Jack
The construction Inge refers to is a high reliability specification. Rhodium provides a very hard, wear resistant substrate. The soft gold provides a lubricant and an oxidation shield. Your specification is a compromise. The increased Knoop hardness is achieved by coplating another metal with the gold The hardening metal is incorporated in the gold bath. If memory serves, usually by adding nickel sulfate or cobalt sulfate to the bath. The 99.7% gold usually contains a small amount of cobalt sulfate.
I'm on the road and don't have access to reference materials, but I believe that the 99.7% gold is a separate plating class from the 93.7% gold per a MIL specification. The high thickness and increased hardness provide switch life. If a softer and/or thinner gold is used one should expect the switch life to decrease. The amount of change in life depends on the types of considerations listed by Inge along with the departure from the specification. The board should also specify a thick deposit of nickel under the gold. Otherwise the pressure from the switch contacts will distort the contact surface and shorten the switch life.
Consider that gold contact fingers are designed for a "few" insertions and extractions through the life of the board (maybe 50 or 100 cycles). A switch is usually rated in the thousands of make and break cycles. Your board fabricator may have to consider locating a subcontractor to perform the nickel and gold plating operations. The search might start by contacting the plating chemisty supplier to determine if they have any customers that purchase the 93.7% gold chemistry.
Don Vischulis
-----Original Message-----
>From: Inge <[log in to unmask]>
>Sent: Apr 14, 2010 5:19 PM
>To: [log in to unmask]
>Subject: Re: [TN] Question for "Old-Timers" about Gold
>
>Jack,
>like you say, it's more to it....gold thickness is not the one and only
>parameter. Other factors are contact asperity, contact geometry, contact
>area vs. current, wipe pressure, switch slide coefficient, underlying
>plating, life expectancy and at least half a dozen more parameters. I was
>specialist on electromechanical and manual switches decades ago, don't
>remember all details, but I can tell you this: if you count on gold
>thickness only, you don't know what you are shipping. 93.7% Au? We did not
>name that Gold at all. We talked about maximum 0.1% impurities. The more
>impurities, the higher the friction coefficient. Well, I don't know what
>level of quality you speak for. I was in the HiRel, so maybe I'm too
>demanding. Anyway, My advice (which you will probably frown at) is
>basic metal ?? your wiper
>300 uinches of pure Nickel
>25 uinches of Rhodium over Nickel
>5 uinches of soft gold (gold works as 'lubricant')
>
>The above based on a qualified switch mechanism. If that is not the case,
>not even 160 uinches of gold will work for a very long life. 160 uinches!
>Never heard of such a lot of gold. When extrem current switching is actual,
>one use to work with mercury switches, microswitches or like.
>
>Don't listen to me, I'm from an era when we dismantled switches and relays
>and measured all contact parameters and stored the test data for minimum 10
>years. And all our own test equipments were sent to a qualfied reference
>lab. Madness?
>
>Inge
>
>--------------------------------------------------
>From: "Jack Olson" <[log in to unmask]>
>Sent: den 14 April 2010 14:14
>To: <[log in to unmask]>
>Subject: [TN] Question for "Old-Timers" about Gold
>
>> ya know, every time I think I know enough about a particular subject,
>> something comes along to make me realize there is more to it....
>>
>> I need to order some boards using a fabrication drawing that hasn't been
>> updated in over fifteen years.
>> We need Hard Gold Electroplating for a rotary switch contacts.
>> One of the notes calls for gold knoop hardness 200-300, thickness 130-230
>> uin. 93.7% purity
>> The last FAI report showed gold thickness of 160 uin. All other historical
>> records are lost in the shifting sands of time
>>
>> Our vendor has knoop hardness 150, 99.7% purity and says gold requirement
>> is
>> too thick.
>>
>> IPC says knoop 150, gold minimum 31.5 uin. for edge connector fingers,
>> which
>> I assume is the same for contacts (doesn't seem to be addressed)
>>
>> Printed Circuits Handbook says knoop 150, thickness 25-50 uin for
>> non-military, 99.8% gold purity
>>
>> Has gold plating changed that much since then?
>> Is there a danger of wear-out if I get 50uin. gold plating when our old
>> boards were 160 uin.?
>>
>> surfin' the learnin' curve,
>> Jack
>>
>>
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