THE IMPACT OF COMPONENT WARPING TO ASSEMBLY PROCESS

RESULTING TO HEAD IN PILLOW AND NON WET OPEN DEFECTS

Henley Zhou / Ranilo Aranda / Isaias Daguio /Jimmy Chen/Jumbo Huang

AEG – Asia / Advanced Manufacturing Engineering

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Content

Introduction

Defect Phenomenon of Head in Pillow (HiP) and Non Wet Open (NWO)

HiP and NWO Failure Mechanism – How it occurs in the Assembly?

Fish Bone Diagram for HiP and NWO

Root Cause – Dynamic Warpage

Warpage Analysis Technique – Shadow Moire’

Workaround Process (HiP and NWO)

X-ray Inspection

Stencil

Profile

Nitrogen

Solder Paste Chemistry

Summary

Warpage Criteria – JEITA

Future Work

Reference

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Introduction

The demand for more features and increased functionality has been moving

towards the use of component form factors such as FCBGA, System on Chip

(SoC), Package on Package (PoP), Fine Pitch Ball Grid Arrays (FBGA), CSP,

etc., confronting the need for high yields.

Major Challenge Defects: Head in Pillow and Non Wet Open

SoC FCBGA

PoP CSP

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Defect Phenomenon of Head in Pillow (HiP)

Head-In-Pillow: is a non-coalescence (Non-wetting)

between the solder ball and the solder paste during

reflow process as shown in figure.

Head-In-Pillow: appears that solder ball touch with

printed solder paste tightly, but they don’t form a

unitary solder bump and that solder ball just like

sited on hollow or prominence.

Head-In-Pillow: has big risk on reliability or lose

effectiveness after sales as it is could pass the

function test in many instances.

X-Ray Image of HiP

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Defect Phenomenon of Non Wet Open (NWO)

Non Wet Open (NWO) - This defect is known by

many other names such as non wet, lifted ball,

hanging ball, ball on pad and ball on land.

They all refer to the same phenomenon that

is defined as a joint that is comprised of one

metallurgical mass formed from the BGA ball

and reflowed solder paste or flux with

incomplete or no coalescence to the PCB

pad.

In most cases there is no evidence of solder

on the PCB lands .

FCBGA NWO Side View Image

NOW Cross section Image

FCBGA NWO X-ray Image

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HiP Failure Mechanism – How it occurs in the Assembly?

BGA solder ball contact with

printed solder paste Gap is developed due to dynamic

warpage induced in heating

process

Solder paste flux was worked out

excessively

0xidation generated on BGA

solder ball surface.

BGA solder ball contact

with solder paste again

Solder paste flux hasn’t

enough activity.

Oxidation film generated

on solder ball and resist

the reflow wetting with

solder paste.

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NWO Failure Mechanism – How it occurs in the Assembly?

BGA solder ball contact with

printed solder paste

As the BGA enters the reflow soldering

oven, its temperature increases, and

flux in the solder paste starts to

activate.

Gap is developed due to dynamic

warpage induced in heating process

lifting off the solder paste from PCB

land pad

Solder paste with low tackiness cannot

maintain to adhere the contact to the

pad and they are lifted off with the balls.

The paste lift occurs in pre-

reflow zone at temperatures

below the melting point of the

solder paste and the BGA ball

around 160 to 190 ºC.

At reflow the paste will melt

into the package solder ball

and when the package

collapses during cool down,

solder ball will sit on the PCB

land causing NWO defect.

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Fish Bone Diagram for HiP and NWO

8 Critical Factors been identified through Fish Bone Analysis

Stencil Design Solder Paste Printing

Placement Accuracy PCB Warpage

Component Warpage Solder Paste Chemistry

Reflow Profile PCB Contamination

HIP

an

d N

WO

Pad Contamination

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Root Cause – Dynamic Warpage

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The warpage behavior of a package which varies as a function of temperature

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Warpage Analysis Technique – Shadow Moire’

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The BGA was subjected to Akrometrix-Thermoire testing from room temperature up to 240C

Result:

Co-planarity ranging from 2.0 mils to 5.8mils (147um)

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Workaround Process – X-ray Inspection (HiP)

2DX image for G24

Ball # G24

Slice 5

Slice 1 Slice 2

Slice 3 Slice 4

AXI 1 images for G24

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• In 5D X-ray inspection, there is no clear difference between good ball and

HiP ball.

Oblique viewing will give a better image

of HiP in 2D X-ray inspection

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Workaround Process – X-ray Inspection (HiP)

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AXI HIP Detection Capability Improvement

Based on HIP characteristics, we focus on the following threshold for HIP detection (a) Open

Neighbour Outlier (b) HIP Outlier (c) Multipass Inspection

Escaped pin G32

Site # of Total Escaped

Defects

Defects Escaped

%

# of Total False

Call

False call

PPM

1 13 27.66% 9 4425

2 7 14.89% 12 5900

3 5 10.64% 7 3441

4 8 17.02% 12 5900

Note: AXI machines have capability to detect

BGA HIP defect with different success

levels.

