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