Electrical – Small ‘blistering’ under surface-mount soldering attempt – is this a problem

hot-airpcbpcb-assemblysolderingsurface-mount

I am very new to SMT soldering – and have started with a small project.

I used resin flux, followed by a small amount of solder paste and then a hot air gun at around 373 degress celcius to flow the solder onto the pins.

I appreciate the images are not zoomed in enough to let you see if there are any bridges (I can assure you – there are not!).

The top side of the board looks like this (right hand edge of board when oriented the way you read the text is where I began – and where the blistering shown below sits under :

enter image description here

The under side – looks like this – it is this area where it looks like 'blistering' on the purple dye of the board has happened — see near C78. Is this normal? Is this due to poor soldering flow? How can I avoid this – other than continually moving the hot air gun and potentially turning the temperature down a bit? :

enter image description here

It turns out – its on this edge where I started so unsurprisingly I had multiple bridges to fix which I think may have caused this. Is the board damaged do you think, or will it likely survive through this early inexperienced attempt?

Best Answer

The photo is a bit too unfocused for me to say for sure, but it almost looks like there was an inclusion under the mask when the board was manufactured. I looks like a bit of air/dust/liquid in there "popped out" through the mask as you overheated the area while trying to re-work the solder bridges.

2 things I'm seeing from the underside shot:

  1. The blister appears to be in an etched gap between copper traces, so as long as it didn't carbonize (thus potentially creating 1 or more shorts), it's likely that it won't severely impair function of the circuit (depending, of course on exactly what signals will be on the adjoining traces...high-frequency signals can be very temperamental when it comes to trace irregularities, but DC usually doesn't care much).
  2. I think I see some heat damage to the vias/thru-holes near C79, C82 & a few of the other caps near the perimeter of your SMD as well. I'm guessing that there was a fairly significant amount of overheating involved, so a few of your components may have been damaged by exceeding their thermal tolerances (probably more likely than the PCB damage effecting performance even).

In the future, I'd recommend practicing with a few low-priced components on disposable PCBs, until you can get your temp/timing down on new soldering techniques...it has helped me avoid many costly mistakes, to be sure. Also, when you have bridges to fix, try letting the part/board cool for a while after the initial solder, before re-heating to fix the bridge. It's a technique that helps keep the heat from "sinking into" your components & causing damage...i.e. lets you have more time to work with a lower risk of damaging expensive components/PCBs. ;)

Related Topic