Ever hear crunchy noises when reflowing cold joints on older hardware?

[Metatronic_Mods]

I discovered quite a few cold solder joints on a vintage synth I was working on earlier. Upon heating the cold joints with my desoldering gun I consistently heard a crunchy scraping noise, and noticed the solder melted unevenly, and seemed to contain distinct grains of a higher melting metal that would take a second or two longer to melt. I did wonder if the uneven, light amber glaze might be a degraded conformal coating of some kind, but I didn't smell anything too "off" and the appearance isn't unlike rosin flux residue.

Has anyone ever had this experience? Any idea what's going on? This board was produced in Japan in either '81 or '82. Anybody know what kind of soldering methods and alloys or coatings, if any, may have been used? Doesn't really matter for the repair, just curious about an unusual (for me) thing.

[Ampera]

Could be pure tin, which afaik has a higher melting point than tin/lead alloy solder (quick google search agrees, 370F for 60/40, 449.5F for pure Sn) . The crunching could be a "tin cry", the sound of the tin crystals deforming (same thing happens if you bend a piece of pure tin), likely caused by thermal stresses/expansion. That's my theory anyways.

Try turning your gun's heat up a bit, if they're big joins that might also help/not help matters (depending on if you like the sounds or not).

[T3sl4co1l]

Yeah, could be something like that.  Or intermetallics, which may imply a copper-rich alloy (worse than SAC305 I guess?).  Or something just really freaking dirty and the grains came out on melting.

Or the cold joints are the result of stress and corrosion and maybe electromigration too, and the crusty stuff is oxides.  Intermetallics can be exaggerated under electromigration, too.

What was the nature of the node -- power, signal, mechanical, some combination?  (Mechanical as in, it's supporting a fairly big component, so, some lead stress involved.)

Tim

[electromotive]

I’ve seen a distinct crystalline structure form in lead-free plumbing solder, which sometimes won’t reflow even with the heat of a torch.

[Alex Eisenhut]

The PCB is how many layers? Gold plated? Any leaky batteries?

[Metatronic_Mods]

What was the nature of the node -- power, signal, mechanical, some combination?  (Mechanical as in, it's supporting a fairly big component, so, some lead stress involved.)

Mechanical for the most part. Larg-ish pins on board mount SP3T's and pots. The rest were mostly on board to board ribbon connections.

wish I still had my material science textbook    it's been a while.. is tin one of the metals that exhibits grain growth in the solid phase over time? what about in the presence of alloying metals?

[Metatronic_Mods]

The PCB is how many layers? Gold plated? Any leaky batteries?

2 layer. No gold plating.  There was a NiCad (I think that's right) for the ROM which definitely crapped its guts all over the logic board, the control board in question is located somewhat near and above the logic board. Are these batteries known to projectile vomit ever?

[TERRA Operative]

NiCd's are bad for puking everywhere when they get old

[vtwin@cox.net]

the only cold joints i hear cracking are my own.

[vk6zgo]

During both the "High Reliability Hand Soldering" course I attended back in the day, we were expressly warned against re flowing faulty joints.
The rule was:- Desolder & remake the joints,

I know that re flowing is fairly common with SMD boards, but it was definitely regarded as a "no-no" for "through hole"components, probably due to the less than ideal contact area available with such devices.

[tautech]

During both the "High Reliability Hand Soldering" course I attended back in the day, we were expressly warned against re flowing faulty joints.
The rule was:- Desolder & remake the joints,

I know that re flowing is fairly common with SMD boards, but it was definitely regarded as a "no-no" for "through hole"components, probably due to the less than ideal contact area available with such devices.

What was the reasoning behind that ? Curious and 

[Siwastaja]

During both the "High Reliability Hand Soldering" course I attended back in the day, we were expressly warned against re flowing faulty joints.
The rule was:- Desolder & remake the joints,

I know that re flowing is fairly common with SMD boards, but it was definitely regarded as a "no-no" for "through hole"components, probably due to the less than ideal contact area available with such devices.

This is why reflowing an entire SMD board with soldering issues in it typically either doesn't fix the problem at all, or fixes it for a very limited time, until it fails again. It's done when you have nothing to lose, or your time has no value.

[CJay]

During both the "High Reliability Hand Soldering" course I attended back in the day, we were expressly warned against re flowing faulty joints.
The rule was:- Desolder & remake the joints,

I know that re flowing is fairly common with SMD boards, but it was definitely regarded as a "no-no" for "through hole"components, probably due to the less than ideal contact area available with such devices.

