Author Topic: Hot air reworking: What I learned  (Read 1310 times)

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Offline Watth

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Hot air reworking: What I learned
« on: November 26, 2016, 02:37:20 pm »
Preamble: The problem
A few months ago, I had a big problem: I made a mistakenly inverted polarity while plugin a compatible AC adapter to my laptop. The PC would work only on the battery power. It wouldn't power from the AC-DC brick, thus wouldn't recharge the battery.
I couldn’t afford a new PC; and buying a replacement motherboard was to complicated and expensive.
I then turned to the awesomest bestest ever web community in electronics :
With the help of user poot36, I could track down the problem to a MOSFET that died. RIP little angel, forever in our hearts.
Hunting for a replacement part
Acquiring the exact part was too long, troublesome. If I remember correctly, it was from sources such as ebay or aliexpress, which didn’t guarantee a genuine part. Nonetheless I easily found an equivalent from more reputable sellers (Farnell and RS, although it was cheaper from the latter).
Replacing the part
Now is the tricky part: the MOSFET comes in the leadless Power 33 package. I was not equipped nor used to this kind of reworking, at that time I had a modest iron with a thermostat on the handle. Although I used it for some basic SMD, it wouldn't do, so alternatively I looked around for shops that did this kind of jobs, but found none.
Investing in equipment
Here I’m lucky my parents proposed to pay for the air reworking I spotted. I bought gel flux (Chip Quick SMD291NL from Farnell) and kapton tape, already had solder paste.

What I learned:
Note: in theses points I might have made wrong assumptions, if so, please let me know!
Training on dead or obsolete motherboard is the best way to start, I deduced:
  • Watch how they do it on Youtube. For example, Louis Rossmann makes it look easy. Of course you know it only looks easy because they’ve become good at it; but for me, seeing Rossmann’s ease and nonchalance at reworking motherboards was quite reassuring, considering what I had to work on was way simpler than what he usually does.
  • When in doubt, add more flux.
  • It’s better to have a generous temperature setting for a short time, rather than slow cooking the comps.
  • Do not pull on the component! You could tear off the copper pad from the substrate. The component will move when it’s ready (If it doesn’t, see the previous point). Try gently nudging it sideways, there are less risks that you’d damage the pad.
  • Kapton (polyimide) tape is a good solution to keep neighbour components from flying away, and deflecting heat from surrounding parts. (Buying is cheaper  from Farnell than on RS.)
    Speaking of which, they didn’t figure a plastic material for connectors that wouldn’t melt at the slightest provocation. In spite of my precautions, some were slightly deformed, but without consequences.
    Did I mention flux? Squeeze some on that board!
When it was time to work on my PC’s motherboard, one thing became clear:
  • It’s damn scary. I had to work on this knowing that if I screwed up, I’d render my pc worthless in a time I couldn’t afford replacing it and had my main activities depending on it.
  • After you remove the dead MOSFET, and place the new one, and you put it all together, and it turns on and it works, it’s a huge relief. Hyyyuuuge.
  • (Almost) anyone can do it, at least for basic stuff. I won’t do reballing anytime soon, but leadless packages are manageable if beforehand you train on dead/obsolete boards.
Because "Matth" was already taken.

Offline amyk

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Re: Hot air reworking: What I learned
« Reply #1 on: November 26, 2016, 03:37:11 pm »


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