This document discusses improving manufacturing yields and reliability for a QFN package. It found that the customer's stencil design led to excess solder causing shorts between pins. Analysis showed the package itself was not an issue. Redesigning the stencil apertures to better match the pad sizes and using a crosshatch pattern for the thermal pad improved solder coverage and increased standoff height to meet reliability targets. Testing different stencil designs validated that the redesigned stencil addressed the shorting issues and improved manufacturing yields.

1. Power Matters
Understanding the Criticality of Stencil Aperture Design and Implementation for a QFN Package
Larry Bright & Greg Caswell
November 2013

2. Power Matters.
Agenda
Overview
Problem Description
Analysis
Stencil Design
Addressing Manufacturing Yields
Improving Reliability
PCB and Stencil Recommendations
Discussion
Conclusion
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3. Power Matters.
Overview
164 pin Dual Row QFN 13x13
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•This presentation will focus on the package in the center of the board
•Manufacturing yields and reliability concerns will be specifically discussed

4. Power Matters.
Problem Description
Customer complaining of low manufacturing yields, primarily due to shorts
•The figure below shows inner pin shorts
•Other boards showed out pin shorts as well
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Bridging (shorts) between pins

5. Power Matters.
Analysis
Initial evaluation on the package itself
•Package warpage measurements
Limited data on Customer board design
•Cross Sectional analysis performed to determine
–Extent and location of shorts
–Via structure and solder escape paths
–Standoff height
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2.46 2.30 1.66 1.53 1.64 2.03 2.16 0.501.001.502.002.503.00OuterInnerThermalThermalThermalInnerOuterMSC Bond-line profile - Top 2.05 1.91 1.46 1.34 1.64 2.16 2.30 0.501.001.502.002.503.00OuterInnerThermalThermalThermalInnerOuterCustomer Bond-line profile - Top

6. Power Matters.
Package Analysis
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TherMoiré tool used to measure warpage across 30°C to 260°C
Sample Size; 3
Pre Bake; 125°C
Convex/Concave

7. Power Matters.
Package Analysis
 Package warpage measurements
• Package within 50um spec across temperature
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8. Power Matters.
Cross-Sectional Analysis
Bridging across outer leads causing shorts
•Bridging was identified in five locations (fours sets of bridged leads) see red arrows and numbers
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9. Power Matters.
Cross-Sectional Analysis
No solder wicking into the Thermal Vias
•Supposition is that the QFN reached reflow temperature before the PCB
•Thermal via holes ~19mils (larger then desired)
•No indication of any via filled material
•No tenting at bottom or top
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10. Power Matters.
Cross-Sectional Analysis
Bond line thickness measurements
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Center Cross Sections (Thermal Pad and 2 Leads) Bond-line Thickness Measurements
Board ID
Part ID
Left Lead (Outer)
Left Lead (Inner)
Thermal Pad
Right Lead (Inner)
Right Lead (Outer)
Manufacturing
A24
1.73
1.38
1.14
1.46
1.65
Manufacturing
A21
1.65
1.54
1.10
1.34
1.54
Evaluation
U2
2.20
N/A
1.30
N/A
2.28
•Thermal pad standoff ~29um (1.14 mils)
•Not meeting IPC target of 2.5-3.0 mils
•Data shows package concave profile

11. Power Matters.
Stencil Design Evaluation
Customer Stencil Design
Customer Thermal Pad Design 
Only allows 34% solder coverage
We recommend a more traditional pattern as show below
Target solder coverage is 65-85%
Minimal 6mil spacing between print areas
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12. Power Matters.
Stencil Design Evaluation
Stencil Comparison
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The top-left column is our stencil design
The bottom-left is an over lay of the Engineering board with our stencil design
The top-right column is the Customer stencil design
The bottom-right is an over lay of the Engineering board with the Customer stencil design
This clearly shows the excess solder we have calculated and are seeing on the actual boards

13. Power Matters.
Stencil Design Evaluation
Customer Stencil Overlay – Close up
•Paste is yellow, copper is Red
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36% Coverage
1.0mm aperture Length
0.70mm Cu Pad Length
0.50mm aperture Length
0.45mm Cu Pad
Length
0.20mm aperture Width
0.25mm Cu Pad Width
0.22mm aperture Width
0.25mm Cu Pad Width

14. Power Matters.
Stencil Design Evaluation
Customer aperture shape oval vs. bullet shape
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Potential for more solder to
Short to adjacent pins
NOTE: Oval within the bullet shape

15. Power Matters.
Addressing Manufacturing Yields
Thermal Pad X-Rays
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Customer Stencil
Our Stencil
Shows Concerns regarding Thermal Pad coverage not an issue
No Bridging or Voids on either board
78% Coverage
36% Coverage
Solder in Thermal Vias
No Solder in Thermal Vias
No Solder in Thermal Vias

16. Power Matters.
Addressing Manufacturing Yields
More X-Rays; Titled Axis
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Customer Stencil
Customer Stencil
Customer Stencil
MSC Stencil
MSC Stencil
MSC Stencil

17. Power Matters.
Addressing Manufacturing Yields
Testing Stencil Designs
•Our Stencil
•X-Ray images show very clean solder coverage with no bridging across any of the pads
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Customer Stencil
Our Stencil
Testing Stencil Designs
•Customer Stencil
•Note extra “heel” of excess solder due to aperture opening too large for copper pad
Excess solder (heel)

18. Power Matters.
Addressing Manufacturing Yields
Top View
•Our stencil
–Much more copper visible
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Our Stencil
Customer Stencil
Top View
•Customer stencil
–Much more solder on copper pads

