Author Topic: Dry bones  (Read 1029 times)

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Offline PerranOakTopic starter

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Dry bones
« on: August 29, 2019, 09:44:34 am »
I was soldering a board yesterday and everything was going fine. Then, for no reason I could see, one joint would not solder! All the joints before it were OK as were all the joints after it.

It was a manufactured PCB and the component was a through-hole T092 tranny - this happened on only one lead - with quality multicore solder and a Hakko FX-888D.

The solder simply "climbed-up" the lead, away from the pad as if allergic to it! Neither flux nor masses of solder helped.  |O

Eventually, it worked after trying time and time again.

Why on Earth does this happen and what should one do about it?
You can release yourself but the only way to go is down!
RJD
 

Offline taydin

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Re: Dry bones
« Reply #1 on: August 29, 2019, 09:51:45 am »
Must have been covered with some kind of oxidation. Those will be really hard to remove with heat, because oxides have a very high melting point. There are acid based fluxes for this type of situation, but those do cause corrosion themselves if the copper traces are exposed.
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Offline amyk

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Re: Dry bones
« Reply #2 on: August 29, 2019, 11:46:13 am »
Was the pad connected to a large ground plane or similar?
 

Offline PerranOakTopic starter

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Re: Dry bones
« Reply #3 on: August 29, 2019, 12:33:06 pm »
taydin: ah, I see.

amyk: no but the lead was quite long.
You can release yourself but the only way to go is down!
RJD
 

Offline MosherIV

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Re: Dry bones
« Reply #4 on: August 29, 2019, 03:22:28 pm »
Hi. I agree with amyk. Most likely a pad with high thermal mass.
You mentioned you eventually did it.
It just takes a long time to get enough heat into the pad to get the solder to melt onto it.
Even the best brand irons will take time.
My personel preference is for Metcal irons, they handle high thermal mass the best, easily out perform other brands.
Just look up the Metcal penny trick. Not many irons can solder together pennies or 2 nuts!
 

Offline PerranOakTopic starter

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Re: Dry bones
« Reply #5 on: August 29, 2019, 04:32:09 pm »
They sound excellent.
There is no tendency to damage components with them though is there?
You can release yourself but the only way to go is down!
RJD
 

Offline MosherIV

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Re: Dry bones
« Reply #6 on: August 29, 2019, 05:51:39 pm »
Yes, it is possible to damage components by over heating them.
Do not worry to much about it, you are probably ok.
 
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Offline Brumby

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Re: Dry bones
« Reply #7 on: August 30, 2019, 01:55:05 am »
In addition to oxides, you may have had other contamination, such as oils from fingerprints or any one of a bucket of other things.

If these are not cleaned off before soldering, you will have a hard time and overheating components - and tracks/pads - becomes a much greater risk.

Some can be cleaned with IPA while others may require other means - including physical scraping.  But if you do scrape, be careful to only attack the contaminants.  Be kind to the pads!
« Last Edit: August 30, 2019, 01:57:08 am by Brumby »
 
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