the impact of component warping to assembly process

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
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
2
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
FCBGA
SoC
PoP
CSP
3
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
4
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 .
NOW Cross section Image
FCBGA NWO X-ray Image
FCBGA NWO Side View Image
5
HiP Failure Mechanism – How it occurs in the Assembly?
 BGA solder ball contact with
printed solder paste
 Gap is developed due to dynamic  BGA solder ball contact
with solder paste again
warpage induced in heating
 Solder paste flux hasn’t
process
enough activity.
 Solder paste flux was worked out
 Oxidation film generated
excessively
on solder ball and resist
 0xidation generated on BGA
the reflow wetting with
solder ball surface.
solder paste.
6
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 prereflow 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.
7
Fish Bone Diagram for HiP and NWO
 8 Critical Factors been identified through Fish Bone Analysis
Pad Contamination
HIP and NWO
 Component Warpage  Reflow Profile
 Stencil Design
 Solder Paste Chemistry  PCB Contamination Solder Paste Printing
 Placement Accuracy
 PCB Warpage
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Root Cause – Dynamic Warpage
 The warpage behavior of a package which varies as a function of temperature
9
9
Warpage Analysis Technique – Shadow Moire’
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)
10
10
Workaround Process – X-ray Inspection (HiP)
 Oblique viewing will give a better image
of HiP in 2D X-ray inspection
Ball # G24
Slice 1
Slice 2
Slice 3
Slice 4
2DX image for G24
Slice 5
AXI 1 images for G24
•
In 5D X-ray inspection, there is no clear difference between good ball and
HiP ball.
11
11
Workaround Process – X-ray Inspection (HiP)
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
Site
1
2
3
4
# of Total Escaped
Defects
Defects Escaped
%
# of Total False
Call
False call
PPM
13
7
5
8
27.66%
14.89%
10.64%
17.02%
9
12
7
12
4425
5900
3441
5900
Note:
AXI machines have capability to detect
BGA HIP defect with different success
levels.
Escaped pin G32
12
12
Workaround Process – X-ray Inspection (NWO)
 Oblique viewing will give a better image of NWO in X-ray
13
Workaround Process – Stencil
 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
14
14
HiP Workaround Process – Profile Optimization
Process Optimization Focused on Two Critical Factors
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.
15
15
NWO Workaround Process – Profile Optimization
 Long soak reflow profile show marginally improved results when compared to short soak
reflow profile (70% defect rate vs. 90% defect rate)
16
16
Workaround Process – Impact of Nitrogen
 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
17
17
Workaround Process – Solder Paste Chemistry
Items
Paste Type Sensitivity to
defect
NWO
Low Tackiness
Low Activity
HiP
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
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.
Tackiness dropped
substantially
Implies good thermal stability of
flux vehicle, tackiness did not
significantly changed
18
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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.
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.
• Still there is room for improvement on the
detection capability of 5DX.
19
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
20
Future Work
Process related issue can be controlled but material related aspects
should be addressed by the industry.
OEM
Material
suppliers
EMS
What is the
maximum
permissible
warpage value to
eliminate HiP &
NWO?
21
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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People Drive Innovation
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