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[Mystic_Merlin]

Hello there,

Today I found some SIMM RAM with 514400 and 814400.
I'd like to have some recommendation to unsolder those ICs. I read that too much heat will damage the chip and that it was better to force the IC out of the board with a screwdriver than overheating it...

I know Kaz is a master at soldering/desoldering those ICs, so if you're reading...

Also, it would be nice to make a listing of compatible ICs for respective copier/memory board. Here's a start for the 20-pin DIP memory chips found on Supercom/Pro fighter memory boards.

Toshiba TC: 514400
Fujitsu MB : 814400
Goldstar GM : 71C4400/71C17400
Hitachi HM : 514400
Hyundai HY : 514400
Micron MT : 4C4001
Mitsubishi M5M: 44400

PS: I recently noticed the Supercom Special / Special Partner are indeed upgradeable to 128M...now is it possible to load a 48M game...

[CrackLtd]

you need a hot air desoldering tool. (not cheap)

[kyuusaku]

They aren't DIP, they're SOJ (small outline J leads).

If you're gonna make your own boards you can also use 4M x 4 bit RAMs (24-pin SOJ).

[Mystic_Merlin]

[rodneyk]

Hi Mystic_Merlin,

A small soldering advice.

The only way to (in a normal way) desolder such chips is to apply enough heat to all it's pins at once, just long enough to loosen the chip. SMD chips are designed to be able to stand this heat for a short period. The shorter, the better of course. With professional tools it's a matter of heating all the pins evenly and quickly with enough airflow and heat from the hot air soldering tool while gently holding the chip with a pair of suitable tweezers. With something as small as a SIMM you can even lift the entire PCB up by the chip with a pair of tweezers. Just a little bit above the table surface. When the chip loosens, the PCB will drop to the table. The direction of hot air, myself I always point it from above, slightly inwards to the component, directed exactly at the pins where you want to heat their solder. It's the trick to move around in such a movement that all pins get heated properly but also evenly. So heat the pins long enough, but still keep moving until they are all hot. Then when the component loosens, lift it with the tweezers with which you are holding it and put it on the table to cool down, or if you use the lifting method, the small PCB will drop when it is loose.

If you don't have the appropriate tools I would suggest to think of a method that can do the same thing. Perhaps a hot air gun that is normally used to strip paint by heating it. If the gun can get hot enough it might do the trick. The heat might have more effect if you direct it through a smaller opening but be careful not to overheat the tool. I think such a tool might not be too expensive compared to SMD tools. It is important to find something that has an electrical heating element inside and is not operating by flame.

The screwdriver method, when I first read it I found that kind of crude. That's not to say it's not working though I would never do that myself with a memory chip. The mechanical stress also might damage something.

For example, I once cut off a small part of a resistor network because I didn't need all of it's resistors, which caused the other resistors near the cutting edge to decrease their resistance due to the mechanical stress. I solved this by cutting a bit further away from the last resistor part that I planned to use. My point is, I don't like stressing components mechanically myself.

Soldering smd chips back to a PCB is a matter of making sure the PCB pads are clean (in case another chip was removed from there previously), positioning the SMD chip in it's place carefully so it's exactly straight(important), then applying a small pressure from above with the tweezers to hold it in it's place and while holding it, heating all pins around with your hot air tool briefly but long enough to melt the solder and let the SMD chip stick to the PCB with the little solder it has left on the bottom of its pins(and often present on unused pads too for example). Let it cool down and the IC is fixed. Next it's best to apply a little bit(but enough because it evaporates too) of no-clean flux to the pins of the IC and using a needle tip soldering iron with 0.35 mm soldering wire to gently solder each pin making sure the solder flows to the pad. After each pin you soldered, clean the tip on a wet sponge so it's clean again and no solder on it. The flux helps preventing the solder from easily flowing two pins together by accident and also helps the solder to flow to the pad and pin. When resoldering ICs, make sure there is not any solder between pins before you position it. Mostly, a small wipe with a soldering tip is enough to remove the solder which is shorting pins. After soldering sometimes it's a good idea to use the hot air gun again to let the solder on all the soldered pins flow once more to make it more evenly distributed. After soldering it's a good idea to clean away the flux if there is any excess flux left. You can use a flux-off spray can or if not available a small cloth with some alcohol on it. If so, make sure to let everything dry and evaporate before testing. The hot air tool helps to dry stuff gently too. In the entire work always use heat with caution, enough, but not more than necessary.

