Gold thickness on connectors is currently a topic of much interest in the industry. How much is enough? What does gold do for the interconnect? The answers depend on the application and resulting technical need.
To de-mystify the topic of gold thickness, it helps to separate the scientific from the speculative elements. First of all, gold is a noble metal; it is an excellent barrier to oxide surface degradation (in the proper thickness), and is second only to silver for conductivity properties. It can be easily formed and is quite ductile.
Because gold is expensive, it’s important to stay within a limited range of thicknesses. The standard for military-grade thickness is 50 millionths of an inch. The minimum gold thickness is something in the range of 4 to 7 millionths of an inch, referred to as flash gold. At flash-gold thicknesses, the cost of the metal itself is overwhelmed by the cost of processing. Relatively speaking, fused tin/lead (tin and lead electroplated and then reflowed) costs roughly the same as flash gold over nickel. While excellent for soldering, flash gold is not thick enough to be a valid wear surface, as in the case of a mated pin and socket.
A workhorse thickness for most mated contact surfaces is 30 millionths of an inch. Industry expert Max Peel of Contact Research (Attleboro, MA) has tested more interconnect problems in his lifetime than any of us will ever see, and he believes that any gold thickness less than 30 millionths of an inch over a barrier layer is problematic.
The importance of a nickel underplate cannot be emphasized enough. Nickel is an important migration or barrier layer where gold is used. Left without a barrier layer, copper will seek oxygen by migrating or “piping” thru the gold to the surface, where it can “bloom” over the gold surface in time and create a surface metal that is easily attacked by oxygen (corroded, as in copper oxide--a long term effect that takes about a year). This is partly due to the relatively course grain structure of a gold deposit. Nickel, on the other hand, is very tightly grained. A deposit of 200 millionths of an inch (a workhorse average for nickel) of this mechanical structure provides a barrier that effectively prevents the copper from piping through to the oxide-rich surface. Many nickel platers put “levelers” into the bath (for a variety of reasons), and these chemistry agents give the nickel a smooth surface finish. When you admire a gold-plated part for it’s reflectiveness, you are actually being impressed by the properties of the nickel.
Could you use solid gold over copper? Yes. In fact, some applications require an absence of nickel and, for those, the thickness of the gold needs to be 100 millionths of an inch or more to effectively block copper piping. Still, due to the relatively high cost of the metal itself, it is far better to use a nickel barrier layer when gold is the final metal surface, which is why that construction is standard practice in the connector business.
While thin or flash gold is a wonderful answer for highly solderable surfaces, thick gold is not. While a small amount of gold in a solder joint makes it superior to the same solder joint without gold, a high amount of gold in a solder joint can cause brittle solder and possible failure, something like a “cold” solder joint.
Lost knowledge
As always in manufacturing, these choices are trade-offs that should be commonly understood. However, what used to be “common knowledge” is not so common anymore. One example is the recent Restriction of Hazardous Substances (RoHS) directive, effectively banning lead from solder joints. The public telephone network infrastructure industry, populated with equipment from companies like Lucent and Nortel, spent two decades identifying the source of and solving the tin whisker issue in their networks. Now RoHS comes along and suddenly the very thing which brought about the solution (lead in the solder joint) is being withdrawn from the solder joint. This untenable situation is what led network service providers to ask for and receive an exemption from RoHS compliance.
That’s where industry associations come in. The connector manufacturing industry has a rich legacy of excellence in component engineering and fact-finding for interconnect solutions. Two major connector companies in America, Amphenol and AMP, at one time had business models that maintained a strong technology-training program for all their sales and support employees. Not long ago, it was de rigueur for the players in our industry to have a solid foundation in interconnect design and fabrication practices-like this discussion on gold-as part of our early careers. Business models evolve (or regress), mergers/acquisitions happen, and change occurs. Today, there is a void in the teaching of the technology “basics” of interconnect engineering. One way for industry leaders to take action is involvement with in industry associations. The International Institute of Connector and Interconnection Technology (IICIT), previously known as the Electronic Connector Study Group, is stepping up to the plate and working to make a difference by educating, training, and facilitating industry discourse. Members and board members of IICIT actively champion the cause of training in our industry to prevent repeating the mistakes of the past in our search for the solutions of tomorrow.
Join us by visiting the IICIT website at www.iicit.org and signing up as a corporate or individual member. Get involved! Help us teach and learn so that issues such as gold-plating thickness recommendations are fully matched to the application need, not the pocketbook or the whim of the plating service.
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DALE REED is the Chairman of the Bylaws Committee for the International Institute of Connector and Interconnect Technology (IICIT), and vice president of sales and marketing at Trompeter, 2555 Townsgate Rd., Suite 300, Westlake Village, CA 91361. He can be reached at Tel: (805) 551-0530; Email: dale.reed@trompeter.com.
