WOW!
This is the clearest and down-to-earth explanation I have ever seen about this topic. Thank you very much for taking the time to write it.
Best regards,
Rigo
Principal Flight Hardware Technology Assurance Engineer
NASA, Goddard Space Flight Center
Workmanship Standards, Code 300
Phone. (301) 286-6129
Fax. (301) 286-6576
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Thursday, May 09, 2013 9:28 AM
To: [log in to unmask]
Subject: Re: [TN] Gold Removal
Good question, Bob.
Gold does not "melt" at soldering temperatures. A dissolution process takes place, and because gold plating on component leads is so thin, the dissolution is typically enough to absorb all of the gold into the solder bath upon contact with the molten solder. This is also how a solder joint is formed with copper and certain other metals. Copper, gold, silver, and nickel do not melt at normal solder temperatures, a small amount is dissolved to form the bond with the solder, called the intermetallic formation.
A double tinning method utilizes the first tin pot to dissolve and absorb the gold into the molten solder, and the second pot is required to ensure the remaining solder is relatively free of gold (the first pot has gold levels increasing over time as the tinning is performed, so the second pot is used to make sure no gold remains). The dual pot method is used where only a small amount of solder is used for tinning, and is thus more easily filled with unwanted "impurities" (other alloys and elements other than Sn63Pb37 or whatever the desired alloy is). The dynamic wave is simply another option for obtaining the same results, where a larger volume of solder is pumped to provide a laminar flow that will ensure the gold is removed and replaced with the target alloy, Sn63Pb37 as an example. Because of the larger solder volume in the dynamic wave method, the gold content is diluted such that it does not go above the limits listed in J-STD-006.
When using a double pot method, the gold content must be monitored in both pots. This is done with a regular pot analysis, and the data is used to determine the frequency of solder replenishment to dilute the gold and other alloys to an acceptable level. The solder test is relatively inexpensive and can be performed by solder companies such as Alpha, Kester, etc. With the laminar flow method, you still need to test at regular intervals, but because of the larger volume of solder it takes longer for the gold and other impurities to go above the specified limits.
Tinning data should be kept to determine the frequency of sampling for test, as well as how often the solder should be refreshed with new solder to dilute the impurities to an acceptable level. A simple tinning log is valuable in determining the history (number of components tinned, what type, and when) to quantify the amount of tinning over time in order to help determine the safe frequency for adding solder that will ensure the alloy will remain within spec, the spec being J-STD-006, including Appendix A and B, which list the allowable levels of "impurities" (it's hard to consider how gold and silver can be considered "impurities", but it's all relevant to the goal of Sn63Pb37 for leaded solder, and nothing else).
J-STD-006 also lists the test methods used to determine the impurities.
The concern with having gold levels above the limits in the J-Standard is that gold in the solder joint will nucleate through the hardened solder joint towards the junction of the component lead and the solder, and/or the pad and the solder (the intermetallic junction). Gold wants to return to this intermetallic boundary, like many other elements seeking equilibrium. So the gold molecules will actually travel through the solder joint over time (nucleation), leaving behind a more porous solder joint, which is called embrittlement, which leads to solder joint fractures and potentially catastrophic electrical failure.
I am not a metallurgist by training, so this explanation may be somewhat crude to those who are, but I think it will serve the purpose of explanation. For all of you metallurgists out there, feel free to expand on this or correct me.
Hope this helps you,
R. Dean Stadem
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert DeQuattro
Sent: Thursday, May 09, 2013 7:28 AM
To: [log in to unmask]
Subject: [TN] Gold Removal
Hello Fellow Technetter's,
I have a question on J-Std-001E 4.5.1 Gold removal which states a double tinning process or dynamic solder wave may be used. Can any of you describe this process. How is the gold actually removed? Or is it just being tinned? Gold melts at over 1000 C so I am a little confused.
Thanks,
Bob
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