Post on 26-Jun-2020
Dr. Milena Vujosevic
Juan L Cruz
Contributors: Guruprasad Arakere, Ru Han, Jeff Cook, Min Pei, Je-Young Chang
Mechanics
Backing plate
DT@T=T1
@T=T2F
F1F2
F = F1 + F2 + … + Fn , n-number of attachment points
Complex multi-physics problem! Can result in failure of solder joints!
2 areas of special concern for solder joints
• die shadow
• package corner
Failure
• Impact of Load Magnitude
• Impact of Attachment configurations
Impact of load magnitude: Design of Experiments
10
Modeling DOE
Parameter:
Range:
Load (lbf)
4lbf -35lbf
Board Thickness
32-130mil
Backing plate Thickness
0.1-0.9mm
Specific Values 4, 10, 15, 25, 35
32 (0.8128mm)
40 (1.016mm)
55 (1.397mm)
93 (2.396mm)
130 (3.302mm)
0.1, 0.9
Modeling DOE designed to comprehend the impact of enabling load and coupling of important design parameters.
Basic assumption: Impact of enabling load on SJR is not independent of other design parameters
Output: SJ damage metric (ISED- Inelastic Strain Energy Density)
Die shadow pack. corner risk• Load with BP• Load without BP
G. Arakere, M. Vujosevic, J. Cook, ‘Temperature Cycling
Performance of Ball Grid Arrays under Thermal Enabling
Load’, ITHERM 2014, Florida, May 2014.
35lb
Mechanism that affect PC bending.
X
Results: Effect on Package Corner (PC)
Load: Impact on PC SJs is significant. Increasing load increases PC risk in TC. The impact is very pronounced for thin boards.
Board-thickness: Thicker boards significantly reduce PC risk
Backing Plate Thickness: Thicker backing plates could slightly increase PC risk (to a larger extent on thin boards); test data:
Q&R MMS Q&R MMS
Q&R MMS35lb
4 lb
Effect on Die Shadow (DS) Region
• Board Thickness: Thicker board increases DS risk in the presence of load (Conforming the legacy knowledge)
• Load: Impact of uniformly distributed load on DS is negligible.
• Backing plate thickness: Impact on DS is negligible.
X
Pareto Chart
Die shadow risk in TC in the presence of load is primarily governed by board thickness. Load and backing plate thickness have smaller impacts.
Q&R MMS
Thin Boards (32mil and 40mil)
• Low loads (4lbf): DS joints are at higher risk compared to PC joints
• Medium/High loads (>10lbf): PC risk could be higher than DS risk even at medium loads. The load at which the risk shifts from DS to PC depends on the board thickness.
Thicker Boards
• DS joints are at significantly higher risk compared to PC joints for all considered load levels
Backing Plate Thickness• For a given load and board thickness, thin BP’s slightly better for PC in TC
Comparison: Package Corner (PC) vs. Die Shadow (DS) Risk
High PC RiskHigh PC Risk
Only DS Risk Only DS RiskBP Thickness = 0.9mm
Q&R MMS
Load (lb)
Brd. Thickness (mm)
Impact of load magnitude: Summary
Impact of load on package corner SJR is significant. Increasing load increases risk in TC. The impact is very pronounced for thin boards.
For a given board thickness there is a load magnitude that shifts critical SJ from die shadow to package corner. For thin boards that magnitude is low/medium loads; thick boards will require a very high load for that shift to occur.
Backing plate thickness has an impact on SJR in corners. Thinner backing plate reduces PC risk. Not having a backing plate is even better. (This impact is especially relevant for thin boards where corner joints are at risk).
14
Package corner SJR risk •Load•Board thickness•Backing plate thickness
Die shadow risk Board thickness
Load vs. Thermal Performance vs. SJR Risk
15
Note: This graph is for illustration purposes only; Every package will have a different graph; Values for resistance and SJ damage are normalized.
