...and Murphy strikes again.

[JimRayden]

Just finished a board just to find that all nine FETs are the wrong way around.

I need to get a punching bag with a picture of my face on it so I wouldn't hurt myself when I find out such things.


Any tips to ease my pain of de- and resoldering all of 'em?

----------
Jimbo

[PerroGrande]

I need to get a punching bag with a picture of my face on it so I wouldn't hurt myself when I find out such things.

I've got to admit that while I feel your pain, I couldn't help but laugh at that line...

Well, desoldering isn't bad, but one does have to be careful.  The things that can go wrong de-soldering are really the same that can go wrong when you solder...

- Too much heat to the component (toasted FET)
- Too much heat to the board (lifted/damaged trace)
- Solder bridges.

First of all, use some solder wick or a "sucker" to remove the solder cleanly.  Do one leg at a time on each FET so they stay a bit cooler.  Check everything after removing the fets and clean any bridges, excess flux, etc. before replacing.   I actually prefer a "sucker" to wick -- requires a little less heat in my experience and you can see what you're doing a bit better.  Your mileage may vary. 

Take your time and all will be well.

[R.G.]

I came up with something to help me over the last few decades. It's a graphic that I laid out and could print when I needed to. When I'd done something similar, I'd print it out, tape it to the wall and follow instructions.

It's an ordinary letter-sized sheet of paper, with the top 2/3 covered by a large bullseye target.

Underneath that, on the lowest 1/3 of the sheet, was the instructions: "Bang head here."

Sometimes it helped. 

[PerroGrande]

R.G. -- you should put that on GEOFEX, in the same section where you have the templates for drilling enclosures...   

[R.G.]

That's not a bad idea! 

[aloupos]

The problem with FETs is that pinouts sometimes vary from manufacturer to manufacturer. 

A good idea might also be to socket them.  I use individual sockets (pins only, round).  Then I test the circuit, and before I box it I solder the FET to the pins.  For me it's the best of both worlds, because you can test different fets in the circuit if you like, but you can also install them "permanently" for a unit that will take some road abuse.  The pins I bought are pretty small and don't take up much vertical space on the board. 

I got them at a local shop, so I'm not sure where to get them online, but they must be around. 

[GonzoFonts]

[JimRayden]

(WARNING! Print out before use!)


----------
Jimbo

[tranceracer]

I've had this happen many times too!
A good technique I've found using a desoldering braid is to cut small 1/8" to 1/4" pieces of braid.  Pinch the small peices of Cu braid between the soldering iron tip and the board.  The solder braid absorbs the solder quite quickly because the length of braid is short.

If I find myself working on one pad to long, move on to another one and come back to it later after it cools down.  If the pad takes more than one piece braid to absorb all the solder, move on to another pad and come back to it after it cools.

If you're desoldering transistors and are worried about heat etc. I've custom fabricated a heat absorber by cutting the tip of an alligator clip to a point and attaching it to the leg of the transistor that's being desoldered.

be careful because too much heat, among other things, will cause the trace to lift off the board.  I've trashed whole boards because of this.  To me it's easier to replace a burnt up transistor than to rebuild the project on a brand new board  ... but that's just me.

-tR

[Paul Perry (Frostwave)]

Everyone has their own approach, but - if the fets have long enough leads - just cut them off a few mm from the board, then solder new fets (or the same ones, again if the leads are long enough) to the stumps. I've got away with this in the past.