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Too rough and too much solder.
Too rough and too much solder.
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Old 13th October 2021, 05:37 PM   #1
nigelwright7557 is offline nigelwright7557  United Kingdom
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Default Too rough and too much solder.

Been busy building up a USB oscilloscope.
It has two SMD devices, one 28 pin a2d and one 64 pin PIC microcontroller.
I have managed to solder the a2d numerous times on previous designs which had PDIP PIC's.
I wanted to move up to a higher sampling speed so found only way was to use a 0.5mm pitch PIC.

My first attempt went badly with plugging USB into PC it threw a hissy fit and told the device was shorting the bus.
I re-soldered the PIC a couple of times but no joy. By this time the pads were starting to look a bit rough.

Had a few pcb's so started a new one with just PIC on it and basic components for PIC to run like crystal etc.
I used a lot less solder this time and was as gentle as I could on the pads.
Still had a couple of shorts but once removed it worked.

Tricks seem to be use plenty of flux paste. Be gentle so0ldering as pads are easily wrecked. Use copper braid to remove excess solder if iron on its own wont do it.

Putting down basic solder to start with runs solder around pads so braiding it doesnt remove all the solder around pin and leaves a good joint.

A good magnifying glass helps detect shorts.
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Last edited by nigelwright7557; 13th October 2021 at 06:50 PM.
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Old 13th October 2021, 10:24 PM   #2
Mark Tillotson is online now Mark Tillotson
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For 0.5mm I'd always use oven or hot air. You need to stencil the solder paste ideally too. 0.5mm is about 3 times more fussy than 0.65mm I reckon, you can't get away with imprecision in solder paste placement or amount.

The golden rule with surface mount rework is less solder, more flux. Bathed in molten flux is correct, it prevents oxide building up on the pins (it only takes one pin to oxidize too much to ruin your day).

With hot air you can rework many times without lifting pads (assuming you are careful with the temperature control).
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