Aging solder joins of power transistors

[djk]

I have been having failures of copperweld leads, both on film caps and resistors (solder joints look fine, no connection when measured with ohm meter).

jneutron, do you have any idea of a reasonable service life to expect for these kind of parts? What type of replacement schedule should I work out for these things?

[audiofan]

I never heard about the problem with gold , how do you solder gold plated connectors ?

[jneutron]

Originally posted by djk
"Sigh. I see a lot of explanations like that."

Maybe you should contact NASA, that solder joint was created for that photo in their workmanship standards manual.

If that is the level of their understanding, then there is very little I can do to help them. Anybody who states that a solder joint which shows the frosted or dull finish is a result of overheating has little if any experience with soldering....experience such as actually sitting in a production line and actually performing the soldering.

I have developed production procedures in a mil environment, for every soldering process known to man (with the exception of fluxless submerged ultrasonic soldering), and the solder materials I have personally used is basically the indalloy catalog, from 60C melts to 395C. The biggest hurdle I've had to face in all that time, is to develop the visual workmanship criteria AND teach the reliability people and the inspectors what it is they are looking at. Because, most of the alloys I've used CANNOT form a shiny surface finish when it solidifies, regardless of the solder process parameters. Hydrogen belt furnace, vacuum DAP sealer, focused infrared, vapor phase....all of these techniques, regardless of operator expertise, are unable to produce a shiny 50/50 lead indium surface..this is just one alloy as an example.

BTW, many times the explanation is inaccurate, but the end result is the same..this is one such case. If the joint looks like that and a eutectic lead tin is being used, something is wrong..the inspector doesn't necessarily need to know the technical details to be able to kick it out. Note: While it's great to have inspectors who will kick out rejects based on appearance alone, it does make it harder when the good solder joints happen to look like the example being used for a bad one. You end up trying to explain to them why what you called a reject in the past, suddenly is no longer a reject? Then you end up trying to explain eutectic vs non, phase diagrams, flux modification of surface energy....things they do not have the background to understand...deer in the headlight..

Man, been there, done that. When you find an inspector who can understand that stuff, gold.....gold I tell ya...keep em, promote em..pay em well..

Originally posted by djk
"The second photo shows cracking like I see on amplifier TO-3 outputs, but without the dull crystalized look. The holes on the relay board look oversized, hard for me to tell exactly what went wrong without a closer look. Relay is brass.

I agree, without the part in hand and under a 30x stereoscope, it's difficult to tell exactly.

Originally posted by djk
"
"any suggestions for reactions other than mailing them to you?"

Tin them, wick them dry, and then use 4% silver solder. The tin will still migrate and fracture, but the silver seems to raise the fracture strength of the joint.

Agreed with your procedure. Tin silver IS stronger, not just seems to be..

Originally posted by djk
"
"never had a gold wired component cause trouble"

You're lucky.

Agreed. Murphy at work.

Originally posted by djk
"
Had a repair tech at work that refused to follow my instructions, had to replace $200 pin diodes after only 30 hours of thermal cycling. The solder joints looked perfect, even under 10X magnification. Absolutely no continuity.

Nature of the beast. Until it slaps em in the face, they don't understand why the instructions are what they are.. This kind of data point is useful to the good techs, they will learn to trust the person giving the instructions.

Cheers, John

[jneutron]

Originally posted by djk
I have been having failures of copperweld leads, both on film caps and resistors (solder joints look fine, no connection when measured with ohm meter).

jneutron, do you have any idea of a reasonable service life to expect for these kind of parts? What type of replacement schedule should I work out for these things?

Sounds weird. Any idea what the underlying plating is?? What is the gold finish?

Sometimes platers will use 10 microinches of nickel as a flash prior to the gold. It acts as a diffusion barrier to the gold. Perhaps the parts are aged, and the underlying nickel has oxidized. Initial soldering may be alloying to the gold with an underlying oxide barrier remaining intact. The oxide layer being so thin, is not visible, and the gold overlaying may be supporting the meniscus and hiding the lack of a metallurgical bond to the nickel.

Same can happen with gold over copper I guess, but I've not used that as it is not good for 6 month shelf solderability nor steam age tests.

If this is the case, do a dip using R flux in a 60/40 pot set to 250C. Remove the lead very slowly, allowing surface tension to pull the soldercoat very thin. Then, inspect for de-wet. If the gold is too thin allowing oxygen to diffuse to the flash, you'll see it here.

If that's the case, you need a two step dip process. First dip is to clear the gold. Second is for refurbishing the de-wet areas. That will need an RA flux....DO NOT DO THIS FOR STRANDED LEADS IN A MIL ENVIRO, WICKING WILL KILL YA.

Repeat the aggresive flux dip until the dewets go away.

This should work for both matte gold and shiny gold, as the RA will clear the organics from the shiny levelers..

Service life is difficult to predict, as it's not clear what the underlying problem is. My guess is an underlying oxide layer being hidden by the gold/solder meniscus.

Originally posted by audiofan
I never heard about the problem with gold , how do you solder gold plated connectors ?

Honestly, I've never had a problem with soldering to gold plated connectors. I use them exclusively for all my audio needs, and have never experienced a failure due to embrittlement (that I am aware of). I think that because of the cost of gold, most vendors go thin, 30 to 50 microinches tops. Much thicker, and the user may not adequately clear the gold during soldering.

For internal use, amps and such, I'd worry about the places where heat is involved.

Cheers, John