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RELOW PROFILE FOR MIXED LEAD FREE AND STANDARD DEVICES

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#70975

RELOW PROFILE FOR MIXED LEAD FREE AND STANDARD DEVICES | 19 September, 2013

We have a PCB to be populated using reflow soldering. The board contains lead free BGA device(11x11, 1mm pitch ) and Standard SMT devices which are not lead free. Since profile temperature is different for both devices, our contract assembly house is not willing to take up this job. How to arrive a common reflow profile ?. Whether same solder paste(being used for Standard SMT devices) can be used for both devices ? .regards.

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#70976

RELOW PROFILE FOR MIXED LEAD FREE AND STANDARD DEVICES | 19 September, 2013

With the Pb-Free BGA on there, you'll want to use pb-free solder paste for assembly. The logical course of action would be to use Pb-free solder paste for the entire assembly, and to reflow at the temperatures necessary for the BGA and Pb-free paste.

You may need to check the "standard" parts to insure that they can withstand the elevated reflow temperatures. When it comes to the actual solder connection, I would expect minimal issues between the Pb-free paste and the end caps of the components.

Then again, you should probably consider converting the entire BOM to lead-free. These days, Pb-free is the standard for the majority of the components on the market, and, most often, they cost less than the leaded parts. Unless there is some pressing reason to continue the assembly as 63/37, moving everything to lead-free is probably the way to go.

Cheers, ..rob

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#70979

RELOW PROFILE FOR MIXED LEAD FREE AND STANDARD DEVICES | 19 September, 2013

If there is only a single lead free BGA on the BOM, I would suggest a "hybrid" profile. Use regular solderpaste (SN63/pb37) and peak your temperature at around 225C. Allow a bit longer TAL to let the two solders co-mingle at the BGA joints. You are at the high window for regluar solder, and at the low window for lead free solder. This is a normal practice during this transition time to full RoHs building.

Another method would be to remove the lead free spheres from the BGA device, and re-ball using regluar solder.

To simply switch to a lead free paste and use the higher temperatures might not be a good idea, in that your regluar solder SMD parts will likely not do well at the temperatures required for lead free paste, around 250C. There is also the PWB to consider in this scenario as well, it might not be rated for lead free reflow temperatures.

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Reflow Oven

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