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

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Bottom side process question

#5552

Bottom side process question | 13 March, 2001

Anyone doing a print-dispense-place-reflow process? I've never heard of it being done this way before today.

Don't know why, actually, as it sounds like a fairly friendly method provided the adhesives used are happy with a standard reflow profile.

And while we're on the subject, can anyone direct me to some good reading on the topic of bottom side assembly, both from the process and design guideline perspectives? Thanks.

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#5572

Bottom side process question | 14 March, 2001

Check Bob Willis' site. He calls this Single Side Multipass Endothermic Elongated Happy Reflow Soldering, er someting like that. Ha ha ha!!! Naw, he calls it Simultaneous Double-Sided Reflow Soldering. That might be something like you are talking about, although I did not understand anything you said. ;-)

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Michael Parker

#5574

Bottom side process question | 14 March, 2001

Some thing to watch out for is the cure spec for the adhesive. Will that match with your solder paste reflow spec? You will have two dissimiliar materials going through endothermic reactions at the same time. You could end up with under cured adhesive hitting a hot zone that is intended for the solder paste and screwing up the adhesive. Why not eliminate the adhesive if the bottom side parts aren't that heavy and drop off during top side reflow?

With a long oven (7 to 10 zones), you might get away with the mixed cure and flux evap happening correctly, but could lose overall process time with a slower belt speed that is needed to cure the epoxy correctly.

Watch the proximity of adhesive to paste deposits. The epoxy will become liquideous, forming a wet puddle momnetarily that could flow into the solder paste deposit. With the paste flux doing the same type of puddling, the 2 materials will try to mix and the result is solder spheres migrated into the epoxy. While the spheres become permanently entrapped in epoxy, you risk creating an electrical path with the spheres for an surface current to follow. Could compromise the components operation and reliability. I've had this happen recently, at first the problem appeared to be cosmetic (entrapped solder spheres) but the final application of the assembly was hi-freq. (6 GHz) RF and the customer was afraid of a chance of failure with leaving the spheres in the epoxy.

Good luck, think it all the way through before charging off and rewarding yourself for having saved a step. Costly rework is just waiting to happen.

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#5577

Bottom side process question | 14 March, 2001

Mike makes a very good [ no, make that excellent!!!] point about reflowing solder at the same time you are curing glue. There is some evidence that the outgasing during the cure of glue can affect the LT reliability of solder connections. [Regardless of what those lying cretins that pose as friendly sales reps say.]

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#5588

Bottom side process question | 15 March, 2001

Like I said, I would only try this if I had some sort of reassurance that the adhesive was compatible with my reflow profiles. Yeah, I know, reassurances coming from some sources can be less than reassuring.

The whole point of this process is to enable you to wave mixed technology boards without worrying so much about shadowing, since there's already a connection made during reflow. It also doesn't require a chip wave, as I'm told, for the same reason. I can only speculate that the existing reflowed joints retain enough solder to maintain an acceptable fillet.

A consultant we have worked with in the past has said he's had good success with this process, so it's something we're just considering as an option *IF* we decide we want to build two sided assemblies.

I'd thought about adhesive -> solder, but flux -> adhesive. Hmmmmm.... Are the chemists working hard to reduce the consequences of these possibilities?

And as to Dave's comment about long term reliability issues due to outgassing from the adhesive, double-hmmmmmmm.......

There is clearly much research to be done.

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#5589

Bottom side process question | 15 March, 2001

That makes us almost even, Dave. I have a hard time believing half of what you say. ;)

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Travis Slaughter

#5591

Bottom side process question | 15 March, 2001

Don�t be afraid of the more standard doubles sided mixed process. Even without a chip wave it works, don�t get me wrong it works better with the chip wave, but even without chances are you will be spending more on the additional steps than you would touching a solder joint here or there. There are several things that can be done to reduce skips like present the board at an angle or better yet chip parts all orientated along the wave. Play with dwell heat etc. Start slow don�t try a bunch of different boards at the same time one with all chip parts on the bottom first. After you get that one figured out try one with sot�s They are a bit harder to avoid the skips on. Then if your feeling brave go for the soic�s.

On a side note anyone ever hear about putting a double sided board over the wave without epoxy? I�m talking solder print, place, reflow, solder print place reflow, stuff, wave, no epoxy. I heard rumors about it and did some experimenting on it didn�t get far but it looked possible.

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Michael Parker

#5594

Bottom side process question | 15 March, 2001

The only way I know of waving the bottom side with SMT parts installed without epoxy would be to use a masking media, Kapton Tape, wave pallet that keeps those parts from being susceptible to gravity once the connections are reflowed. If there is another way, let's here it! Could be the flucksing fumes got to my head and I didn't pay attention that day in class!

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Travis Slaughter

#5598

Bottom side process question | 15 March, 2001

Like I said it was only a rumor I heard but intrigued me enough to play around with it one day. The lambda wave worked better than the chip wave did with makes me believe it was the surface tension of the solder not melting the already soldered parts or a shadowing affect only melting one side at a time. It was most likely the later because tant caps and tsop�s held up the best in my very short uncontrolled experiment of which I have not had a chance to get back to with any real effort. I was just hoping someone here had heard of it and could clue me in a bit, most likely just a pipe dream but wouldn�t it be nice if it worked.

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#5599

Bottom side process question | 15 March, 2001

Wazis havin' "a hard time believing half of what you say"? I demand a least 90% from peeps!!!

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Michael Parker

#5614

Bottom side process question | 16 March, 2001

Shadowing sounds about right to me also. Really dicey experiment to run if you are dropping expensive tantalums just to save a little coin in epoxy and processing time.

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blnorman

#5616

Bottom side process question | 16 March, 2001

We screen print epoxy adhesive, place components then run the board through our adhesive oven (2-5 min cure at 150�C). This is in addition to reflow.

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#5617

Bottom side process question | 16 March, 2001

That's what we do also. It's the "print, dispense" sequence that's peculiar, but that's what you need to do if you are going to eliminate the glue cure cycle.

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#5620

Bottom side process question | 16 March, 2001

Steve, I did this several years ago on a terribly designed board that left a lot of joints unsoldered after the wave. It works OK. This was a bog Irish board with nothing fancier than SOP packages. Glue was Amicon D125. Happy St. Patrick's Day everyone. Darby.

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