Recapping a NeXT Cube

[powellb]

I need your experienced advice.

Some background: I have recapped about half a dozen Macintosh logic boards, never with a single issue. I don't have a desoldering gun, but I will typically just use flux and solder wick to draw out the solder from under the SMD electrolytics. Always has been fine.

I started working on my NeXT Cube that I have had for 30+ years. Although I don't see any sign of leakage from the SMD caps, I figured it is time to replace with tantalum. I set my iron to 330C added a little flux gel, grabbed the wick, and added heat. As I did these alternately to each side, the traces and pads around the capacitor lifted right from the board.

I used silver conductive paste to glue the pads and a tantalum cap to the pcb as well as hold the traces down. The board is working fine and I tested everything around that cap, but I stopped there. I don't want to go through this with every cap.

Any suggestions on how to do the rest of the caps? I have never seen traces (let alone pads) just separate from the board like that. I know a pad could separate with force, but this was not (IMO) excessive heat. A heat gun would probably be worse since you can't concentrate on one spot, I would think many more traces will lift off (also, some of the caps are in REALLY tight spaces with chips and traces all around them).

Anyone have experience with NeXT boards? Is there something about their manufacture that traces act like stickers on the PCB?

Thanks!

 

[Byrd]

I would try to pre warm the board, more heat (350 - 370), flux then reflow pads with fresh solder. Are you doing all the caps (quick Google says there are about 7 SMT caps and 3-4 through hole caps)?

 

[KennyPowers]

I recapped a non-turbo 040 NeXT Cube board with tantalum polymers and didn't have any trouble with pads or traces lifting. I removed the old SMD caps by applying flux to the pads and lifting them off with these cheap hot tweezers tinned and set to 350c. Yes, a few of them are in tight spots...

 

[powellb]

I would try to pre warm the board, more heat (350 - 370), flux then reflow pads with fresh solder. Are you doing all the caps (quick Google says there are about 7 SMT caps and 3-4 through hole caps)?

Thanks! I guess, it is the more heat that worries me as everything just lifted from 330C heat (though it could be counter-intuitive to me). I think reflowing is a great idea. I plan to redo all of the caps, but the first went so poorly, I've stopped there. The through holes should be straight forward, but I also thought the surface mounted would be straight forward...

 

[powellb]

I recapped a non-turbo 040 NeXT Cube board with tantalum polymers and didn't have any trouble with pads or traces lifting. I removed the old SMD caps by applying flux to the pads and lifting them off with these cheap hot tweezers tinned and set to 350c. Yes, a few of them are in tight spots...

Thanks! I'll take a look at the tweezers.

 

[zigzagjoe]

I definitely wouldn't try to hotair them off. Tweezers work well if you have enough clearance to deploy them. Personally, I just snip the capacitors off (cutting inline with the leads). Usually no issues with lifted pads as in my experience this applies the least stress to the pads.

 

[pbertolo]

My personal technique for SMD: firmly grab the capsule with needle nose pliers, push hard downward and then - at a the same time - twist the capsule by 90 deg or more. The connectors will break by shear and the capsule will come off. You will then remove the plastic base and easily desolder the remains of the connectors from the pads. I did many, many boards (including Cubes) over the years, without any setback.

 

[powellb]

Thanks all, I have ordered some soldering tweezers and will try again sometime soon.

Regarding snipping or twisting them off, I will likely resort to that for any troublesome caps that don't want to lift with the tweezers.

 

[powellb]

Sorry for the delay, but I want to update everyone. I wasn't able to get to this all summer, but today I cleared a couple of hours and was able to replace the SMD electrolytics without any further issue. I guess that first time was the anomaly. I ended up getting some solder tweezers, and I used flux very liberally. Turned out I didn't have proper replacement radial caps, so I ordered some high-quality (I actually have a bunch of cheap caps that had the right specs for those, but I want quality ones for this) to replace those next week.

I quickly plugged it back up, and it went to the monitor ROM, so all is good!

Thanks again for your suggestions.