old solder joints: redo them or leave??

[jaunty]

I'm trying to do some maintenance and repair on a 1970s digital delay unit that's quite valuable and members on another forum were sternly warning me against trying to re-do solder joints on the daughter cards which mostly contain 7400 logic and RAM chips ... to ME the joints look old and decrepit and need some re-doing as they are VERY grey bumpy and curdled looking (even wrinkly in some places) and flow like cold solder when heated (you know ... they kind of break up instead of melting ... so i figured maybe there was some chemical or electrolytic action that resulted in some kind of deterioration of the joints over time and I'd hoped i could improve general reliability by reflowing/redoing them.

the people on the other forum who warned me not to touch them suggested they SHOULD look like this and did since new ... though i find that kind of hard to believe ... what say you?

[alsetalokin4017]

Well, some of those do look OK to me and some (maybe just light and angle) don't.

What was the reason given for _not_ reflowing?

I think if you know what you're doing, have a good ESD-safe iron, use the right temperature, the right solder and the right flux ... and remember to turn off the equipment and discharge the big capacitors     ... what could go wrong?

[james_s]

Personally I would try touching up at least the worse looking ones. If the board is double sided with plated through holes though then cracked solder joints are rarely an issue. Single sided they're much more likely to cause problems.

[jaunty]

alsetalokin4017


oh i had no idea that being 'ESD safe' was even an option with irons - i thought a wrist grounding strap was the only way to go - i only have a humble Weller WES51 soldering station - hmmmm... research time i guess - thanks for the info

i think - as for the 'advice' - it's just an astoundingly complicated/scary machine to work on with many many points of failure and no-longer made custom logic etc ... so perhaps that's why ... though i was hoping maybe because there were SO MANY solder joints that maybe - just maybe - this was something that people often overlooked and might bring some benefit

[jaunty]

ok thanks James ... though i AM curious what happens to the metallurgy (composition) of solder over the years ... and if impurities etc migrate though to create 'cold' joints over time

[james_s]

ESD safe iron is one that has an earth grounded tip, if it has a grounded power cord then it's virtually certain to be ESD safe. If in doubt, check with an ohm meter.

I don't know what happens from a metallurgic standpoint, I think the solder can oxidize though. I know I've encountered old crusty looking solder that is reluctant to melt and flow. It's even worse if an electrolytic capacitor leaks on it.

[jaunty]

yes these certainly have been very reluctant to melt and when they do - the impurities on the surface break up like islands or wrinkle etc ... it struck me that 'old' solder like this was very likely to be much more brittle and prone to mechanical stresses ... seems to me though that over constant usage many metals can experience a form of elctrolysis (like in high school physics experiments doing electroplating or making batteries etc) - and impurities can migrate through the solder joint - perhaps even the tin separates to a degree from the lead - but that's only wild speculation

[Bashstreet]

The old saying... if it is not broken do not fix it 
Now some of these joints do look suspect but it might be the lighting/angle.

I would leave them alone unless there is confirmed cold joint(s) if there are then proceed normally and redo them.
What ever you decide best of luck 

[jaunty]

The old saying... if it is not broken do not fix it 
Now some of these joints do look suspect but it might be the lighting/angle.

I would leave them alone unless there is confirmed cold joint(s) if there are then proceed normally and redo them.
What ever you decide best of luck 

thanks - i always thought that any 'grey coloured' joint esp with a funny texture like that was necessarily 'cold'  - and that the metal had somehow degraded and lost conductivity and strength over time (?)

[Gregg]

I remember fixing a number of Radio Shack electronic items by merely re-soldering every joint. It takes expensive equipment and a lot of time to tell what the connection integrity may be under a crappy solder joint. 
If there is something specifically susceptible to heat on your board, it would be wise to not re-solder it until it becomes necessary but for the rest I would do them one at a time in different areas to minimize heating the board.  Use a known good active flux, add good solder to dilute the bad, let cool while you do the same to another area of the board, come back and wick or suck up the old solder / new solder combination, let cool again and finally solder with good 63/37 solder; give the board s good cleaning with IPA.

[Bashstreet]

The old saying... if it is not broken do not fix it 
Now some of these joints do look suspect but it might be the lighting/angle.

I would leave them alone unless there is confirmed cold joint(s) if there are then proceed normally and redo them.
What ever you decide best of luck 

thanks - i always thought that any 'grey coloured' joint esp with a funny texture like that was necessarily 'cold'  - and that the metal had somehow degraded and lost conductivity and strength over time (?)

The joints in my opinion do not portray age/heat type degradation rather it appears these joints were overheated during the original soldering.
If the joints have survived this long there is not much point to redo them unless there is a observable fault(s) (ie pin separation)

Do all the joints on the board appear similar?

[Mr. Scram]

It sounds like you plan on fixing it until it's broken. If it works, leave it alone unless you have good reason to suspect issues will arise soon.

[T3sl4co1l]

ok thanks James ... though i AM curious what happens to the metallurgy (composition) of solder over the years ... and if impurities etc migrate though to create 'cold' joints over time

AFAIK, the culprits are
1. Cyclic stress causing fatigue and cracking
2. Corrosion

If these are old enough to be leaded solder, it's somewhat less susceptible to fatigue, but exhibits creep, in which case static stress is a problem, too.

Also, is this a 1-layer board, or more?  PTH?

Single side solder, no PTH, is very weak: the solder fillet bears the entire component lead strain, with little or no support from the hole.  It's very common to see cold solder joints on such equipment.

While there is an intermetallic layer (Cu-Sn species) on the surface of leads and pads, it is usually thin and strong, and not changing over time.  Thickness of the layer depends on time*temp during soldering, which must be kept low for this reason, but -- much more significantly -- also because everything else involved (PCB and component resins) breaks down quickly at those temperatures.

A graphic illustration of intermetallics might be disassembling copper water piping, if you've ever had to do that.  The joints are usually pretty tightly fitting to begin with, filled with lead-free solder (tin with a few percent antimony), and heated with a hand torch.  Disassembly can be tricky.  The interface layer is sticky and squeaky in texture.  If you aren't pulling and twisting straight on axis, it may not move at all, and it's easy to miss the melting point and keep on heating the joint.  Which grows the interface layer, only making the joint tighter and tighter.  An overheated joint can be effectively diffusion-brazed together and actually impossible to disassemble!

Again, this isn't significant in normal PCB assemblies, even with very fine pitch components.

Gold plating can be significant, but that's a matter for another thread

Tim

[tkamiya]

It looks worse because of magnification.  If I were to work on that, I'd use my hot air gun and reheat all joints.  It worked effectively for me when wide area needs to be touched up.

[jaunty]

Also, is this a 1-layer board, or more?  PTH?

Single side solder, no PTH, is very weak: the solder fillet bears the entire component lead strain, with little or no support from the hole.  It's very common to see cold solder joints on such equipment.

it's just your garden variety 70s era double sided through hole board - single layer

[jaunty]

It looks worse because of magnification.  If I were to work on that, I'd use my hot air gun and reheat all joints.  It worked effectively for me when wide area needs to be touched up.

to my mind it actually looks BETTER in the photo (far shinier joints - much less dark grey) ... than through the microscope it was taken through (via camera lens too - so there's lots of fog and flare) thanks

[tkamiya]

Well.....  ok.  I guess that's a matter of opinion.

At minimum, the middle round on very top need to be re-wetted.  Solder around wire is no longer adhered to the wire.  Now, if that's the only one like that, then I would just fix what is broken.  But if it is wide spread, then I would use (my trusty) air rework gun to reheat joints.