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Warp PCBA- how to repair ** Question #2

Yngwie

#7260

Warp PCBA- how to repair | 15 July, 2001

What is the best method to repair Warped PCBA. Obviuosly twisting it back to make it flat would be a disasterous. The method that I've tried wold be mounting the assembled board to the flat holder with the clamp and bake them at 120 Deg C for about 4 hours. What is the best method other than this. Should the baking temperature be higher than the Tg of the FR4 ? say 135 deg C?

2nd question, currently I'm running WS and would like to change to no-clean process. I've proposed this to my customer but they insist to look at the unbiased article that listing out the benefits of going no-cleaned vs clean process. I'm sure the cost does ? Anyone has an article on NO-CLEAN VS CLEAN ?

Hope to hear from you Experts. thanx.

cheers... Danial

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#7270

Warp PCBA- how to repair ** Question #1 | 16 July, 2001

The method you suggest is about the only way to �clean-up� this terrible situation. Realistically, this is a "memory" type condition induced by design, build symmetry, "cross ply construction", excessive cool down rates during lamination or reflow at assembly. [Read � it may not go away / it may go away, but later return / it may go away.]

Comments on your dewarping plan are: * Our board fab and we �dewarp� multilayer boards at 300�F. Of course, we do this on bare boards in a �pressing� action that wouldn�t work on a stuffed board. After our fab or we dewarp boards, the dewarper uses a electric engraving pencil to etch a 1/4� �W� within 1/2� of a global fiducial on side 1. * We�d be uneasy about dewarping an assembled board at Tg held in a clamped or pinned position, because of the potential of the center of the board to sag in the center worse than the original warping. Do not assume the Tg of your board is 135�C. There is no standard Tg for FR-4. * Appreciating that you built these boards for a customer, you built the boards out of specification, and your customer probably will FREAK when they find-out what you�ve done; consider talking your customer into waiving the defect on these boards. * Yano in real life, even without requesting a waiver, you probably should notify your customer that you�ve dewarped their boards, because dewarping is so radically different from your routine production process. This suggestion carries more importance as the number of layers of the bare board increases.

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#7271

Warp PCBA- how to repair ** Question #2 | 16 July, 2001

You�re correct. If you attach $5 to each board, your customer will be happy to approve your process change. Consider:

�Process Improvement Strategies: Implementation Of A No-Clean Process" http://www.smta.org/knowledge/proceedings_abstract.cfm?PROCEEDING_ID=358

IPC TP-1115 �Selection & Implementation Strategy for A Low-Residue No-Clean Process� http://webvision.ipc.org/scripts/mgrqispi.dll?APPNAME=IPCWEB&PRGNAME=TOCFRAME&ARGUMENTS=-N,-N,-A,-A,-N50

Find "Evaluation of Low Residue Soldering for Military and Commercial Applications: A Report from the Low-Residue Soldering Task Force". Sandia Labs completed the study in June 1995. If you cannot negotiate the Sandia labyrinth, it may be available from EMPF, because they used to get involved in that kind of stuff. Check with Caldon W. Driscoll ACI USA 610-362-1200 cdriscoll@aciusa.org

Finally, check the fine SMTnet Archives.

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