Why desolder the old?

[Fizzycapola]

Re: Why desolder the old?

Why cant I just heat up the solder pull the capasitor out and reheat the solder and put in the new capasitor..

You can do it that way provided you keep the leads full length to avoid
transmitting heat into the cap body and you pull the leads through (with
long nose pliers with breaks for each few millimetres achieved to allow
built up heat to dissipate - pushing the leads through didn't work for me -
it just bent the leads alot).

Once the solder is molten it's reached it's thermal capacity and any more heating will force the heat to start flowing into sensitive parts of the board at
a component killing 450°C and the capacitor body destroying them through heat damage instantly.

As long as you don't cause heat damage to the board or capacitor there
shouldn't be any problems. Of course Troda's advice bypasses these risks.

The other technique of just ripping off the capacitors and soldering new ones
onto the existing leads, even I won't condone that though, leads must be
as short as possible for maximum performance. Although I admit
I've been having thoughts of placing component sockets on the board so I
can simply slide out and slide in different capacitors on the fly, to see which capacitors give the best performance.

All said, I don't suggest you listen to me over Troda's, he's a electronics
professional and I can't read circuit diagrams designed for children without
suffering brain failure.

[Topcat]

Re: Why desolder the old?

All said, I don't suggest you listen to me over Troda's, he's a electronics
professional and I can't read circuit diagrams designed for children without
suffering brain failure.

lol.... I don't know if I'd go that far with Trodas.... afterall, he was installing caps backward.....

[Spacedye69]

Re: Why desolder the old?

From my experience with my underpowered 40w iron, I add new solder before I try to pull the cap out, and if the hole does not clear after 2 tries with my sucker, I add more solder and the hole comes clean. A little time consuming, but it works great untill I buy a real solder station.

[gonzo0815]

Re: Why desolder the old?

I mostly push the new caps with short leads through the uncleaned holes. With most modern boards, cleaning the holes is a waste of time for me, unless i spend some money to get a propper soldering station. I usually only ned two or tree times of heating to get it in.So far i have no problems. Of course, i am carefull to not overheat the caps.

[Fizzycapola]

Re: Why desolder the old?

lol.... I don't know if I'd go that far with Trodas.... afterall, he was installing caps backward.....

OMG how did that happen ... LOL I meant to say Topcat in my post. Sorry. I must have GSC caps inside my head or something not the first time on this forum either ... I think my brain is pushed too hard on this forum

[trodas]

Re: Why desolder the old?

Topcat -

lol.... I don't know if I'd go that far with Trodas.... afterall, -he was installing caps backward...

LOL Yea, that's true, once I did that. But you do it too, so, what's your point?

Anyway, I do agree that cleaning at least one hole is matadory. Then I do shortening a little the cap's wires - if need - so the wire that go into the cleaned hole is longer. Now just allign the shorter wire so it go to the still not clean hole, hold the cap and slightly press and heat the hole up from other side, till the cap pass thru to position. Make sure it is straight and cut the remaining wires and the solder the rest.

Becuase it is usualy very hard to clean some holes in mobo, that are on big cooper layer areas on the PCB. So in that case I just clean only one to save time

Maybe someone shall do a picture guide of the job

[Topcat]

Re: Why desolder the old?

Topcat -

LOL Yea, that's true, once I did that. But you do it too, so, what's your point?

I'd be lying if I said I never did it.... lol

[Spacedye69]

Re: Why desolder the old?

I have never...LOOSERS!!!! Lots of ASUS boards too.

[grand_amatic]

Re: Why desolder the old?

Well I did the bad thing!! I have ordered some capacitors from chris and I havent gotten them yet. I had some important work to do on my computer (TWL Match) and I had to get my board running until I get the new caps. I took 2 6.3 2200's out of a spare 350watt psu and soldered them in place of the junk 6.3 3300's. I didnt clean the holes just heated them up and pulled out the old, heated again and slid in the used ones. Motherboard works great and now we are 23 in TWL 2vs2. If anyone plays Americas Army you can hang with us at www.nawclan.org
When i get my new caps I will replace the used ones but I needed a quick fix and it worked great. I also monitored my voltages and they all seemed stable while the game app was open. Thanks for everything guys and Ill be looking on here for help from now on. Keith aka CPT. EagLeTaLoN

[Topcat]

Re: Why desolder the old?

If they're overdue, PM me your name and the date you ordered, and I'll get the ship date and tracking number for you.

*edit*

Go check your mailbox, they arrived the day you posted that...

[pentium]

Re: Why desolder the old?

- a hot enough soldering iron

He is being serious. Even I have a problem.
We recommend something at least 35 watts. The source by shorted city sells lots of the weaker irons. Anything below 35 watts will hae lots-o-trouble desoldering parts.

[AKBessy]

Re: Why desolder the old?

willawake, thanks for those web pages on the capacitor lab - good learning for us unlearned people. :-) Are those your hands in those pictures in the capacitor lab? If so, your hands are too clean! LOL

[Tom41]

Re: Why desolder the old?

If I have to replace a component by soldering (last thing I did was a DVD remote control, where the IR-LED had failed - soldered in a new one), what I do is heat the solder until it becomes molten and pull the old leads out. I then allow the solder to cool down, at least 1 minute after it's become solid again. Depending on the amount of solder still in the holes, I either leave it at that or put a little more solder into where the holes are before I start the process of putting the replacement component in.

