reducing head in pillow defects

an Alent plc Company

Reducing Head in Pillow

Defects

September 2009

HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company

Introduction

• Potential sources of Head in Pillow Defects

• What is the mechanism of failure

• Evaluation of Package Warpage – Measurement

– Industry Standards

• How Can Solder Paste Reduce HIP Defects? – Print Volume

– Wetting Force/Speed

– Paste Activation Level

• Future Experiments


What is Head in Pillow?

Board Side

Component Side

http://images.google.com/imgres?imgurl=http://cache4.asset-cache.net/xc/200199707-001.jpg%3Fv%3D1%26c%3DNewsMaker%26k%3D2%26d%3DE35B54D5385E960E199B9A004EE3C8471F6F6178A68B340C&imgrefurl=http://www.gettyimages.com/detail/200199707-001/Taxi&usg=__nFRzwCYkF0e_Fkconf5zKdYsbrg=&h=413&w=413&sz=39&hl=en&start=185&um=1&tbnid=s9JmNma3_IVOxM:&tbnh=125&tbnw=125&prev=/images%3Fq%3Dpicture%2Bof%2Bhead%2Bin%2Bpillow%26ndsp%3D20%26hl%3Den%26rlz%3D1T4ADBR_enUS276US276%26sa%3DN%26start%3D180%26um%3D1


Difficult Problem

In circuit testing may not detect the HIP defect

Z Direction X-Ray may not detect the HIP defect

Devices that are exposed to Drop Shock or Thermal Cycling in

field unfortunately Will over time experience this defect

Head in Pillow =

Where the solder joint

and the sphere are

touching but no

intermetallic layer is

formed


Head in Pillow Defect C&E

Overview

Warpage of

substrate

Inconsistent

bump size

Insufficient solder

paste volume

Printing misalignment

Inaccurate XY

placement

Insufficient

placement force

inadequate reflow

profile that results in

component & PCB

warpage

Lifting of BGA bumps

due to wetting force

Excessive Peak

Temperature

Too much TAL

Opportunities of defects from the different stages of a SMT process


Head in Pillow Defect Mechanism

•BGA Oxidizes

•Flux Activity Being

consumed

•Flux Becomes Liquid

•BGA Oxidizes

•Flux Activity Being

Used

•Oxidized Sphere

rejoins the Paste

deposit with Limited

Activity


Jeita Maximum Warpage Standards


Measuring BGA Warpage

Schematic of Shadow Moiré Method


Shadow Moiré Test Unit


Moiré Computer Generated Image


How Can Solder Paste be

Engineered to Reduce HIP?

I


Alpha Response

Solder Paste that…

produces higher and more consistent volume

deposition

has a greater resistance to longer soak profiles

has faster wetting speed

• Gage R&R testing for determining in-house Head

on Pillow defects to confirm solder paste

effectiveness


0.25mm round BGAs, 0.1mm laser cut stencil

OM338 T45

-20

0

20

40

60

80

100

120

140

160

1 7 13

Board #

Vo

lum

e (

cu

mil

)

Average - Boards 1-6 (Initial) Average - Boards 7-12 (wipe)

Average - Boards 13-18 (pause) Minimum vol. - Boards 1-6

Minimum vol. - Boards 7-12 Minimum vol. - Boards 13-18

450-31

-20

0

20

40

60

80

100

120

140

160

1 7 13

Board #

Vo

lum

e (

cu

mil

)

Average - Boards 1-6 (Initial) Average - Boards 7-12 (wipe)

Average - Boards 13-18 (pause) Minimum vol. - Boards 1-6

Minimum vol. - Boards 7-12 Minimum vol. - Boards 13-18

Print Volume

Increased print Volume and Volume Repeatability

Improved Paste Print Provides 40% More Paste Height!

Standard Paste Grizzly Bear


Preventing Defect with Paste

Print Height

Probable Defect

Defect

Prevented

70µ Deposit

100µ Deposit


Wetting Force

• Test Method

– Malcom Wetting

Balance Test

• Equipment

– Malcom SWB-2

• Test Procedure

– See Malcom SWB-2

Operating Manual


Wetting Force

Average

-12.00

-10.00

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

0 2 4 6 8

Wetting time , SecW

ett

ing

Fo

rce

, m

N

Average

-12.00

-10.00

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

0 2 4 6 8

Wetting time , Sec

We

ttin

g F

orc

e, m

N

Grizzly Bear Standard Paste

Goal: Increased Wetting Force and Speed


Head in Pillow

Test Method


Creating HIP Defects

The Occurrence of the HIP defects average 200 ppm based on customer

feedback

Issue: How do we create a test method that creates higher DPMO

(Defects per million opportunities) so we can test for it?

Good Joint

HIP Joint


Manage Reflow and Warpage

Time, Temperature, Z-Axis Location


HIP Test Equipment

• Test Vehicle (TV)

• BGA338 Dummy component

• BGA rework station

• Fabricated TV holder and fixture for holding the

dummy BGA with vacuum.

• Thermocouple data acquisition

• Solenoid Valve


HIP Experimental Set-Up


HIP Experimental Set-Up

1. The BGA component is held a few mm above

the test vehicle by applying vacuum to the cup.

2. The BGA is then dropped on to the test vehicle

by releasing vacuum.


Reflow Profile Indicator


Drop Component at Precise Time

During Reflow


Experimental Results

• Test Highly Activated Paste vs. Very Low

Activity Paste and Measure HIP DPMO.


Head in Pillow Resistant Paste

Good reflow at corner pads

Only one HIP Defect was

Observed after prying the BGA

from Test Vehicle


Less Activated Paste

After prying BGA from TV

Obvious HIP joint in the

center of the BGA block!


Initial results suggests that flux

chemistry does affect HIP

More Active Paste Less Active Paste


Future Work ZhuoMao BGA Rework -- HIP A reflow profile

0

50

100

150

200

250

0 50 100 150 200 250 300 350

Time sec

Te

mp

era

ture

°C

lower left corner

upper right corner

lower BGA row

center BGA block

217°C

TC positions

Under the BGA

Reflow Profile vs. Head in Pillow Defect Rate


Summary

Component Size/Warpage Can Affect HIP Defect Rate

Key attributes for Solder Paste to reduce HIP DPMO’s

Taller, More Consistent Paste Deposits

Faster, Stronger Wetting Force

Enhanced paste flux Activity and Activity Duration

High Gage R&R HIP Test to Confirm Paste Properties


For more information click the

link below:

Alpha Products

http://blog.cooksonelectronics.com/how-do-you-add-value-to-your-assembly-process/?utm_campaign=All-Products

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