Poor solder joints

[SonicSteve]

Hi guys,
I've been meaning to ask this for some time now and just haven't.

Why do some solder joints not bond well. Often I will replace 5-8 caps at a time. Some joints are harder to solder though. It seems that the solder doesn't fill the hole on these ones. With a little wiggling I can literally cause the joint to come loose.
I then have to pull the capacitor off clean the hole and keep trying till I see the solder fill the hole.
Does anyone know why some of the joints do this?

[willawake]

Re: Poor solder joints

what iron are you using, what temperature
what solder

[SonicSteve]

Re: Poor solder joints

what iron are you using, what temperature
what solder

I'm using a weller wlc100 with the spg40 with a chisle tip. The iron base is set to slightly more than 4 out of 5. So I don't know exactly what temp it is.

The solder is light duty rosin core. 60/40 .8mm or .032"

Should anything like rubbing alcohol or water be used to help clean the yellowish residue?

[Rainbow]

Re: Poor solder joints

The solder joint is bad usually because of low temperature. Both parts that needed to be soldered together must be hot enough.

[willawake]

Re: Poor solder joints

the iron should be ok for soldering. i was thinking if it is defective but if you can remove caps easily then it must be hot enough.

i would suggest to try a different solder. otherwise you will have to modify your technique.

[dood]

Re: Poor solder joints

I use that same iron, and on most joints I leave it right between 3 and 4. However, if you have a joint that's located on a large grounding plane, the ground will soak up a lot of the heat fast, so you need to bump up the temp and work faster to get a good joint.

[SonicSteve]

Re: Poor solder joints

Here is my technique,

1. remove bad cap (using a desoldering iron with pump)
2. clean the holes out with a pin
3. scrap off any yellow residue from old solder joints
4. Clean with Rubbing alcohol
5. replace caps
a. put solder on iron tip, heat the capacitor lead with iron and solder.
b. apply solder to joint area till hole fills.

Any tips?

[willawake]

Re: Poor solder joints

a. put solder on iron tip, heat the capacitor lead with iron and solder.
b. apply solder to joint area till hole fills.

no you must heat up both the lead and the pad around the hole with the iron and touch the solder to the pad.

basically if you dont heat both, the flux will form on the cold part and the solder will form around the flux.

[SonicSteve]

Re: Poor solder joints

no you must heat up both the lead and the pad around the hole with the iron and touch the solder to the pad.

basically if you dont heat both, the flux will form on the cold part and the solder will form around the flux.

I do try to heat up the pad, I'm just a bit paranoid as to damaging the board by leaving the hot iron too long.

[willawake]

Re: Poor solder joints

its very doubtful the board would get damaged while soldering. more likely to get damaged while removing caps.

i guess its 3-5 seconds to make a nice joint.

[SonicSteve]

Re: Poor solder joints

Thanks,
I'll give it a go with that advice. The flux explanation does explain a few things. I guess the pad just wasn't quite hot enough. The iron I know is plenty hot, it will melt the solder on contact without delay.

Is there a definite rule as to how long is too long to leave the iron touching the boards pad?