1. Definition Stage Project
FCBGA Package Warpage
Definition Stage Project
Raiyo Aspandiar – INTEL
HDP User Group Member Meeting
Host: Dell/IBM
Austin, Texas
September 26, 2012
Presented by John Davignon
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Purpose
Mitigate Area Array Solder Joint Quality Defects Generated by Package and / or Board Warpage during SMT Reflow Soldering
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Background
Package/board Warpage increasing trends
Driven by thinner package substrates and thinner die
Package/Board contacts getting smaller and closer thereby reducing ability to overcome increased Warpage.
Solder Joint Quality Impact of increasing Package and Warpage.
With advent of lead free soldering, the assembly temperatures have increased and the warpage impact has been exacerbated.
Thinner die the rigidness is loss and it can not control the substrate warpage.
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4. Examples of Warpage Induced Defects for Area Array Solder Joints
Package Substrate
Die
Board
Head-on-Pillow Open
Non –Wet Open
Oxide layer or too solid to coelesce prior to soldification.
All these are for FCBGA, Molded packages, QFN,
Stretched joint
Head on pillow
bridging
Various Solder Joint Defects can occur during SMT Reflow Soldering due to Excessive BGA component and/or Board Warpage
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Real Time Video of FCBGA Solder Joint Mechanism in Assembly Line Reflow Oven
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Project Scope
- What is IN the Scope
FCBGA Package just as its entering the SMT Reflow Oven
Die
FC BGA Package Substrate
Package Substrate
Project Focus Area
Printed Circuit Board
Reflow Process
Temperature –Time Profile
Oven Atmosphere
Solder Paste
Rheology
Wetting
Metallurgy
Activator chemistry
Volume printed on land
Surface Finish
OSP/ENIG/ImAg/ENEPIG/ etc
Package Termination
Geometry (ball, pillar, column, etc)
Metallurgy (SAC, low Ag SAC, BiSnAg, other)
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Project Scope
- What is OUT of the Scope
Package Warpage Mitigation
Laminate Type
Stack-up
Die Thickness
Die Size
Package Warpage Measurement
Metrologies
Specifications
Die
FC BGA Package Substrate
Package Substrate
Printed Circuit Board
Board Warpage Mitigation
Laminate Type
Stack-up
Board Warpage Measurement
Metrologies
Specifications
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Objective
Establish a limit for dynamic package warpage that can be mitigated during board assembly without impacting solder joint quality
Need to set a baseline for today, and set what the limit is for all gaps. It will be non-package or non-PCB related
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Project Goals
Identify mitigation paths for solder joint yield loss caused during the SMT reflow soldering process specifically due to the excessive warpage of package and/or boards
Evaluate these mitigation paths for their effectiveness in increasing solder joint yield despite high levels of package and/or board warpage
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Technical Discussion
Potential Mitigation Paths
Alternate solder paste and package solder ball metallurgies, i.e., low temperature solders
Alternate solder temp profiles (steep ramp, soak, spike reflow profile, for instance)
Alternate package termination geometries (instead of balls)
Optimized solder paste printing geometries 
New solder paste formulations
Tacky Fluxes, applied by dipping
Each mitigation path can comprise a separate project proposal or all can be combined into one project
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11. What has been Done to Date
Project Scope Identified
Key Variables Identified to Control and Monitor during SMT Reflow Soldering
Gap between Ball and Paste
Researched various methods developed in the Industry to Simulate Major Solder Joint Defects such as HoP and NWO
Three methods selected for further Evaluation and Development
Modified Plexus’ Method of Adhesive and Solder Pre-form under Package
Cisco’s Rework Equipment Use Method
Real Time observation of Solder Joint Formation within in-line reflow oven
15 -17
18-25
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12. What is Planned For the Future
Develop Metrology: Explore the feasibility of using Cisco Rework Method and Modified Plexus Adhesive Method for use with a full array package instead of a single solder ball at a time
Develop Real Time Monitoring: Explore possibility of using real time monitoring of solder joint formation within reflow oven with high dynamic warpage components, and subsequently characterize the dynamic warpage of problematic packages already in use today
Brainstorm Mitigation paths: for excessive warpage of package and/or boards induced SMT solder joint yield loss and select potential candidates for assessment
Select Mitigation Path: Obtain Components and Design and Procure Boards for Evaluation of Mitigation Paths
Assess Affectiveness of Mitigation Path: Design and Run Experiments to evaluate present capability of materials and processes and feasibility of the Warpage-induced Defects’ Mitigation Paths
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Team Members –to date
Akrometrix
Alcatel-Lucent
Arlon
ASE
Celestica
Ciena
Cisco
Curtiss-Wright
Ericsson
Flextronics
Fujitsu
Hitachi-Chemical
IBM
Intel
Juniper
Multek
Nihon-Superior
Panasonic
Phillips
Plexus
Rockwell
TTM Tech
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Back-up
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15. Effect of Typical Dynamic Warpage Characteristics of FCBGA packages on Solder Joint Formation
Post Solder Ball Collapse Point
Maximum Ball-Board Separation Point
Temp
Start of Significant Pull Way of Ball from Board
reflow
Cool down
soak
`Flat Package` Point
Ramp up
Post Solidification Point
Room Temperature
Head-on-Pillow
Open
Time
The typical Dynamic Warpage of a FCBGA package entails
For a corner solder joint…….contact first, then separation between the solder ball and the solder paste
For a center solder joint……separation first, then contact between the solder ball and the solder paste
This contact + separation or vice versa sequence creates solder joint defects
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16. Gap in Center joint of Package Gap in Corner Joint of Package
Variation of Gap between Bottom of Solder Ball and Top of Solder Paste (when solid or molten)
`Flat Package` Point
Start of Significant Pull Away of Ball from Board
Temp
Maximum Ball-Board Separation Point