[rodneyk]

Hi Mystic_Merlin,

For replacing or adding RAM chips I would advise anyone to check the type of RAM for "Fast Page" DRAM, or "EDO" DRAM type, to add or exchange with the same type. I am not sure if they can be exchanged with the other types or mixed, I would advise against that after reading their properties related to refresh cycles etc. Since SIMMs are readily available it makes sense to find SIMMS with the same type, either fastpage or edo, as the chips you are adding to or replacing.

Regarding access time, make sure the added or replaced chip has the same access time or lower access time value to make sure it can handle the speed. Access time is often written on chips with -6 -7 or -8 which in this case stands for 60, 70 and 80ns access times.

[kyuusaku]

All copier RAM is FPM because FPM became standard before copiers existed, not that they use it. EDO also requires an illegal condition so it won't conflict.

[Mystic_Merlin]

Thanks guys and thanks a lot rodneyk for such precision in your explanations. It is very valuable information. I sure understand it's way more technical to solder/unsolder SOJs. But hey, we're here to learn new stuff
Actually I trained myself with the screwdriver trick on some useless SIMM boards and the result was...mmm...inconsistent?...It mostly depends how well these ICs are soldered, some would go easy depending on the quality of the soldering/pcb.
Well, I sure wouldn't try that one too often and I'll practice the "official method" if I come across the proper tools one of these days. I watched some interesting vdos on youtube btw...like the oven trick

[rodneyk]

Hi Mystic_Merlin,

Yeah I think it's important to be clear about technical things. Not everyone can be a technical expert and sharing knowledge and experience to help others are some of the important things that forums are all about after all. The interest is there so I think it's useful. If the information is unclear or incomplete, it is also less useful.

I would encourage enthousiasts to try to learn SMD soldering, it looks more difficult than it really is. A steady hand is most important so don't lift any heavy objects just before doing it. If you observe the right procedures the chance of success is big. I would advise during first attempts to solder back chips using a powerful TL light source around the PCB and using a magnifying glass on a stand to see every detail clearly. Then you can get a feeling for how the soldering tin flows. It would be good to practise on an empty simm PCB to solder one or two chips back onto it to get a feeling for it.

Also, I would advise to use lead solder and not silver solder because lead solder is much easier to use because of it's lower melting point. Afterwards just wash your hands when you're done. The exposure to lead through evaporation is so small that it can be neglected as long as common sense is used. For professionals who solder all day long it's a different matter of course.

I see about the "oven trick". I can imagine how they would do that.
With that method it's also a matter of determining the ideal exposure time in the oven to effectively loosen all chips at once. I suppose when heating the oven properly first, then inserting the PCB and determining the ideal minimum time would be one method to consider. With that method it's also a matter of exposing as briefly as possible to avoid damaging the chips. Though I believe the exposure time will be longer in case of an oven compared to a directed hot air tool. I suppose it would need to be absolutely maximum temperature that the oven can make otherwise it might take too long to heat the PCB. I wouldn't recommend it unless you have no alternative.

The cheap hot air tool I mean is something like in the picture. It's temperature is 300-500 degrees / 1600 Watt power and it has a over-temperature safety protection so you can even experiment with different home made nozzles to get the heat more directed and concentrated. It seems to me more gentle to the components than an oven. The tool in the example costs 35 euro new. A used one should be even much cheaper.

Since I moved from one workshop to another recently with my work, I now don't have access to my own hot air and desoldering iron anymore. I am thinking of buying my own soldering set because I recently saw a relatively cheap set on the internet:
http://www.eleshop.nl/aoyue-2702-4in1-station-p-186.html
(I don't mean to advertise, I just want to illustrate that it need not cost thousands of euro's anymore)

I came across this website because I am on a mailing list of Elector magazine who promoted this equipment a while ago. I like it because the desoldering tool which comes with this set is something I used before which turned out to be very useful in that it's large heating element can keep it's heat properly during the desoldering action which is useful to desolder pins which have large ground planes around them. It's the same tool as the brand "Hakko". The hot air tool of that one I have never used before but I suppose with a suitable nozzle it should work fine. Otherwise I don't have any experience with this brand myself.