RAM Desoldering Tips

[rich_r]

I am having some trouble in desoldering RAM chips from Spectrum pcbs. I often end up pulling at least one track from the board when the chip comes out. I have trashed one pcb using a manual solder sucker, so I invested in a temperature controlled vaccum gun type desolder tool.

I have had no problems desoldering larger chips from Spectrum boards (eg Z80, ROM, PCF1306P). But RAM chips seems to prove tricky for me. It's like the tracks and through hole plating aren't fixed to the pcb very well around that area, or the pins are mechanically jammed into the plating.

My best success seems to be if I set the desolder gun to 350C, apply a bit of flux and remove most of the solder from the pins. Then resolder them with fresh (lead) solder and suck them again to get the last drops of solder off. But then the chip is still solidly attached, even though I can see daylight round most of the pins.
So then I push the pins with a soldering iron as best I can into the centre of the holes and into the board a little. Obviously I'm not daft enough to try levering chips out with a screwdriver (at least with more than very gentle pressure), but it just seems that when it does come free, one or two legs have the through hole plating stuck to the leg and the track lifts off the board.

I've been soldering and desoldering things for 40 years and nothing has ever given me so much trouble.

So - any tips? Have I got my desolder gun too hot? If I try at 300C, then it doesn't always melt the solder quickly and I worry I'm having to hold it on the pin too long. But I'm willing to try anything to save destroying my Spectrum any more than it currently is.

[Namtip]

Do you want to keep the chips?

If not, clip the chip off of its legs first, then tweezer out individual legs while holding the iron against fresh solder. Then suck the solder out of the PTH.

[dfzx]

You're doing everything right. My conclusion from my own attempts to get those ICs out is that sometimes, quite often in fact, it's just not possible. A desoldering gun is more convenient than a solder sucker, but it's not a magic weapon.

The problem as I see it is that when the IC was inserted, its legs were bent slightly inward or outward from the perfect 90deg to the PCB. The chip was then pushed in and wiggled into the socket with a decent shove of someone's thumb. It's exactly as you say: the pins are mechanically jammed into the plating. Handily, at the the time, that meant there was a definite interference fit, IC legs springing laterally against the inside of PCB holes, and the board could be turned over and soldered without the IC falling out.

40 years later, that interference is still there, and the IC isn't going to be inclined to simply fall out when you remove the solder. (I've seen other ICs in other boards do exactly that.) Actually, it's going to be very difficult to remove all the solder. There only needs to be speck between the leg and the inside of the hole, and you won't be able to get the IC out without undue force. Flooding the joint with new solder in order to make an airtight seal for the vacuum pump to suck against is a good start, but getting absolutely all the solder out is sometimes just not possible, and when you can't the IC won't budge.

You can sometimes work out what pins are sticking. Do just as you are doing, and when you've got as much of the solder out as you can, for each pin press the tip of the iron against the end of the leg and ensure it moves. Then try again with tweezers. If there's a speck of solder holding the leg in place it'll move when the hot iron is against it, but not when it's cold. That should allow you to work out which pins are still stuck with solder. If there's only one or two, that information might help.

The one thing you don't mention is hot air. I've had a bit of success with that approach. Use hot air from the underside of the board, keeping the blower moving to prevent burning the board. You need to be a bit patient, allowing the two lines of the socket to heat right up. It might take a minute or more. The issue remains that when the solder is molten on all the legs, you still need to apply just enough force to the IC to overcome that interference fit and prise it out without getting frustrated and yanking it.

Given the relative values of the PCB and the DRAM chips, frustrating as it may be, destroying the IC is always the better way to go. Retroleum have "new old stock" for £1.85 a chip.

[1024MAK]

Yeah, why are you trying to save knackered DRAM chips?

I strongly recommend cutting all the pins on the DRAM chips, then removing each pin individually.

BTW, I set my iron to 340°C and my desoldering station to 380°C.

Nearly all new DIL/DIP chips are manufactured with splayed out legs. If a tool is used to insert them into the board, the legs will only be "aligned" enough so that they can go into the holes on the board. As soon as the tool is removed, the legs will in most cases, hold the chip in place.

As others have said, it only takes a spot of solder to hold a pin, leg or lead to the through-hole plating. As said above, try wiggling the leg forwards and backwards with a screwdriver, if it won't wiggle, it's likely stuck. So then try moving it with the tip of your iron.

Oh, and BTW, none of these problems with removing chips are new. This has always been the case with double sided or multilayer boards with plated through holes.

Mark

[roganjosh]

Also, if you do cut the legs of the chips, then don't be tempted to try removing the remaining bits of the pins from the PCB using your vacuum desoldering gun. It's entirely possible they'll get stuck in the gun and necessitate buying a new gun unit.

[oO cozy Oo]

I`ve had success using my de-solder gun with adding flux, then I use a solder iron on the pins and then slowly wobble the pins when they are hot, as mentioned before the pins are bent most of the time, and I`ve used solder wick on the topside by the pins.

[rich_r]

Many thanks everyone, that's given me some encouragement.

I don't know if these chips are actually faulty, hence I'm trying to save them. Diagnostic board implies a fault in lower RAM on random fill, but doesn't give any clues as to which bit. No real clues from the display pixels either.

But I agree - the value of a few 4116s is far lower than the value of the pcb. So I think I might cut the pins if any don't come free easily. One they're all out I'll solder in sockets (and repair the couple of tracks I've pulled off with thin wire on the bottom of the board). I do have a few spare 4116s so can afford to lose some. I will give the pins a wiggle with my soldering iron to see if they move (rather than just bend) before attempting to put any upward pressure on them. Might try the gun a little hotter too.

It's so annoying that I've had no issues on the larger chips on this board, however maybe they're not pushed in so firmly due to having more pins so grip more easily on insertion. The PCF1306P on this board for example did just drop out once the solder was sucked out with just a little finger pressure to loosen it. Also I guess a lot of what I've been desoldering over the years have been capacitors which have round pins, so not so likely to mechanically bind into the holes.

I will get this Spectrum working again, one way or another

[rich_r]

I had another go this morning. Desolder gun temperature increased to 380C. Wiggled the gun a lot more whilst sucking the solder to move the leg about. Resoldered with fresh solder, then wiggled and sucked that again. Then with a soldering iron (set at 330C), gently pushed each leg left and right, up and down in each hole until it felt loose. Then a slight pull on the chip with my fingernails (not levering with a screwdriver) to see if it moved a little. Then push gently on each pin with the soldering iron until they all sink through the board. At that point I could easily pull the chip free with no damage So just three tracks to repair on IC13 and I think I'm now able to fit sockets and install my spare set of DRAM chips.

[winston]

Hot air is my go-to method for desoldering chips (works as well on old boards full of PTH chips as it does for newer boards with SMD).

Clamp the board vertically, gently warm the solder side in the region of the chip(s) you want to remove, then as the solder starts to melt ease the chip(s) off from the other side with tweezers. Then go around and clean up the through holes with solder wick. This method has never failed me.