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reworking brds by flowing the board thru' oven for the 3rd times

zam_bri

#5627

reworking brds by flowing the board thru' oven for the 3rd times | 19 March, 2001

Our process mostly are double sided reflow and we are using water soluble paste on ENTEK PCB.

Rework WIP has been accumulated since the last 4 months, and upon clearing, one of the method that the test guy use are to reflow the PCB again for one more time and they found that by doing this it has somehow, some of the board have passed the Test without having them to do any touch-up. I have strongly against the process as this will expose the board for the 3rd time reflow and will definately jeopardised the compoenents' reliability. I don't have a strong point to argue to what this process could have leads to. Appreciate if anybody could support my point.

Cheers...

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#5629

reworking brds by flowing the board thru' oven for the 3rd times | 19 March, 2001

One very important consideration in your situation is to determine if you have moisture sensitive components on these boards.

If you have any device classified level 3 or higher then you have a very high level of risk associated with moisture sensitivity. A level 3 component has a maximum floor life of one week prior to reflow once the components are removed from their dry bags. If you run these boards through an additional reflow without baking them first this may result in internal cracks and delaminations, maybe even popcorning. These are insidious latent defects that can pass electrical test but may result in early life failures in the field.

The default baking conditions are 48 hours at 125C. You may obtain more details by consulting the joint IPC/JEDEC standard J-STD-033 : Standard for Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices. This document can be downloaded for free at http://www.jedec.org

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Steve

#5632

reworking brds by flowing the board thru' oven for the 3rd times | 19 March, 2001

Another issue is that once solder paste has been reflowed, the majority of the flux is gone. If it is reflowed again without solder flux, the risk of introducing oxidation to the component leads is increased, thus creating a weak solder joint.

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#5665

reworking brds by flowing the board thru' oven for the 3rd times | 20 March, 2001

Another consideration is that heat damage is accumulative. If you have temperature sensitive components, that third reflow may be bringing them close to their failure point.

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