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

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Pad and Stencil Design

floydl

#17611

Pad and Stencil Design | 17 September, 2001

Based on your research, what is the pad and stencil design that you recommend for 0201 assembly?

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Jeff Schake

#17620

Pad and Stencil Design | 17 September, 2001

The results from an experiment comparing 27 different combinations of pad dimensions concluded that the pad design with 15 mil pad length, 12 mil pad width, and 9 mil pad separation produced the best assembly yields.

A 5 mil thick laser cut stencil was ultimately selected based on the premise that:

- 6 mil thick stencil may not produce acceptable transfer efficiency for such a small aperture size (i.e. approaching area ratio violation threshold).

- 4 mil thick stencil may not comply with the larger solder volume requirements for other common surface mount components.

The aperture design on the 5 mil thick stencil was 15 mils long x 11 mils wide (rectangular openings).

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#17624

Pad and Stencil Design | 17 September, 2001

What about the material of the stencil? Nickel formed? stainless steel? even more materials are available, some with different ways to form the apetertures.

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Jeff Schake

#17639

Pad and Stencil Design | 18 September, 2001

A laser cut stainless steel stencil was used exclusively for the evaluation of the complete 0201 assembly process, which addressed printing, placement, and reflow. However, I have also conducted off-line 0201 print tests using a nickel electroformed stencil. The print deposits using the electroformed stencil do appear to look nicer and may produce marginally better volume repeatability and transfer efficiency. You will probably be able to get away with using the under stencil cleaning system less frequently with an electroformed stencil compared to laser cut. These marginal gains may not, I suspect, have an overwhelming influence on the overall assembly yield. Hence, armed with what I know currently, my decision to use laser vs. electroformed for 0201 assembly would be primarily driven by cost.

-Jeff

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davidd

#17644

Paste Inspection Requirements | 19 September, 2001

Hi Jake, in your experience, what to do feel is the level of automated 3D paste height or volumetric inspection required moving to the 0201 process control requirements and defect detection.

Dave

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jschake

#17649

Paste Inspection Requirements | 19 September, 2001

I�ll volunteer to answer this one for Jake. In order to prove the performance and capability of the stencil printing process, I think this level of inspection detail is required. However, in a production scenario where a known printing performance level has already been defined:

* If the 3D paste inspection equipment is available to use in production, by all means use it. As long as throughput doesn�t take a big hit, this is an effective means to monitor the process.

* I think basic paste inspection is necessary on the critical areas of the board that are suspected to produce flaws first, by whatever means is possible and available in order to ensure high process yields. What I mean by �basic� is simple paste presence vs. paste absence where the threshold defining each can be uniquely defined. The vision systems on the advanced stencil printing systems offer these sort of basic algorithms in addition to checking the stencil apertures for blockage. This information can be monitored offline on a network PC, if set up that way, providing this sort of realtime health-check information on demand. Keep in mind the penalty for using the printer for inspection is longer cycle time.

-Jeff

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George Verboven

#18834

Pad and Stencil Design | 9 February, 2002

For more info about Metal Stencil Overview. also look at: http://www.tkb-4u.com/articles/printing/metalstenciloverview/metalstenciloverview.php

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