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Workaround Process – X-ray Inspection (NWO)

Oblique viewing will give a better image of NWO in X-ray

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Workaround Process – Stencil

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Designing the stencil aperture with 2 – 4mils larger than the pad

Increase the stencil aperture opening to deliver more paste to targeted areas

most affected by component warpage

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HiP Workaround Process – Profile Optimization

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Reflow Profile

Analyze the formation mechanism of Head-In-Pillow

defect, we can find that the separation of BGA bumps

from solder paste leads to excessive oxidation of BGA

bumps during heating process (Soak1 + Soak2), and

the exhaustion of flux activation result to less capability

to remove the oxidation and form the coalescence of

solder joint, so the reflow profile optimization to

decrease flux exhaustion is very important.

Solder Paste Chemistry

The resistances against flux burn-off and flux

oxidation barrier capability are very important to

prevent Head-In-Pillow defects. With comparing to

the conventional solder paste, the HIP developed

solder paste can reserve enough flux and with good

performance to remove the oxidation film between

solder bumps and solder paste during the further

heating process.

Process Optimization Focused on Two Critical Factors

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NWO Workaround Process – Profile Optimization

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Long soak reflow profile show marginally improved results when compared to short soak

reflow profile (70% defect rate vs. 90% defect rate)

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Workaround Process – Impact of Nitrogen

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For HiP mitigation, it gives a wider process window for certain component technologies

and in some cases mandatory.

While for NWO, N2 does not provide significant margin of improvement to mitigate the

defects

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Workaround Process – Solder Paste Chemistry

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Non Wet Open

Solder paste properties sensitive to cause NWO defects like the activity of the flux and high temperature

tackiness

Ability of paste to stick/adhere and maintain contact between PCB land and BGA ball

Propose a screening tests, like solder paste bake test or tape in cavity test to be able to rank different

pastes with respect to the risk of NWO defect formation.

Items NWO HiP

Paste Type Sensitivity to

defect

Low Tackiness

Low Activity

Flux oxidation barrier capability

Resistance to burn-off

Head in Pillow

The resistances against flux burn-off and flux oxidation barrier capability are very important to mitigate

Head-In-Pillow defects

Special solder paste formulation developed for HiP

Implies good thermal stability of

flux vehicle, tackiness did not

significantly changed

Tackiness dropped

substantially

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Summary

HiP • War-page of BGA/CSP package and PCB

substrate together with flux consumption will easily lead to Head-In-Pillow defects.

• Reflow profile optimization with lower peak temperature and shorten the ramp up time from 190C to 220C with can reduce HIP defect

• Use the solder paste with good resistances against flux burn-off and good flux oxidation barrier capability can reduce HIP defect.

• X-ray with oblique viewing and specifically setting can be used to verify the HIP defect improvement effectiveness.

• Still there is room for improvement on the detection capability of 5DX.

NWO • Due to dynamic War-page of BGA/CSP

package will easily lead to Non Wet Open defects

• Solder paste is the significant factor which impact with the BGA NWO issue

• Solder paste selection chemistry such as activity of the flux and high temperature tackiness are critical in mitigating NWO defect

• Optimize the reflow and long soak time will marginally help for the NWO issue.

• Increase the solder paste volume will help to reduce the NWO issue.

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Warpage Criteria - JEITA

Standard of Japan Electronics and Information Technology Industries Association- JEITA

The warpage specifications are too loose and apparently allows poor contact between

ball and solder paste

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Future Work

What is the maximum

permissible warpage value to eliminate HiP &

NWO?

EMS

OEM

Material suppliers

Process related issue can be controlled but material related aspects

should be addressed by the industry.

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References [1] Alex Chan, Paul Brown, Richard Coyle, Lars Bruno, Anne-Kathrine Knoph, Thilo Sack, David Geiger,

David Mendez and et al “Collaboration Between OEM and EMS to Combat Head on Pillowing Defects:

Part 2 – Warpage Acceptance Proposal, SMTA China East Proceedings, 2014”

[2] Dudi Amir, Satyajit Walwadkar, Srinivasa Aravamudhan, and Lilia May “THE CHALLENGES OF NON

WET OPEN BGA SOLDER DEFECT”

[3] Henley Zhou, William Uy, Jumbo Huang “Two Different Ways of Resolving BGA Head-in-Pillow

Defects”

[4] Alejandro Castellanos, Dr. Zhen (Jane) Feng, David Geiger, and Murad Kurwa “ Head-In-Pillow X-ray

Inspection, SMT Magazine, May 2014

[5] Henley Zhou, Isaias Daguio, Jimmy Chen, Akron Lee “The Effects of Nitrogen on Solder Joint Quality

And Systematic Approach of Reducing Consumption in Printed Circuit Board Assembly, SMTA China

South, Aug 2014”

[6] Dr. Avi Rochman & Johnny Chen ”Head in Pillow (HIP) Root Cause Analysis and Corrective Actions

Guideline, WW Flextronics Best Practice”

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