TECHNET Archives

May 2013

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Thu, 9 May 2013 09:13:07 -0500
Content-Type:
text/plain
Parts/Attachments:
text/plain (128 lines)
Hi Dean - you may not be a metallurgist but you must have stayed at a 
Holiday Inn recently. Very very good process details. Let me change your 
last paragraph just a bit.

Gold and tin will form a intermetallic compound (IMC) - AuSn4 - when the 
solder joint composition contains 3-5 weight % gold. This IMC is very 
brittle and will crack during vibration, drop shock or thermal cycling 
causing the solder joint to degrade and fail (not a porosity issue). The 
good thing is that gold diffuses into solder at a rate of 100 microinches 
per second so when the procedures Dean listed are followed, there is very 
little chance of having a gold embrittlement issue. The process goal is to 
not allow the gold content in the solder to exceed the 3-5% range.   The 
IPC JSTD 001 Handbook and the IPC AJ 820A Handbook have a very good 
section on this issue.

Dave



From:   "Stadem, Richard D." <[log in to unmask]>
To:     <[log in to unmask]>
Date:   05/09/2013 08:30 AM
Subject:        Re: [TN] Gold Removal
Sent by:        TechNet <[log in to unmask]>



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

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________




______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________

ATOM RSS1 RSS2