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Questions on underfill process

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Questions on underfill process | 8 June, 2007

We are looking at a specialized method of direct soldering ceramic circuit modules (maybe 40mm square) to a mother board with solder joints on only 2 of the 4 sides. These modules would be oriented horizontally, or mezzanine like, to the mother board. We are concerned about the CTE mismatch between the ceramic module and FR4 mother board and are researching the �underfill� process used in BGA�s, but we have no real first hand experience or knowledge:

1)Is this process fairly standard in the industry?

2)Is this underfill process something for which most CEM�s would have the dispensing equipment?

3)Is the �underfill� typically added after post soldering inspection?

4)What are the minimum and maximum �gaps� for underfill to work?

5)What is the maximum �flow length� for the underfill

6)How is it cured?

Thanks in advance

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Questions on underfill process | 8 June, 2007

Jim: I have limited knowledge, but here's what I know:

1. Yes, it's a standard process. In today's SMTNet, it's rarely mentioned, and few people reply to postings.

2. Nope. It's a "specialty" piece of equipment for most CEM's. Depending on your product volume, you can get a table top batch unit for fairly cheap.

3. Yes. From my experience, we had a CSP device with small bumps where we couldn't screen print the solder paste. Plus, we were populating this device on a flex circuit. So, what we did, we dunked the device in a flux tank (this is an option on most P&P equipment), and reflowed the part (the bumps). THEN, after everything was soldered, we dispensed the capillary underfill material and then heat-cured it in an oven.

I can't answer 4 & 5, but you can get in touch with, say, Henkel-Loctite technical support.

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Questions on underfill process | 11 June, 2007


Another good resource would be Asymtek ( I have had a lot of luck with their dispense equipment for underfilling applications. They have run tests in the past using various methods for our company to determine what type of epoxy worked best and which equipment produced the best results. To answer your other question, to my knowledge people either use cappilary flow epoxy or a no-flow which is deposited on the PCB after screen printing prior to part placement and cured using the reflow process. Hope this helps

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Questions on underfill process | 11 June, 2007

Sirl, who are you to say we hardly reply to poastings? Look below and see how many replies you see. then you can reply yourself with the reason why you failed to burn any of us.

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