Attempt at Correlating Wetting Forces and Solder Spreading

1
Attempt at Correlating Wetting Forces and Solder
Spreading

• Bev Christian and Matt Stevens
• Research in Motion

2
Test Vehicle

• Older, discontinued revision of production
  circuit board
• FR4
• ENIG finish Au 0.08 microns, Ni 3.34 microns
• Each panel consisted of two boards, one used for
  spread testing and one used for wetting balance

3
Test Vehicle

• 42 pads of 20 different sizes chosen
• Fell into different size categories
• small, medium large and extra large
• Boards cut into 12 sections for wetting force
  testing

4
Stencil

• 3 mil thick
• Stainless steel
• Laser cut
• Apertures just a fraction of pad size

5
Wetting Balance

• Multicore MUST System II (now sold by Concoat)
• Outfitted with a 2 mm globule block
• 50 mg of 60/40 Sn/Pb solder
• 235C
• Flux ACTIEC 5
• Speed 0.4 mm/sec
• Depth 0.01 mm
• Sample brought to solder globule at 45 angle

6
Experimental Variables

• Pad size
• RT or Heat conditioning (125C)
• Atmospheric aging (air or nitric acid fumes)
• Performed three factor, two level DOE
• Pad size - small extra large
• Heat 0 or 7 days
• Acid 0 or 4 hours

7
Preparing to Measure Spread

• Measured board pads using a CMM (/- 0.001 mm)
• Calibrated American National Institute of Health
  IMageJ image analysis software with above
  information
• R2 of 0.991 and 0.9993 at 0.7X and 2x,
  respectively
• Measured stencil openings

8
Calibration of ImageJ
9
Calibration of ImageJ
10
Measured using CMM 10 pads of each of the 42
sizes chosen mean pad variation was 0.010 mm2
(0.49)
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Aperture Sizes
12
Aperture Sizes

• Apertures are on average 3.6 of pad size
• Range from 1.7 5.4

13
Spreading
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Chart Key

• P Pad size
• H Heat effect
• A Acid effect

15
Summary of Solder Spread Results
16
Summary of Solder Spread Results

• All main effects 2 factor interactions are
  significant
• Order of significance is
• Acid
• Heat
• Heat Acid
• Pad size Heat
• Pad size Acid
• Pad size

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Summary of Solder Spread Results

• Change in spread
• Small pads 988 acid gt 21
• 988 heat gt 0
• 988 heat acid gt 0
• Extra large 233 acid gt 26
• 233 heat gt 205
• 233 heat acid gt 24

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Main Effect Charts

• These figures use all data points collected,
  averaging across all values of the other two test
  variables.
• One of the problems with this, is that there are
  no 1, 2 or 3 day acid aged samples.
• A further chart is shown to account for this by
  taking out all acid aged results for the pad size
  chart.

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Main Effect of Pad Size on Solder Spread
20
Main Effect of Heat Conditioning on Solder Spread
21
Main Effect of Acid Conditioning on Solder Spread
22
Main Effect of Heat Conditioning on Solder Spread
(no acid aged samples)
23
Root Time Effect of Heat Conditioning on Solder
Spread (no Acid Samples)
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Root Time Effect of Acid Conditioning on Solder
Spread
25
Wetting Force Results

• 42 pad types
• 23 wetted
• 17 consistently did not, irrespective of
  treatment
• 2 always cut during preparation

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Summary of Wetting Force Results
27
The Only Significant Factor!
28
Solder Spreading and Wetting Force Relationships
to Pad Size and Conditioning
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Conclusions

• Stencil openings of 0.02 mm2 were too small to
  leave solder on the pads
• Stencil openings should have been centered on the
  pads
• For the samples used, heating times greater than
  approximately three days had no further effect on
  degrading spreading
• The same can be said for acid conditioning
  greater than 4 hours

30
Next Steps

• Replace acid conditioning with humidity
  conditioning
• Try with other finishes
• Interest other companies, etc. in taking part in
  a larger study

31
References

• IPC J-003- Joint Industry Standard
  Solderability Tests for Printed Circuit Boards,
  IPC, April 1992.
• Frear, D.R., Yost, F. and Hosking, F., The
  mechanics of Solder Alloy Wetting and Spreading,
  Kluwer Academic Publishers, January 1993.

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References

• MUST System II Wetting Balance Users Manual
  Multicore Systems, 1994.
• Cullen, D., Going Beneath the Surface of Surface
  Finishes, accessed via http//www.circuitree.com/
  CDA/ArticleInformation/coverstory/BNPCoverStoryIte
  m/0,2135,86472,00.html

33
References

• Tran, M.Q., Design of Experiments Investigation
  of Multiple factors, RIM Internal Training, Fall
  2002.
• Christian, B., Bailey, K., Solderability Testing
  of Printed Circuit Boards, IPC Printed Circuit
  Expo 1999, Long Beach California, March 1999.