6 ms·

PCB Rework and Repair Guide [pdf]

168 points by varjag 8mo ago

barfiure 8mo ago
Watch the PACE videos as well, they’re on YouTube and they also cover how to remove conformal coating and using different tools.
amelius 8mo ago
Missing: how to solder a wire to the thermal pad on the bottom side of an IC. Assume there are components on the back side.
- eimrine 8mo ago
  Isn't it just ground? If not, can you drill a corner of the chip? Most of chip material is just plastic.
  - Kirby64 8mo ago
    Not always. Also, drilling the corner of the chip is a bad idea, as you'll likely hit one of the corner pads. There's not enough room.
- dlcarrier 8mo ago
  If it's QFN, you're probably best deadbugging it and jumpering all of the pins individually, but first make sure that you need the connection. QFP wireframes all have a center pad, whether or not it's electrically connected to the die or needed for thermal dissipation. If it's an SO package, e.g. SSOP, TSOP, etc., Desolder the IC, add the jumper wire, bend the pins down enough to account for the thickness of the jumper wire, and resolder the IC. Either way, make sure you have enough thermal mass connected to it for thermal dissipation. If there's components on the opposite side of the PCB, it's probably not much.
lnsru 8mo ago
Almost 30 years old. Old good times without BGAs and modern barely visible components. While some basics are still applicable the modern problems are not covered at all.
- varjag 8mo ago
  It's great for working on vintage equipment, stuff that might need (and warrant) that kind of repair. Less so if you run a cell phone repair shop.
  - nxobject 8mo ago
    There are a surprising number of Indian cell phone repair shops with YouTube channels that do feats of soldering, like repairing torn flat-flex cables… I bow down at their craft. If I ever get a sabbatical I’ll go to India and ask to be an apprentice.
- Vexs 8mo ago
  I don't think that modern boards are really repairable at all beyond component replacement- 4+ layer stackups being the big reason. If there's a way to do anything to those boards besides total replacement I'd be super interested to know. The techniques here are also way beyond basics I think- like, you look at most guides for repair and it's "idk just solder some bodge wires on there, here's what a good joint should look like"
  - dsvf 8mo ago
    Andrew Zonenberg posted a Twitter thread a year or two ago where he fixed a missing PCB trace some layers down a PCB, with a stereo microscope, precision mill and very steady hands. Edit: here's the thread. It's a 6 layer PCB with a short on L5 that needs to be fixed from the L1 side. https://xcancel.com/azonenberg/status/1468825231225540611#m https://xcancel.com/azonenberg/status/1468825231225540611#m
    - dmbche 8mo ago
donquichotte 8mo ago
Beautiful illustrations!
eimrine 8mo ago
I have a problem of dismounting brittle SATA ports, maybe I just need more preheating but they are sitting really tight in the board.
- dlcarrier 8mo ago
  SATA ports are often rated for only tens to hundreds of cycles, so they're often made out of fallapartium. If you're replacing one, it's easiest to cut all of the plastic off first, then desolder the contacts one at a time. Sometimes you can even pull the plastic off, without damaging it. It's usually necessary to preheat the board.
- fuzzfactor 8mo ago
  Things like that are often the only through-hole components on a PCB. I like to melt the original solder one contact or area at a time, add a little bit of fresh flux-containing solder if needed, and as soon as it gets to the consistency of mercury, vacuum it clean out with a good soldersucker.
emporas 8mo ago
> Destructive static charges are induced on nearby conductors, such as human skin, and delivered in the form of sparks passing between conductors, such as when the surface of printed board assembly is touched by a person having a static charge potential. [..] It is important to note that usually the static damage level for components cannot be felt by humans. (Less than 3,000 volts.) Less than 3000 volts cannot be felt by humans? Should be 3000 millivolts right? i.e. 3 volts...
- bayindirh 8mo ago
  Considering static discharge is a spark, I assume 3.000 volts is the correct scale. When I wince from a static discharge generally there's visible spark. That's not 3 volts.
- georgefrowny 8mo ago
  It's 3000V, but the energy delivered can be absolutely minute on a human scale, even for the highly sensitive nervous system. But metal oxide layers (what static discharge is often blowing holes in) are not on a human scale, they're atoms thick. And highly insulating, which leads to teravolts/metre field gradients. For discharge that you can feel and see, the energy is even higher, but damage can be done far below this level.