19. Power Matters.
Addressing Manufacturing Yields
Top View – High Mag. 50x
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Alternating Inner and Outer Pins
Solder paste residual after cleaning
Very clean solder joint and cu pad
Customer Stencil
MSC Stencil
Alternating Inner and Outer Pins

20. Power Matters.
Addressing Manufacturing Yields
Edge View
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Inner Pins
Outer Pins
Alternating Inner and Outer Pins
Alternating Inner and Outer Pins
Outer Pins
Inner Pins
Customer Stencil
MSC Stencil

21. Power Matters.
Addressing Manufacturing Yields
Edge View – High Mag. 50x
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Alternating Inner and Outer Pins
Outer Pins
Outer Pins
Inner Pins
Inner Pins
Customer Stencil
MSC Stencil
Alternating Inner and Outer Pins

22. Power Matters.
Addressing Manufacturing Yields
Preparing additional Cross Sections
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Recommend 7 data points per line; Outer, Inner, Thermal (Left, Center, Right,) Inner, Outer

23. Power Matters.
Addressing Manufacturing Yields
Cross Section Customer Standoff Detail
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Location 1 51.52 um
Location 2 49.20 um
Location 3 37.63 um
Location 4
35.89 um
Location 5
41.10 um
Location 6 56.15 um
Location 7 59.04 um
Location 15
52.10 um
Location 16 48.63 um
Location 17 37.05 um
Location 18 34.15 um
Location 19
41.68 um
Location 20
54.99 um
Location 21 58.47 um
Location 8 43.99 um
Location 9
41.68 um
Location 10 30.10 um
Location 11 28.94 um
Location 12 35.89 um
Location 13
49.20 um
Location 14
53.26 um

24. Power Matters.
Discussion
Stencil DOE Assessment
•The board with 78% coverage has voiding in a random pattern and shows that 8 of the 12 vias have some measure of solder in the holes
•There is little push out on the I/O joints in the cross sections indicating that the overall volume of paste there is on target.
•Note bulbous joints on the I/O pads indicating that the part again was pulled down by the thermal pad and the paste was pushed out on the I/Os
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MSC
MSC
MSC
Cust
CUST

25. Power Matters.
Discussion
Stencil DOE Assessment continued
•The volume of paste in the vias indicates that this reflow profile brings the PCB to temp just prior to the QFN.
•This is what I would expect of a reflow profile
•SMTA journal indicates several benefits to using a crosshatch pattern thermal pad.
–Helps avoid depositing solder directly over thermal vias
–Allows excellent escape paths for volatiles
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26. Power Matters.
Improving Reliability
Further Studies
–The next set of evaluations involved gathering IPC9701 data (Performance Test Methods and Qualification Requirements for Surface Mount Solder)
–Twelve reliability boards were used
–Three (3) Type-7 plugged vias using MSC stencil
–Three (3) open vias using MSC stencil
–Three (3) Type-7 plugged vias using hybrid stencil
–Three (3) open vias using hybrid stencil
–Four (4) boards (2 each) subjected to Temp Cycle
–One each to be cross sectioned; total 4 boards
–Serialized-Daisy Chain packages with Shodowmorie test will be used
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27. Power Matters.
Improving Reliability
Ensure use condition exceeds expected life
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28. Power Matters.
Improving Reliability
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3.51 3.33
2.85 2.64
2.89
3.26 3.42
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Outer Inner Thermal Thermal Thermal Inner Outer
DfR 4 Open Vias- Middle (Bd 1-U12)
-33% Slope or 0.87 drop
2.16 2.01
1.75
1.46
1.82
2.32 2.42
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Outer Inner Thermal Thermal Thermal Inner Outer
Hybrid 25 Open Vias- Middle (Bd 7-U5)
-66% Slope or 0.96 drop
4.06
3.76
3.40
2.96 3.10
3.49 3.60
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Outer Inner Thermal Thermal Thermal Inner Outer
DfR 4 Filled Vias- Middle (Bd 2-U7)
-37% Slope or 1.09 drop
3.37 3.17
2.83 2.92
3.46
3.90 4.10
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Outer Inner Thermal Thermal Thermal Inner Outer
Hybrid 25 Filled Vias- Middle (Bd 8-U5)
-45% Slope or 1.28 drop

29. Power Matters.
Recommendations
PCB
•The lower number of vias the better
•Smaller Thermal via drill holes
–(8-10mil is better than 17-19mils)
•Smaller outer pads (0.5) and/or cover edge with solder mask
•Filled vias or place the vias in the streets between the print zones, if possible
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30. Power Matters.
Recommendations
Stencil
•Minimum of 5mil (0.12mm) Laser Cut Stencil with electropolished aperture walls
•Based on a copper pad 0.25x0.70mm outer and 0.25x0.45mm inner the recommended aperture configuration is 0.25x0.50mm outer and 0.25x0.45mm inner. Use oval shape to get optimum paste release and minimize clogging
•Use a stainless steel squeegee applied at 45-60ºangle.
•Squeegee speed must be reduced to fill the aperture during print cycle
•Thermal pad coverage needs to target 65-85% coverage to achieve 2.5-3.0 stand-off, using a 3x3 array with each opening noted herein
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31. Power Matters.
Conclusion
The customer approached Microsemi with a board-level manufacturing yield issue
The data supports that adjusting the stencil apertures on the I/O’s help address the issues with shorts
•However, there are many other process variobale that also need to be control
The data shows that proper thermal pad stencil design can achieve the desired stand- off height to achieve high reliability
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32. Power Matters.
Thank You!
Questions?
Larry Bright@ 512-228-5600
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