Post Solder Ball Collapse Point
Post Solidification Point
Gap in Center joint of Package
Gap in Corner Joint of Package
Gap
Sharp Decrease in Gap due to collapse of central solder joints
Room Temperature
Time
Besides first contact , then separation for the corner ball, there is a sharp decrease in the gap as the solder balls melt and collapse in the reflow zone within the reflow soldering oven
This sharp drop and gap can facilitate coalescence of the molten solder ball and molten solder paste if paste flux is still active
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Key Points
Any method of simulating BGA package warpage during reflow soldering needs to address the following to simulate the conditions for corner solder joints
Initial Contact of the BGA solder ball with the solder paste and subsequent separation before or during the soak zone or ramp region
Sharp drop in the gap between the molten solder ball and molten paste when the solder balls melt and collapse in the reflow zone region
Any method of simulating BGA package warpage during reflow soldering needs to address the following to simulate the conditions for central solder joints
No contact between the BGA solder ball and solder paste before or during the ramp zone or soak zone until the flat package point is reached
Contact developed between the solder ball and solder paste later in the soak zone
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18. Simulation for the Corner and Outer Row Solder Joints of the BGA
Requirements
Initial Contact of the BGA solder ball with the solder paste and subsequent separation before or during the soak zone or ramp region
Sharp drop in the gap between the molten solder ball and molten paste when the solder balls melt and collapse in the reflow zone region
SAC Solder Pre-form and Solder Paste or Flux
Package
Room Temp ~20C
Adhesive
Gap < 0
Initial set up entails amount (thickness) of adhesive adjusted to ensure negative gap between the solder balls and the paste;
Board
Soak Zone Temp ~160C
Gap > 0
Expansion of the adhesive in its thickness direction raised the balls from the paste and creates a positive gap
Board
Melting of solder preforms in the reflow zone and the subsequent flow of the molten solder into PTH will dramatically drop the stand-off height of the package and eliminate the gap
Reflow Zone Temp ~240C
Gap < 0
Board
The amount of gap and rate at which it is created in the ramp zone of the profile is controlled by the expansion coefficient of the adhesive in its thickness direction
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One Proposal to Simulate Gap Variation between the BGA Solder Ball and the Solder Paste on the PCB Land
BGA Package
BGA Solder Balls
Adhesive
Solder Paste
Gap > 0
Board
Solder Preform
Function of Adhesive: To simulate the increase in the gap by changing its length with increasing temperature due to its coefficient of thermal expansion
Function of Solder Preform: To melt at reflow temperature and flow into the PTH it is placed over and simulate the sudden decrease in the gap that occurs when the balls collapse after becoming molten
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20. Balls and solder paste melt while in contact
4/28/2017
Simulation for the Central Solder Joints of the BGA
Requirements
No contact between the BGA solder ball and solder paste before or during the ramp zone or soak zone until the flat package point is reached
Contact developed between the solder ball and solder paste later in the soak zone
Low Temp Solder Pre-form and Solder Paste or Flux
Package
Adhesive
Initial set up entails a gap between the solder balls and the paste; gap amount controlled by amount (thickness) of adhesive
Room Temp ~20C
Gap at RT > 0
Soak Zone Temp ~160C
When the low temp solder pre-form melts balls lowered and touch SAC solder paste
Gap < 0
Reflow Zone Temp ~240C
Gap < 0
Balls and solder paste melt while in contact
The temperature at which the solder ball and paste make contact can be varied by using different solders with different melting points in the SAC profile soak zone
Intel Confidential

21. Requirements for Adhesive Under Component to Simulate the Gap between the Ball and the Paste during SMT Reflow Soldering
Adhesive
Expansion of the adhesive in its thickness direction raises the balls from the paste and creates a positive gap
Minimum Gap to simulate problematic warpage is 150 microns
Gap > 0
Board
High Coefficient of Linear Expansion after Cure;
Sufficient Hardness/Modulus/Rigidity to push the package up as the adhesive expands
Survive temperatures up to 250C after cure
Dispensable or screen printable
Key question: What is the range of Linear Expansion that is needed for the adhesive?
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22. Cisco’s SRT Rework Equipment Method for Head-on-Pillow Testing
BGA rework equipment was also used to reflow the solder ball and the solder paste, while controlling the movement of the solder ball with respect to the solder paste on the pad.
The purpose of controlling the movement is to mimic the movement of
the corner balls of a BGA as they warp during the reflow process.
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23. Conditions for Cisco’s Method
C1: Initially there is no initial contact between the solder ball and the paste. Then at 200 ° C during the cooling down phase, the solder ball is pulled down slowly (1 mil/second) so that it makes contact with the solder paste.
C2: Initially there is no initial contact between the solder ball and paste. Then at 190 °C during the cooling down phase, the solder ball is pulled down slowly so that it makes contact with the solder paste. This condition is similar to C1 but there is less contact time
before solidification.
Note: When initial contact is made between ball and paste, no H-o-P defects were formed
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24. Photo’s of Cisco’s Method for Evaluating H-o-P defects
Before Solder Paste Reflow
After Solder Paste Reflow
Aspects of Actual Reflow Soldering of high warpage BGAs NOT Simulated in this Test
Lifting up of the solder ball from the solder paste during initial temperature ramp (though this can be done with the equipment)
Un-constrained Ball Collapse when the solder melts
Temperature Profile simulated with hot air rework machine instead of a reflow oven
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25. Malcom RCA-1 Observation Monitor
Observes and Records the components on a PCB as it goes through the Reflow Oven
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