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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Double Sided Reflow

John S

#33252

Double Sided Reflow | 17 March, 2005

We are moving into using fine pitch components in a double sided reflow process. In one of our test runs, we noted that some of the QFP's would move while reflowed on the bottom on the second pass and become defective. Is this being done widely with fine pitch on both sides of the PCB? This is an automotive product, so reliability and yield expectations are super high. Thanks John

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Bob R.

#33256

Double Sided Reflow | 17 March, 2005

I can't answer your question directly, but we're an automotive supplier and we don't do dual reflow with QFPs because of their moisture sensitivity level. We don't have the processes in place to manage the time between first and second pass so we'd be in danger of reflowing a QFP that had sat hour past MSL requirements. I know many assembly houses have processes to handle this, either by nitrogen/dry storage between reflows or by monitoring time between reflows.

Just something to be aware of.

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#33257

Double Sided Reflow | 17 March, 2005

This happens. You may need to put a dot of chip bonder on the corner of the floating QFP after first pass.

What size are the QFP that are moving?

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Rob

#33258

Double Sided Reflow | 17 March, 2005

Or...

If you have an inline glue dispensor, after printing put a glue dot in the centre of the footprint & this should hold it.

Rob.

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#33259

Double Sided Reflow | 17 March, 2005

Hi John, This problem is interesting. I've never had a problem with solder shorts caused by subjecting a device to reflow a second time. I've built many double sided SMT assemblies with fine pitch devices on top and bottom. Some things to possibly look at... Is the same pitch located on the primary side? If so, are they shorting? If not, check your bottom side blower settings. I've had problems with off shifts turning up the convection too high thinking they are doing us a favor. This has caused shorts for me in the past. Another possible cause is a "jerky" conveyor. Verify that your conveyor runs smoothly without "hitching". Sorry I can't be of more help. I think adding adhesive is a bandaid for some other problem. I just wish I knew what the problem was.

Regards, Justin

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Dougs

#33260

Double Sided Reflow | 17 March, 2005

I'd agree with Justin, we do double sided reflow on most of our boards with fine pitch QFP's both sides, you do have a problem somewhere else, what size is you board? Is it warping slightly? What kind of oven are you using? Is it convection? fans or air amp?

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RDR

#33266

Double Sided Reflow | 17 March, 2005

Are you running the second side on an edge hold or just laying it on the belt?

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KEN

#33270

Double Sided Reflow | 17 March, 2005

Check your furnace for "level".

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Dreamsniper

#33284

Double Sided Reflow | 18 March, 2005

We are producing control products for the Aerospace Industry both Commercial and Military. I have TQFP 100 fine Pitch in one of my PWB's Solder Side and have no misalignment problems or defects on my part after 2nd reflow. I control my reflow profile to between 213 to 220 deg. C Peak with not more than 60 secs dwell. We make sure that once we run the job it's not gonna exceed 3 days until the product is finished in our SMT Line (both top and bottom). Our advantage is that we do not run high volume and run only 50 pwb's per job at the most. I know that in the automotive industry they do it in high volume. We can't afford to lose 1 PWB after the SMT process because our PWB's with populated components are expensive. That's why we control our schedule. We run slow though.

regards,

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Grant

#33299

Double Sided Reflow | 20 March, 2005

Hi,

Can you guys cascade the line, with a board flipper, printer, placer and oven all in the same line, so you can do both sides in the same pass? The product then leaves the line finished.

Regards,

Grant

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py (france)

#33322

Double Sided Reflow | 21 March, 2005

it's definitively not possible.. printer/chip shooter/pick and place/reflow and printer/chip shooter/pick and place/reflow.

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Grant

#33348

Double Sided Reflow | 22 March, 2005

Hi,

Yes it costs more, but would get full speed production. We design our own products, so can fit them to manufacturing which is nice. Currently that means no underside components at all!

Regards,

Grant

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John S

#33355

Double Sided Reflow | 22 March, 2005

Thanks for the advice. In this scenario we are designing our own product. Currently we are building at a prototype level with no appreciable wait between reflow passes. The part in question is a 64 pin QFP. We use a BTU VIP 98 with edge conveyor. The PCB is 0.032" FR4, so it is susceptible to bowing. The next rev should be 0.062". Thanks John S

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