168 points by varjag 8mo ago

Watch the PACE videos as well, they’re on YouTube and they also cover how to remove conformal coating and using different tools.

amelius 8mo ago

Missing: how to solder a wire to the thermal pad on the bottom side of an IC. Assume there are components on the back side.

eimrine 8mo ago
Isn't it just ground? If not, can you drill a corner of the chip? Most of chip material is just plastic.
- Kirby64 8mo ago
  Not always. Also, drilling the corner of the chip is a bad idea, as you'll likely hit one of the corner pads. There's not enough room.
dlcarrier 8mo ago
If it's QFN, you're probably best deadbugging it and jumpering all of the pins individually, but first make sure that you need the connection. QFP wireframes all have a center pad, whether or not it's electrically connected to the die or needed for thermal dissipation. If it's an SO package, e.g. SSOP, TSOP, etc., Desolder the IC, add the jumper wire, bend the pins down enough to account for the thickness of the jumper wire, and resolder the IC. Either way, make sure you have enough thermal mass connected to it for thermal dissipation. If there's components on the opposite side of the PCB, it's probably not much.

lnsru 8mo ago

Almost 30 years old. Old good times without BGAs and modern barely visible components. While some basics are still applicable the modern problems are not covered at all.

varjag 8mo ago
It's great for working on vintage equipment, stuff that might need (and warrant) that kind of repair. Less so if you run a cell phone repair shop.
- nxobject 8mo ago
  There are a surprising number of Indian cell phone repair shops with YouTube channels that do feats of soldering, like repairing torn flat-flex cables… I bow down at their craft. If I ever get a sabbatical I’ll go to India and ask to be an apprentice.
Vexs 8mo ago
I don't think that modern boards are really repairable at all beyond component replacement- 4+ layer stackups being the big reason. If there's a way to do anything to those boards besides total replacement I'd be super interested to know. The techniques here are also way beyond basics I think- like, you look at most guides for repair and it's "idk just solder some bodge wires on there, here's what a good joint should look like"
- dsvf 8mo ago
  Andrew Zonenberg posted a Twitter thread a year or two ago where he fixed a missing PCB trace some layers down a PCB, with a stereo microscope, precision mill and very steady hands. Edit: here's the thread. It's a 6 layer PCB with a short on L5 that needs to be fixed from the L1 side. https://xcancel.com/azonenberg/status/1468825231225540611#m https://xcancel.com/azonenberg/status/1468825231225540611#m
  - dmbche 8mo ago

donquichotte 8mo ago

Beautiful illustrations!

eimrine 8mo ago

I have a problem of dismounting brittle SATA ports, maybe I just need more preheating but they are sitting really tight in the board.

dlcarrier 8mo ago
SATA ports are often rated for only tens to hundreds of cycles, so they're often made out of fallapartium. If you're replacing one, it's easiest to cut all of the plastic off first, then desolder the contacts one at a time. Sometimes you can even pull the plastic off, without damaging it. It's usually necessary to preheat the board.
fuzzfactor 8mo ago
Things like that are often the only through-hole components on a PCB. I like to melt the original solder one contact or area at a time, add a little bit of fresh flux-containing solder if needed, and as soon as it gets to the consistency of mercury, vacuum it clean out with a good soldersucker.

emporas 8mo ago

> Destructive static charges are induced on nearby conductors, such as human skin, and delivered in the form of sparks passing between conductors, such as when the surface of printed board assembly is touched by a person having a static charge potential. [..] It is important to note that usually the static damage level for components cannot be felt by humans. (Less than 3,000 volts.) Less than 3000 volts cannot be felt by humans? Should be 3000 millivolts right? i.e. 3 volts...

bayindirh 8mo ago
Considering static discharge is a spark, I assume 3.000 volts is the correct scale. When I wince from a static discharge generally there's visible spark. That's not 3 volts.
georgefrowny 8mo ago
It's 3000V, but the energy delivered can be absolutely minute on a human scale, even for the highly sensitive nervous system. But metal oxide layers (what static discharge is often blowing holes in) are not on a human scale, they're atoms thick. And highly insulating, which leads to teravolts/metre field gradients. For discharge that you can feel and see, the energy is even higher, but damage can be done far below this level.