Small 'blistering' under surface-mount soldering attempt - is this a problem?

Question

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 :

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? :

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?

Answer 1

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. ;)

Answer 2

I'm sorry, but I can't see what you're referring to in the images. But, if I'm understanding correctly, the "bubbling" is visible on the bottom side underneath the reworked IC "U3"?

If so, I expect the PCB itself will be fine. The solder mask can delaminate from the substrate without breaking connections. And even the FR4 layers can come apart a bit without trace damage.

However, I worry about U3. The bubbling shows that you got things much too hot. You may well have damaged the IC.

I'm guessing that your purple PCBs are from OSHPark (I love OSHPark!) and were fairly inexpensive. If I was in your situation I would simply start again on another board with a new U3. It would solve both possible problems (the PCB or the IC) and might save a lot of time and headache.