solder to gold or copper ??

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arficus

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Joined 2009
#1
Some CMC bakelite tube sockets just arrived for an ST70 rebuild. Gold plated copper pins. I've read abot "issues" with soldering to gold, gold migrating, etc... Would it be advisable to sand off the gold plating (down to the copper) in the areas to be soldered?
Thanks.
 
#4
KatieandDad said:
Gold doesn't migrate, it's Silver that migrates.

http://www.ami.ac.uk/courses/topics/0158_emgr/
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FYI the tin in solder will dissolve gold really well to form a very brittle AuSn alloy. It is a well known and documented issue called gold embrittlement. Dramatic temperature gradients or changes can actually cause parts to fall right off the board due to gold embrittlement. This is the reason that true high end parts, like used in RF, will have gold plating on the contact surfaces that need to be low resistance but the soldering surfaces will not. Even the boards often have good plating on exposed parts but solder pads are gold free. If they aren't gold free they will usually strip the gold from the pads before soldering. It doesn't take much gold, the accepted value is 3%, though some studies have seen issues at as low as1-2%, to make the joint dramatically more prone to failure.

I was reading about this the other day and came across this link:
http://www.semlab.com/goldembrittlementofsolderjoints.pdf

Case History #1
A failure analysis effort in September 2003 [5] raised some interesting questions regarding the relationship of gold embrittlement to voiding is SMT solder joints. Thevclient reported that SMT components were literally falling off of the PCBA. Examination of the fracture surfaces in the SEM (Fig. 3) revealed that the joints were heavily voided and that the voids contained some entrapped soldering flux. Closer examination revealed that there were thin Au-Sn intermetallic platelets widely distributed over the fracture surface and in the surfaces of the voids. Most of the Au-Sn IMC in these solder joints appeared to be in a very thin (~ 0.1-0.2 microns) plate morphology, which could be expected to exaggerate the embrittlement effect for a given amount of gold compared with coarser plates or needle morphologies. It was hypothesized that the numerous thin Au-Sn IMC plates trapped solder paste volatiles during reflow resulting in the voided condition of the solder joints. It was unclear what specific conditions lead to the observed morphology of the IMC in these joints. Calculation of the gold concentration in these joints based on the 33 microinches measured on unassembled PWBs gave 1.65 wt%Au, which is significantly lower than the 3 wt% threshold for gold embrittlement. This suggests that there may be embrittlement and other process issues for lower concentrations of gold if the IMC segregates to certain areas of the solder joint.
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#5
You need to distinguish between "flash" gold and thicker gold coatings.

Flash gold is only of cosmetic value. It is too thin to make troubles during soldering.
If the thickness is not stated or the parts are not very expensive the gold coating is flash gold.

Thicker gold coatings (2-5 um) are used for plug contacts. It has nothing to do with the conductability but with the chemical resistance against oxidations.

Even this coatings are not pure gold, as pure gold is too soft.
Cheaper contact choatings are usually silver coated. This has the advantage of a lower resistance but may make troubles in sulphur containing atmosphere.

You can order PCB with (flash) gold coatings. They have a very flat face which may be important for SMD soldering.

Cheers,
Udo
 
#7
I think from my reading that it is the ratio of gold to solder that determines whether there is a risk of gold embrittlement in the soldered joint.

Lots of solder very effectively reduces the gold proportion provided the gold is thin to start with.
Gold flash is, I believe, a very thin coating.

On the other hand, the gold on plated RCA sockets is probably in the "thick" category.
I solder to these regularly and have not had a case of joints falling off, if one gets the barrel hot enough for the solder to flow.
Maybe it's the extra heat that removes the embrittlement risk?

The barrel of the RCA is the best place for the Return Wire of the twisted signal connection.
The barrel is also the best place for the RF attenuator at the input to receiving equipment.
I stopped using the solder tag, that comes with RCA sockets, decades ago because of high loop area in the signal wiring.

Is that the way to spell embrittlement? It not in my spellchecker!
 
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#8
Most other common electronic materials are not susceptible to migration: iron, nickel, and tin because of their low solubility in water;
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This has been reported as a main factor in MLC failure due to tin and silver migration.
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can you resolve this apparent contradiction with respect to tin?

Tin is a problem and yet the first quote from Katie&Dad's link seems to be saying that tin is not sufficiently soluble in water.
 
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#15
Probably SN 62% Pb 36% Ag 1.5% Cu 0.5% or something very near...If you look for a phase diagram for solders with these four metals there are only certain ratios that are eutectic. To much copper would make the joints brittle, and its only real function is to limit copper dissolution into the copper.
 
#17
It is a nice solder to solder with, it flows well and protects your solder tip if it is copper. Many years ago when there was a lot manual soldering in production departments we used SAVBIT (tin/lead/copper eutectic) for the above reasons. It will also be good for soldering very fine copper wires as there will be minimum or zero copper dissolution.
 
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