Tinning gold plated leads ( DIP, etc)

John: What�s the matter? Free gold � good thing for you, bad thing for your supplier. :^)

Pre-tinning of gold plated leads is important to: � Prevent gold embrittlement of the solder joint � Determine if the leads are solderable prior to assembly (i.e. gold can be plated over oxidized nickel). SMTnet Archives are rift with horror stories about corroded NI in ENIG pad coatings.

We've seen several instances of gold embrittlement associated with soldering to gold plated leads and gold thick film pads. The volume fraction of brittle AuSn4 intermetallics tend to be high due to the 4:1 ratio of Sn to Au in the intermetallic compound. Check into "Solder Joint Reliability," John Lau, Van Nostrand Reinhold, pp. 205 - 212.

As for the amount of gold that you can live with, take a look at section 5.4 in the J-STD-001 Handbook - it does a nice job of covering the details of gold removal and why.

J-STD-001 5.4.1, Gold Removal: � Prescribes the removal of all gold from components and solderable surfaces prior to assembly. � Specifies that gold has to be removed from 95% of the to-be-soldered surface. � Endorses either a double dip (two solder pots) immersion or the use of a flowing solder wave as proven methods for removal of the gold plating and replacing it with tin/lead. � Promotes the development of documented data showing that your method of removing the gold is sufficient for allowing reliable assembly operations.

Although, not a requirement, consider developing a PM plan for this gold dipping pot to demonstrate the control of the solder purity over time and use. � Do NOT pre-tin gold leads in your solder pot or other pot used for routine lead tinning. � Rather than developing an elaborate PM plan for this gold dipping pot, consider dumping the pot in your reclaim barrel on a reasonable schedule.

Consider sacrificing a couple of components and do a destructive physical analysis to measure the thickness of the gold plating. � If plating on the components is less than 50 microinches, solder some connections without tinning of the components. Then perform a DPA of connections soldered without gold removal to determine the quantity of gold in the solder connection. � Leaded components where the leads have some compliance and greater than 3% gold in the connection will cause you problems in any environment, unless the Tce (CTE) of the components and PWB are closely matched (less than 8 ppm/�C differential). � If the gold content is less than 3%, leads have some degree of compliance, and your processes are tightly controlled; you should talk to your customer about discontinuing the (dual or single) tinning process. � If the gold is greater than 50 microinches on the (as received) leads, then you probably need to continue tinning the components.

Some lead tinning service suppliers are: � XL Addenda 708.971.8843fax8846 Gene Binkowski � Six Sigma 1940 Concourse Dr San Jose, CA 95131 (408) 526-1350 Fax (408) 943-0447 sales@solderquik.com � Corfin Industries 7B Raymond Ave, Unit 7 Salem, NH 03079 603-893-9900 fax 6800 www.corfin.net admin@corfin.net Thomas Hamel � Chip Pro 10390 E. Lakeview Dr #206 Scottsdale, AZ 85258 480-860-5790 Fax 8366 Processing Facility 1610 No. Freeway 35E #214 Carrollton, TX 75006 972-242-9455 fax 8497 www.chipro.com

My2� Dave F

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