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Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Board support issues

Rick Taylor

#17617

Board support issues | 17 September, 2001

We are printing 0.012" apertures on a .012" Solder mask defined pad in a fine pitch BGA application. The board is 0.032" thick with very little room on the bottom to place support pins under these areas. Our first pilot proved extremely difficult to print and I suspect this is the main reason. We would like to provide our design engineers with a specification of how far/close we need to provide support in order to consistently print, but have never been faced with a reason to do it (hence, no internal study has been completed). Do you know of a study or have knowledge of something like this?

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Jeff Schake

#17635

Board support issues | 18 September, 2001

Hey, where's the 0201s? Just kidding.

Off the top of my head, I do not recall seeing results of such a test. Nonetheless, I�ll consult with my peers and let you know if we come up with something that could assist you.

That said, with a few more details that you provide I may be able to do some troubleshooting. To understand your application better, I offer you the following list of questions. Answer what you can and I will try to help.

1. Why are you not able to place support pins under the BGAs? Are there components on the underside that interfere, or is it something else?

2. How close are you able to place a support pin to the BGA site?

3. What is the tooling pin diameter you are using?

4. What is the size of your board?

5. How many pins are you using to support the board?

6. Can you elaborate on the difficulties that you experienced with your pilot build? (Describe the print results you observed.)

7. Solder paste information? (particle size, metal load, alloy)

8. Stencil thickness, technology?

9. Squeegee? (material, length)

10. Print speed, pressure, print gap, separation speed?

-Jeff

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Rick Taylor

#17637

Board support issues | 18 September, 2001

Sorry about not being 0201 related, but I figured the aperture size we were looking at would need similar considerations...thanks.

I will try to answer the questions below: 1. The board is completely packed with components on the other side..very little room to provide a support pin. Also, the sites are not near a web (where the individual board connects to the array material) so it is flimsy.

2. The worst case I have now is about 16mm.

3. I have any number of support pins dia. to choose from.

4. The board is 800mm x40mm, in a 4-up array at 215mm x 115mm. 5. As many as I can fit (guessing about 10-12 2mm dia pins located mostly on the perimeter. 6. Poor release of the paste (clogging >50%) only over the appertures where there was poor support. Most prints took 2 knead strokes to print OK, but this caused fine pitch leaded devices to almost short. stencil had to be be wiped after kneading. 2 similar devices with exactly the same padstack/stencil printed fine.

7. We used Multicore RP-15 mixed alloy, type 3, 89.5% metal loading. Indium 92J has also been used with similar results.

8. 5 mils, SS, laser and electropolished (IRI).

9. SS squeegee, 12"

10. 25mm/s, 7Kg, on contact printing, 1.9mm/s sep speed up to 2.1 mm.

We have limited production experience with devices of this type (with most of the same settings/paste above) and they cannot be printed by a novice, but this is the first product where we cannot (as the design stands now) provide the kind of support we have been able to in the other products.

Hope this helps and thanks. Rick

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Jeff Schake

#17648

Board support issues | 19 September, 2001

You provided excellent detail, and unfortunately I do not see anything out of the ordinary except the tooling issue. This brings us back to the original tooling related questions you had asked before. My apologies for questioning your process integrity.

Might there be a way for you to test the printing of a bare board without any components on the underside? I suggest printing this using the tooling setup that you suspect is giving you trouble. I assume the expected outcome of this would be to find poor paste release in the areas where the board support is missing (i.e. fine pitch BGAs). If the print looks good, then you may be dealing with a different issue. I�ll wait to hear back from you before getting too involved with the scenarios. Clean the same board and print it again, this time having the tooling pins equally distributed on the table to ensure uniform coverage and best overall board support. What happens now? Hopefully, you will have proved the problem is tooling related, as suspected. If so, then you may want to consider a conformal tooling system to provide the support you require on the bottomside of your substrate. I have already addressed questions about this, listed under the subject heading �FormFlex� posted by James. Other conformal tooling system questions were addressed in the previous OnBoard forum where tooling options was main focus for discussion.

-Jeff

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Rick Taylor

#17656

Board support issues | 20 September, 2001

Sorry for not getting back sooner.. a virus attacked our network yesterday. The experiment you have outlined is what I had next on my list...just trying to see if anyone had done it before. We will be experimenting with 0.004" and 0.005" thick stencils, varying the support distance from the problem area, and then measuring the printability over time either using an SVS or just visually looking at how much the apertures clog over time. I have to blow the dust off the SVS and my own memory of its operation if I go that route! Thanks for free advice.

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Jeff Schake

#17659

Board support issues | 20 September, 2001

I'm not sure how soon you are planning to perform this experiment, but it certainly would be interesting to get a snapshot of any results you could feed back to give us all a sense of closure regarding this issue. Is this possible? My sympathy is with you as I image you blowing the dust off the SVS and contemplate the programming task that lies ahead...

-Jeff

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Rick Taylor

#17660

Board support issues | 20 September, 2001

No problem on the feedback. Should I just reply to the same thread or email you directly at DEK? This is my first time using this site, but I imagine they have the capability to attach files, pictures, etc. I will probably be completing the study in about 2-3 weeks.

-Rick

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Jeff Schake

#17662

Board support issues | 20 September, 2001

So that the entire forum benefits, I recommend that you post your findings to the main forum discussion page, instead of replying to this thread. You may even find that others who have not participated in the OnBoard Forum have similar experiences to share. I suggest referencing this thread for anyone that may not understand the history of what you are talking about. I�ll be looking out for it.

-Jeff

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