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Solder paste particle size

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luiHS:
 
Hi
I've been having trouble with very fine pitch component welds, specifically with QFN package, some microcontrollers, and an accelerometer.

Seen under the microscope, after welding in the oven, some small balls can be seen. The problem is that in very fine pitch components, these little balls that remain between pins seem to touch and create short circuits or the equivalent of a resistor. In some boards that causes them to end up burning and in others it does not work properly.

I have been reading about the types of solder paste and it seems that according to the particle thickness they are classified from T1 to T8, with T8 being the finest particle. I have been using T4 solder paste (Loctite MP218S), and I thought I could try to buy a T5, like Chip Quick SMD291AX10T5 or another.

Do you think that to solder very fine pitch chips it is essential to use T5 or even higher solder paste?.
Soon I will be working with BGA (0.8mm pitch) and I would not like to have this problem too.

Cerebus:
Grade 3 or 4 paste should be good to at least 0.5mm pitch - I have no problems with 0.5mm QFNs with grade 3 paste.

Sounds more like a temperature profile problem. A guess, but it's a guess, is too fast an initial ramp-up causing water/solvent in the paste to boil and spit paste all over the place.

Stale paste is always a good suspect too - is your paste (1) within date, (2) properly stored - usually meaning refrigerated, (3) if refrigerated given enough time to warm up before use?

luiHS:

--- Quote from: Cerebus on September 22, 2021, 01:38:27 am ---Grade 3 or 4 paste should be good to at least 0.5mm pitch - I have no problems with 0.5mm QFNs with grade 3 paste.

Sounds more like a temperature profile problem. A guess, but it's a guess, is too fast an initial ramp-up causing water/solvent in the paste to boil and spit paste all over the place.

Stale paste is always a good suspect too - is your paste (1) within date, (2) properly stored - usually meaning refrigerated, (3) if refrigerated given enough time to warm up before use?

--- End quote ---


The temperature curves I use are those of the oven, I have two ovens, T962A and T962C. I use curves 1 and 2, with 2 being the traditional one recommended by all electronic component manufacturers.

I have already used this paste on several occasions from different batches, and the problem is the same, it is not an expired paste or in bad condition, the problem also appears in a recently purchased bottle.

I always store it in the fridge to make it last longer. The only thing I have never thought about is letting the temperature of the solder paste rise, after taking it out of the fridge, before using it. I will have to try it, it did not occur to me, this could be the problem.

However I will buy a solder paste T5 pot to test with the finer pitch chips.

48X24X48X:
Have been using T4 SAC305 and Sn64Bi35Ag1. Both work fine on QFN down to 0.4mm pitch. 1 thing I'm very particular is having the solder mask bridge between the pins. With green solder mask PCB offered by JLCPCB at 0.4mm pitch, this is just enough to obey the minimum rule but sadly their other color offering including their new purple offering couldn't. I don't refrigerate my paste but I don't keep my paste for long either. About 3 months, I would swap them out.

Clif:
the unsuitable stencil aperture design will lead to solder ball issue, can you show maore information about the defect picture and stencil design?

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