I tend to put the new lead on the solder pad and heat the solder until it becomes molten, constantly pushing on the lead. The moment the solder melts, the lead will go through the hole and I can then remove the soldering iron before the heat spreads and damages anything else on the board. Like I said above, I leave at least 1 minute after it's become solid before applying the iron again for other leads.

After soldering, it's a good idea to make sure you don't have 'dry' or loose joints. Basically if the component wobbles a lot when you try to move it, there is insufficient solder and you should add some more.

I've never had anything on the board damaged due to heat from my fairly weak soldering iron - my hands, on the other hand, still bear scars! Having said that though, the original solder from the factory solder bath is usually fairly 'hard' and needs the iron to be held on for at least 10 seconds before it melts. I've never had to replace capacitors either, but that may be about to change as I attempt my first recap (the P3 machine with busted Tayeh caps)!

[AKBessy]

Re: Why desolder the old?

Tom41, how "weak" of an iron are you talking about? I am tempted to try using my 40 watt - but wasn't sure what I will be looking at for time that the iron has to sit on the original solder before it starts warming up. 10 sec., 20 sec., more time?

I guess that the most important thing for me to do since I have never soldered on a board before is to find a board of some sort so that I can damage it if I need too ;-).

[bryanhunt]

Re: Why desolder the old?

I am usually generouse with new solder when recapping. Sometimes i use some SnAg leadfree, but even then i usually ad some 0,5mm leaded solder to. Especially for desoldering i usually ad some of the 0,5mm leaded solder, to allow easy melting of the solder. On most boards i can`t get the old solder to melt untill i add some fresh ones with new flux.

I know this is an old post but I just wanted to make sure people had a word of caution when it comes to mixing solder formulations.

Solders are typically bi-metalic compounds that have been rigorously tested to meet desired properties. When you introduce other elements into the mix those properties change. The major one we are concerned about is the eutectic temperature. This is the point when the solder as an aqueous solution begins to cool it will solidify almost universally. Adding another element to the mix will alter that point or likely remove it all together.

In the above example with tin-silver and tin-lead solders the typical mixing problem manafests itself with the tin and silver having a greater affinity for each other than the lead. Lead also being denser will migrate to the lower portion of the joint while the tin-silver mixture floats on top. As the joint cools the tin-silver will typically reach its eutectic point first (having a higher concentration of tin than it should) leaving the lead below still molten. When the lead eventually cools it of course contracts and creates a void below the tin-silver outer layer. Besides creating a weaker physical joint there is the possibly the void may lay in the current path in which case it will reduce the cross-sectional area which of course ups the resistance of the joint and heat created when current is flowing through it. Later on reworking these boards becomes an issue as you have to use a hotter iron than is typical for either tin-silver or tin-lead.

Up until about 2002 you could almost rest assured the board you were working with would be a standard leaded board so you should be fine reworking with normal solder. When I mean normal solder I mean electronic solders, avoid anything with an acidic flux (normally found at auto parts and home supply stores), use only no-clean or fluxless (harder to work with because you'll have to apply flux seperately, then wash). If you are unsure what solder was used on the boad do your best to remove as much as the old solder as possible and use whatever solder you are most comfortable with.

If you're interested in some lead-contaminated-joint micrographs:



Edit: spelling

[gonzo0815]

Re: Why desolder the old?

Interesting.
It would be interesting too, how this is reflected in real world applications (i mean big solder joints at diskrete components with enough solder, as usuall in repair service).
I can understand, that if you do SMD soldering or you are using the usuall thin solderlayer (for economicall reasons...) that this will cause bad solderjoints.
But if the solderjoints are adequat, i think this should not be a big problem.

I know many profesionall techs, wich do some repair on Grundig TV sets.
Grundig have had a lead free soldering prozess implemented for a long time, but with very bad result.
So most of the guys and me too, do resolder the whole pcb or at least all important parts with leaded solder, and usually if this is made carefully once, there are no more problems of this kind.
I duno wich alloy Grundig was using, but AFAIK it was pure tin, wich would explain, why there is no sutch problem.
Any way, if you do repair something, you usually are generouse with solder.

If i buy a new soldering station some day, then may be i switch completely over to an leadfree alloy, but the SnAg4 is a little expensive for the usual old junk TV set`s (well, CRT will probably die very soon).
The older SnPbAg HF alloy solered with cooper tip is from that point of view not a good choice too.

[bryanhunt]

Re: Why desolder the old?

In pretty much all respects nothing comes close to the ideal properties of the old sn/pb alloy so its not surprising most of the early adopters have had problems (we switched over pretty much on the last day we could). This even includes the new lead free alloys and processes (first off they stress the electrical components more due to the heat required in the ovens).

Typically join contamination causes problems the worse in areas of higher mechanical or thermal stress. This means most often a through-hole component will be the first to exhibit signs of failing followed by some very heavy SMD devices such as large tantalums. Your run of the mill surface mount parts like 805, 603, 402 should be fine.

Basically if you're going to use lead just get rid of all the sn/ag junk. Its not worth the hassle. From an enviromental standpoint silver is just as much of a problem as lead so it would seem like the switch is purely political.

(I develop ICT and FFT systems for class 3 automotive electronics)