Pre-heating your motherboard.

[Neo2_000]

Hi y'All.

I have just bought a new temperature controlled soldering station.

When de-soldering PCB joints positioned in large copper lands, I find it takes considerable time (6 to 8 seconds) to get the solder to melt because of the considerable heat drain, even at high temperatures.

So I am going to experiment with pre-heating the motherboard using home appliances like my gas oven or a hot air hair-dryer.

Has anyone tried this before or can anyone offer advice from personal experience?

Must be someone else out there bent enough to try this before!

[Topcat]

Re: Pre-heating your motherboard.

Don't do that. Simply reflow some new solder into the joint, that'll melt it fast.

[willawake]

Re: Pre-heating your motherboard.

i dont like either idea, with an oven it would be difficult to control the exact temperature and also if your oven is anything like mine the temperature is not constant all over the oven and is excessive in some places. plus you would be cooking some components which are sensitive to certain temperatures. bad idea.

like i said before, the hairdryer probably would not be hot enough. paint stripper heat gun would be good and there are references on the net to people doing smt work with them. i assume it would be best to heat up the rear of the board using it. still dangerous though unless you know how much heat you are applying.

KC8 was talking about pre-heating one time.

[kc8adu]

Re: Pre-heating your motherboard.

using a heat gun in moderation is fine.
just watch out for plastic hsf clips like used on chipsets.
i have done this on 6-8 layer boards where a really hot iron threatened to lift traces before the solder would melt completely down the hole so the desoldring machine could suck it out.
if you severely overdo it you can have smt stuff lying on your bench afterwards!

[Neo2_000]

Re: Pre-heating your motherboard.

Thanks to all for helping with advice.

I bought the iron from Computronics in Perth.

I spoke to a rep from Computronics today and explained to him the difficulties I was experiencing: Even after applying heat for considerable time with the iron on a difficult joint, the cap will often emerge with a solder cast of the joint attached to the lead. I find this very worrying and this is what he advised.

He reckons that I should only be running the Hakko at 350 degrees C.

We discussed the optional "high thermal mass soldering tips" which provide more stable heat delivery and I ordered two different styles.

He also offered to sell me a professional pre-heating unit for A$450 which I graciously declined. He said that they work on heat convection supplying a temeprature of 100 to 150 degrees C and take about 2 to 3 minutes to heat up a board.

Based on this I have tested my free 1000W hair dryer. It will heat up a temperature probe to 100 degrees C in 30 seconds. Another 30 seconds will produce 130 degrees C which is approaching it's limits. This is also within the suggested range recommended by kc8adu using his heatgun method so I will give it a try.

Looking at the gas oven, it's minimum setting is 120 degrees C. If I were to preheat the oven to this, then turn it off and let it stablize before placing the motherboard inside for a conservative minute or two, I am thinking that this would be a reasonable replica of the professional heating unit. But maybe the griller would be better since it would not supply heat to the component side of the board. I can just hear the laughter as you are reading this!

I will experiment with some old discard boards before committing to any one method but admittedly the hair-dryer would be more convenient if it works.

Topcat: Don't do that. Simply reflow some new solder into the joint, that'll melt it fast.

I will also give this a try. For the sake of comparison, I would be interested to know what soldering station are you using TC and it's power rating.

Dickin around with all this funky stuff - I feel just like the guys on Myth-busters lol - assuming that you guys get that show on your TV.

Feel free to offer any comments, criticisms, sarcassams, ridicule, referrals, etc. etc.

[willawake]

Re: Pre-heating your motherboard.

the rep is spot on. nichicon recommend 350oC+/-10oC for 3 second.

and that would be recommended for installing new caps. the old ones you dont care about damaging. for removal, there is more concern about dwelling too long and damaging traces so fast in and fast out at high temp.

i hope you have more success with your new tips. i feel bad that you are not happy with your new station. I think you will have more success adding solder to the joints before you remove the caps. personally i cant work without doing that.

your hairdryer is unexpectedly hot, tell us how it works. the oven is still a BAD idea....

[Neo2_000]

Re: Pre-heating your motherboard.

Thanks Willawake, talks a bit about pre-heating (caps?) there too.

Pre-heationg : Less than 150°C, 90 seconds max.

Don't get me wrong, it's not that I am unhappy with the new iron. I realise that I just have to work out the correct proceedures to use it properly. On regular joints, it works great. It's just the ones in the large copper lands that present a problem at this stage.

In contrast to my old Scope Iron, I feel that the motherboard is very safe when using my new iron but your point about dwelling too long on a joint and the associated risk of damaging the bord is my main concern. The resulting solder cast on the extracted cap lead after six to eight seconds on a difficult joint at 450 degrees worries me. So my aim is to reduce the dwell time and improve the solder flow.

I think you will have more success adding solder to the joints before you remove the caps. personally i cant work without doing that.

In regards to applying solder to the joint. Are you just applying it to the tip of the iron before use or do you actually apply it to the joint jsut as you would when soldering a new component in place?

Using this technique, how long would you typically dwell on de-soldering a large landmass joint and what would you consider to be the maximum safe dwell time at 450 degrees C?

Sorry for sounding like a rank novice but this is just what I am with this new iron.

[willawake]

Re: Pre-heating your motherboard.

you should always apply a small amount of solder to the tip of the iron and then wipe it off on the sponge (or flick off the excess) before starting. the procedure is called tinning the iron. i am sure you know that anyway

you heat up the joint and then add a dab of new solder directly to the existing solder on the joint.

Using this technique, how long would you typically dwell on de-soldering a large landmass joint and what would you consider to be the maximum safe dwell time at 450 degrees C?

still figuring out that one myself.