What was the reasoning behind that ? Curious and 

My understanding of it was that dry joints could contain all sorts of contaminants due to arcing, corrosion, metal leaching etc. so removing the old solder is necessary to ensure the quality of the joint.

It also looks a lot prettier

[GlennSprigg]

During both the "High Reliability Hand Soldering" course I attended back in the day, we were expressly warned against re flowing faulty joints.
The rule was:- Desolder & remake the joints,

I know that re flowing is fairly common with SMD boards, but it was definitely regarded as a "no-no" for "through hole"components, probably due to the less than ideal contact area available with such devices.

Most things are good, in a perfect world. And BOY am I not perfect !!   
Sometimes, we are limited to what we have at hand. (I don't have specialized 're-flow' equipment).
Visible Surface-Mount chip pins are one thing. When there are hundreds of solder-pads underneath
a complex chip, then my only option is/was Archaic methods!! haha...

6-8 years ago, I, (and numerous others globally!), there was a common fault with the Video Chip in
HP Laptops. I 'repaired' probably 15 to 20 of them, using the kitchen oven.
Stripped down to bare main PCB, I placed them on top of Socket-Set 'Sockets, with other heavy metal
'sockets' sitting on top of the problematic 'chips', and baked in the oven for 15 mins.
ALL of them came back to life.... 

[vk6zgo]

During both the "High Reliability Hand Soldering" course I attended back in the day, we were expressly warned against re flowing faulty joints.
The rule was:- Desolder & remake the joints,

I know that re flowing is fairly common with SMD boards, but it was definitely regarded as a "no-no" for "through hole"components, probably due to the less than ideal contact area available with such devices.

What was the reasoning behind that ? Curious and 

My understanding of it was that dry joints could contain all sorts of contaminants due to arcing, corrosion, metal leaching etc. so removing the old solder is necessary to ensure the quality of the joint.

It also looks a lot prettier

Exactly!
Also, there would normally be no flux available from the original joint.

Nice clean component leads & pads, plus flux, make for a good joint, & the whole process doesn't take any longer than farting around re flowing.

[Siwastaja]

During both the "High Reliability Hand Soldering" course I attended back in the day, we were expressly warned against re flowing faulty joints.
The rule was:- Desolder & remake the joints,

I know that re flowing is fairly common with SMD boards, but it was definitely regarded as a "no-no" for "through hole"components, probably due to the less than ideal contact area available with such devices.

What was the reasoning behind that ? Curious and 

If you reflow the old connection, you have no flux at all, and no wetting to the bad joints; and because they are likely to be oxidized, you would need extra flux if anything.

If you add a little bit of new solder (or bare flux), you'll do better, but it's likely it's not enough for really reliable process: it's still less flux than normal, in a situation requiring possibly more than normal.

By removing all the solder and applying completely new one, you are guaranteed to apply flux in the right concentration with the new solder. In addition, you can inspect and clean the surfaces, which could be quite valuable because there might be a reason why the original joint failed.

[Metatronic_Mods]

During both the "High Reliability Hand Soldering" course I attended back in the day, we were expressly warned against re flowing faulty joints.
The rule was:- Desolder & remake the joints,

I know that re flowing is fairly common with SMD boards, but it was definitely regarded as a "no-no" for "through hole"components, probably due to the less than ideal contact area available with such devices.

I've always heard "reflow" used interchangeably with melt    But I did use a desoldering tool.

[GreyWoolfe]

the only cold joints i hear cracking are my own.

No kidding!! I am right there with you.

[Housedad]

Yep, every morning when I stretch.

[T3sl4co1l]

Yeah, it's the crusty stuff that you can't just melt away.  Rough surfaces from mechanical fatigue or electromigration or sparking have lots of oxides in the joint.  For sure, you can't just melt it, but melting it with some gooey flux applied first, isn't necessarily going to get rid of the crusty stuff floating within the solder joint itself.  So the joint can be weaker (and lower current capacity) than a properly made one.

It of course helps to have PTH (plated through holes), where solder fills the space between pin and hole.  Single layer NPTH boards (and sometimes two layer NPTH boards, where vias are made with rivets -- Sony used these a lot) have problems a lot more often.

For hi-rel purposes, you want to clean out and replace the old solder.  For, say, recovering an Xbox, it's not worth spending more time than to just flux and heat.

Tim