Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

16 mil pitch QFP solder bridge

C.J. Long


16 mil pitch QFP solder bridge | 7 October, 2000

Good morning, All:

I am from PCB shop and one of our customer is experiencing solder bridge on 16 mil pitch QFP( immersion gold finish). Land width requirement is 8 +/- 1 mil measured over top of of land and our mean is 7 mil. Bottom land width is 8 - 8.3 mil which is quite good. IPC standard definition of land width is over bottom of land. Measring top width is more difficult and with greater measurement error due to light reflection from corner and hard to focus. I do not understand other customer of my company follow IPC definition and are without solder bridge. Why this customer is experiencing high defect rate.

If there anyone helping me to clarify folowing:

1.How to identify short circuit is from substrate or assembly process. 2. What is reommended metal stencil design rule for fine pitch SMT. Any suggestion is hight appreciated.


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Re: 16 mil pitch QFP solder bridge | 7 October, 2000

1. It is hard to tell, but your PCB vendor should test the PCB. 2. I think the important thing is the width of the apertures. You should have it laser-cut, and control the width no more than 90% of half pitch.

Good luck.

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Re: 16 mil pitch QFP solder bridge | 7 October, 2000

Tray the electroform stencil.

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Re: 16 mil pitch QFP solder bridge | 8 October, 2000

We are working with a 16mil TSOP Land width=0.2mm(8mil) and use:

1.Type 4 particle size solder paste. 2.Laser cut electropolished stencil: Aperture width=0.19mm (we tried lower than this but had solder insufficiency). Thickness=0.005"(130 microns)in order to get closer to 1.5 Aspect ratio for better paste release.

Check the quality of the cut on the stencil and measure the width of apertures by yourself.

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Re: 16 mil pitch QFP solder bridge | 9 October, 2000

CJ: Arturo and the others make good suggestions. Consider: IPC-7525 Stencil Design Guidelines

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Dason C


Re: 16 mil pitch QFP solder bridge | 10 October, 2000

Hi! CJ. Please advise what is your pitch size ie 15.7 mil or what is your capabiliy can you build the board with pitch 15.75 mil (please note that the actual dimension of the package is 0.4mm), and how many lead on the package. The pad width of Industrial standard for the 16 mil pitch is 8 mil.

If you customer design the stencil based on 8 mil pad width and I suppose that he had insufficient problem instead of solder bridging due to the paste is not easy to print with 8 mil width or less. Please check your customer, what is the stencil thickness, aperature size, any alignemne issue on the stencil printer or P&P. What kind of the paste, metal content and visocity, any smearing problem during printing......

DFM for both Assmebly/Fab house, I will recommend my customer to change the pad width from 8 mil to 11 mil and using 5 mil polished stencil with the aperture width 10 mil. Also, check the pitch size s/b 15.7 mil.

I hope that I can help you to resolve the problem.


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