Gold plated contacts soldered to printed circuit boards 1

[trippj]

Good Morning:
I am inquiring about any metallurgical or otherwise reactions that may take place as a result of using tin-lead solder to fix gold plated contact leads to printed circuit boards?

 

[kenvlach]

There are too many variables (chiefly, solder and substrate compositions) to give a simple answer. The gold is usually very thin and only serves to protect the substrate alloy from oxidation prior to soldering. During soldering, the gold dissolves and the liquid solder bonds to and alloys with the substrate metal. The NIST has a lot of info on solder system phase diagrams, useful for checking for solid solubilities and intermetallic formation:

http://www.metallurgy.nist.gov/phase/solder/solder.html

One problem is intermetallic compound (IMC) formation between tin of the solder and nickel. When electroless nickel (Ni + 1-12% P alloy) (EN is used commonly as a barrier layer beneath the gold), is soldered nickel diffuses into the solder, forming an NiSn IMC, and leaving a P-enriched, weakened interface. A good reference (has photos) for this reaction (although gold was not used in this example) is

http://www.alphametals.com/products/solder_solutions/pdf/P-Ni Case Study.pdf

Another problem is whisker growth. A photo of eutectic composition Pb-Sn whiskers grown from a semiconductor-heat sink soldered connection.

http://nepp.nasa.gov/whisker/photos/pom/2003dec.htm

The solder manufacturers Alpha Metals and Kester have technical info on their websites:

http://www.alphametals.com/products/solder_solutions/tech_art.html

http://kester.com/en-us/technical/knowledgebase.aspx

AMP (Tyco Electronics) also has technical information on electronics connections:

http://www.amp.com/products/technology/tech_articles.stm

There are many other metallurgical issues, such as thermal fatigue, void formation from the Kirkendall effect, and whisker growth, although most work is on lead-free solders. E.g., see the June 2003 issue of JOM, etc.

http://doc.tms.org/servlet/ProductCatalog?container=JOM+2003+June

http://nepp.nasa.gov/whisker

http://www.calce.umd.edu/lead-free/tin-whiskers/TINWHISKERMITIGATION.pdf

 

[trippj]

Thanks kenvlach, for your response. We will be changing to tinned contacts in place of gold.

Regards,
Jim