurement I

NozzleOrientation

I (wI respect 10 die)

Machine Dispense accuracy (x,y) I (b)

Figure 4. (a) Macro map of Die Bond Process, (b) Nested cause and effect diagram.

From the identified 23 KPIV's a validation plan was performed using a resolution 4 fractional factorial screening design of experiment, see figure 5.

Epoxy Tailing

Item InputVlrt.ble,

POltlqUIIU delay

HI VIC dellY

Z·prl Movlup Ollay

Nou.1I Dilmlll'

Break Till Velocity Ratio Break Till Offill 1 Break Till Delay 1

Break Till Offill 2

Breek Teil DeilY 2 Z Ready Poaltlon

Z Llvel Search Hllght

Ollpenle OeilY

Type ofExper1ment Oealgn

Simple Sin (Numbe, of Rlplleltea,

NO. of Llvel, Ringe .20t020ml

95t0105ml

Oto looml

Oto 100ml

-100 lO 0 entl

010200%

, to 1000enti

110100ml

0101000 entl Oto lOami

010 2500 entl

010 500 enll

Oto lOami

Figure 5. Resolution 4 Screening DOE

3. Results and Discussion

The most significant factors at 95% confidence level are narrowed down to 11. And to back up the DOE result, Diebond step sequence was mapped and then captured using a high speed camera. At this point, the Epoxy Bridging phenomenon was really made evident. Figures 6a shows the JMP software result while 6b illustrates or points the exact impact of each KPIV's influence through the high speed camera. The z-Ievel offset and position are the most critical. When set low normally for productivity purpose, the epoxy

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tail does not break and when the dispense head moves the tail will be dragged causing the epoxy to bridge the Leadframe flag and the lead post.

(a)

IlIdMthml p-Value <.0001" <.OOOP "'.0001"-0.0002" 0.0004'" 0.0014· 0.0032· 0.0045" 0.0081" 0.0188· 0.0203* 0.0284" 0.0391" 0.0542 0.0756 0.0770 0.0770 0.0842

• • • • • •.• • • • • • • • • • • • • • • • • • • • • J. • • • • • • •.• ••• • • • • • • •.• • • • • • • • • • • • • • • • • • • • •.• • • Ih.tIiI :

ere.kill dtllrl ' dtllyl : : '

(b)

Figure 6. (a) Result of Resolution 4 Fractional Factorial DOE, 11 factors is significant, (b) Mapping of the 11 significant factors and actual verification using high speed camera.

Tr'lWlms)

After confirmations that all 11 KPIV's are valid, using JMP software, optimize range of KPIV's were determined and then locked in each of the Diebond technologies program.

'00'

10"

1000

-1000

·1000

/'

/ ."

ZLlMtlOflsei

HoIilVtftFoKIOI Cr.rtd)! o OZLMIOII'set == 0 o 0ZRnctyPo$�on "00 o 0 prHqueeu delay � 10 o o BrtakmDelay2 <C== 10 o Oe,eakT,WDtlayl <C== 10 o 0 BreakT.�V.10(IttR8�O � 0 o OZ-pr.MOYt\JPOelly == " o OSrtakTallOI'Se!1 � 1000 o ODl5P.nSlOtliY � 0

a 09reakTdOl'ut2 SOD o ONOlZltOiameter � 0

....... COol�OII Cwrel�Y LoU •• T.HlitilCX '15 1843428 0

L 1'00 :::' i • t �,!' ',' .. . !oo .-;,-

:1020: .0100101010. 110 ,.

[ Summary of Fit RSqu,re

RSqu,reAdj

RoolMeanSQuareError

Mean of Response T"IbI�"-1M

Obse"'tions(orSumW�s) "00II'�"1!1I9fdtMl

Figure 7. Optimized level for significant KPIV's to zero out Epoxy Bridging.

. .... "

0.96898g 0.956583 5.865113 11.69641

64

Other than optimized parameters, the DMAIC study has also revealed quick wins or just does it improvement activities like standardization of nozzle adapters and planarity check. These two improvement actions are illustrated in figure 8.

(a)

Validation Resultfor PM Nozzle Planarity

Epoxy Pattern and Coverage with Planed Nozzle

(b)

Good Alignment .". Jig Pattern C .. )

Not Good Alignment : ....... Jig Pattern :, .../'

Figure 8. Quick win activities from DMAIC, (a) nozzle adapter improvement, (b) Nozzle planarity check.

Monitoring of Epoxy bridging showed tremendous improvement immediately after implementation of the optimum settings for all Diebond technologies.

DB PROCESS DATE Epoxy Bridging Internal RBA

Month

Figure 9. Improvements in Trend of Epoxy Bridging RBA.

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4. Conclusions

Growth is inevitable which brings changes and challenges in assembly manufacturing like that of Diebond process complexities with respect to devices. These challenges however can be overcome through Six Sigma which consists of rich parametric tools to optimize a process. In this study it uncovers KPIV's which were not considered in previous studies that will allow standardization for all Diebond process - z level offset and position.

Acknowledgements

The authors wish to thank the following key OSPI leaders and employees behind the success of this study;

Emmie Frani, for providing needed support, resources and inspirational talk that motivated members to contribute and deliver their action completion on time.

Ronnie Manubay, for guidance in terms Process Engineering approach to problem solving.

References

l. JMP Design of Experiments, Release 7 Copyright © 2007, SAS Institute Inc., Cary, NC, USA ISBN 978-

1-59994-413-5.

2. The Lean Six Sigma, Michael A. George, McGraw­Hill,2002.

3. Diebond Machine Process Capability Study, July 2010, ON Semiconductor Philippines Inc.

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