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Power Matters
Understanding the Criticality of Stencil Aperture Design and Implementation for a QFN Package
Larry Bright & Greg Caswell November 2013
Power Matters.
Agenda
Overview
Problem Description
Analysis
Stencil Design
Addressing Manufacturing Yields
Improving Reliability
PCB and Stencil Recommendations
Discussion
Conclusion
2
Power Matters.
Overview
164 pin Dual Row QFN 13x13
3
• This presentation will focus on the package in the center of the board
• Manufacturing yields and reliability concerns will be specifically discussed
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
4
Bridging (shorts) between pins
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
0.50
1.00
1.50
2.00
2.50
3.00
Outer Inner Thermal Thermal Thermal Inner Outer
MSC Bond-line profile - Top
2.05 1.91
1.46 1.34 1.64
2.16 2.30
0.50
1.00
1.50
2.00
2.50
3.00
Outer Inner Thermal Thermal Thermal Inner Outer
Customer Bond-line profile - Top
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
Power Matters.
Package Analysis
Package warpage measurements • Package within 50um spec across temperature
7
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
8
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
9
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
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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Power Matters.
Stencil Design Evaluation
Stencil Comparison
12
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
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
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
Power Matters.
Addressing Manufacturing Yields
Thermal Pad X-Rays
15
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
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
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)
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
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
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
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
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
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
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
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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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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Power Matters.
Improving Reliability
Ensure use condition exceeds expected life
27
Power Matters.
Improving Reliability
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3.51 3.332.85 2.64
2.893.26 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 2.011.75
1.461.82
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
4.063.76
3.402.96 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 3.172.83 2.92
3.463.90 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
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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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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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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Power Matters.
Thank You!
Questions? Larry Bright@ 512-228-5600
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