Tool for design tradeoff decisions
• Impact of Load
• Impact of Attachment configurations
enablingLoad(5,25)
Support_Span(36,64)
enablingLoad*Support_Span
Term
0.0205147
0.0021237
-0.0002600
Orthog
Estimate
Pareto Plot of Transformed Estimates
Effect of Attachment Points Span and Location
• Larger span greater SJ damage risk.
• Load has much larger impact on SJ stress then the span
X
Pareto ChartPackage corner SJ Damage
• Comprehend impact of enabling load support span, and attachment point number and location on FCBGA package SJR under temperature cycling (TC) conditions
Factors and Levels
Load
(lbf)
Support Span
(mm)
Support
Location
(mm)
4, 15, 25
36, 48, 64
(Square tooling
hole location)
14, 20, 26, 32
Support Span
Changing Support Location
BGA Map
(1/4)
Die
Edge
Results: Span vs. Load Impact
Span: 20mm
Span: 32mm Span: 26mm
Span: 14mm
Results: Effect of Support Location
BGA Map
Die
Edge
PC Joint
Stresses in top edge row SJs for different support span
Edge SJs
Peak stress in SJs
Increasing Cylindrical Bending
B
A
CD
Span: 20mm
Increasing support span increases Corner SJ damage
Partial Spherical Bend: Bending along the package corner is highest risk for PC joints
Cylindrical Bend: Less bending along the package corner. Lower risk for PC joints
A B
C D
A B C D
Rectangular Package: Effect of Support Location
Support Location: Decreasing support span decreases PC damage (A - D). Trend same as square FF pkg.
Effect of Bending Mode:
• Cylindrical Bend: Less bending along the package corner. Lower risk for PC joints
• Tooling Hole along Pkg. shorter Edge: Distance from bending axis is larger therefore larger bending moment resulting in slightly higher PC damage
Worst case Tooling Hole Location: Tooling holes along the pkg. diag. or pkg. shorter edge (~0-2mm inside the corners)
A
B
A
CD
B
C D
E F
Min. Distance to Pkg. Corner/Edge from Hole = 6mm
E F
Y-Coordinate
Y
X
X-Coordinate
Increasing Cylindrical Bending
Bending Axis
Highest SJ Damage Risk: Tooling holes along pkg. corner or pkg. shorter edge
Results: Effect of Support Type
Package corner risk for a given load:
2-point > 3-point >4-point
X4-pt Support, Load 25lb
Board Thickness =32milLoad=25lbf
3-pt Support, Load 25lb
3-pt Support, Load = 75lbf
2-pt Support, Load 25lb
Force=F
Force=F/4 Force=F/3 Force=F/2
NOTE: 3-pt Support, 75lb vs 25lb:For large load, SJs along the edge close to the tooling hole can be at risk!
Corner SJ risk
Attachment location: key conclusions
• Support Span: Decreasing support span will improve SJR
• Support Location
Package SJR can be improved by designing tooling along package edge possibly away from corners
Mode of package bending causes asymmetry in damage at corners
• Support Type
A 4-pt tooling hole can reduce package risk compared to a 2 or 3-pt support
Summary
Impact of Thermal enabling load on BGAs is complex
Understanding of the fundamental physics is necessary
It is not just the load magnitude that matters! Board thickness, package FF, backing plate, attachment points number and location are all important
Mechanical considerations (in addition to thermal considerations) are essential for a successful design!
22
SJR –Solder Joint Reliability
Test Data: Effect of Backing Plate Thickness
0.4 mm
0.6 mm
0.9 mm
Fixtures With Backing Plate: Package Corner Bending•Low Enabling Load
High Enabling Load
Reaction Force, Ra=0
• The reaction force (Ra) at the posts causes localized bending of the package corner in the presence of load. Larger load ->large F ->more is PC bending and PC joints become critical
What other factors can increases F?
• BP Thickness: Thicker BP > Stiffer BP > Large F
• Board Thickness: For a given F, thin boards bend more, thereby more critical PC joints
Reaction Force, F