I believe Dr. Jen's explanation requires some clarification.
While oxides are insulators, they are rarely perfect insulators; some
current flow exists and the amount depends on the thickness of the oxide
layer as well as the voltage. Tin oxidizes more rapidly than silver
(though not as rapidly as copper) and palladium oxidizes less rapidly
than silver. So the oxide layer (assuming the unlikely scenario than all
materials have been stored under identical conditions for the same
period of time) of copper will be greater than tin, which will be
greater than silver, which in turn will be greater than palladium. Gold,
as Dr. Jen notes, does not oxidize but does form other compounds such as
sulphides which basically have the same electrical conductive attributes
as oxides.
The core issue is not whether the metals employed oxidize (except for
gold, they will) but whether the oxide is removed during the bonding
operation. Without complete deoxidation of the metal surface(s) to be
bonded, there will never be an intermetallic (metallurgical, in Dr.
Jen's terminology) bond and the connection will be inherently
unreliable.
It should also be noted in relation to this subject that one type of
common "soldered" connection is made over an oxidized surface without
removing the oxide. The connection is winding terminations of wire
coils. The oxidized wire ends are inserted into very hot solder (up to
500°C) and emerge with the appearance of proper wetting. This production
technique, which leaves an oxide barrier between the solder coating and
the base metal, explains the high failure rate of coils at test and in
operation.
Jim Smith
Managing Director
Cambridge Management Sciences, Inc.
4285 45th St. S.
St. Petersburg, FL 33711-4431
Tel: (727)866-6502 ext. 21
Fax: (727)867-7890
eMail: [log in to unmask]
"Jen, Harry" wrote:
>
> In IPC-D-279 document "Design Guidelines for Reliable Surface Mount
> Technology Printed Board Assemblies", page 22, it says that only gold,
> silver and palladium should be used for finish if silver polymer is used.
> The reason is other surface finish, even tin-lead, will form non-conductive
> oxide and block the conduction path. Silver oxide is conductive and gold
> and palladium are quite noble. Copper oxide, on the other hand, is not
> conductive. This is why a metallurgical bonding is required and silver
> epoxy should not be used on copper for more reliable process.
>
> Hei-Ruey Harry Jen, Ph D
> Principal Reliability Engineer
> M/A-Com
> 1011 Pawtucket Blvd. G31
> Lowell, MA 01854
> [log in to unmask]
> 978-442-4804
> 978-442-4787 (FAX)
>
> > -----Original Message-----
> > From: James Canner [SMTP:[log in to unmask]]
> > Sent: Wednesday, July 12, 2000 3:33 PM
> > To: [log in to unmask]
> > Subject: Re: [LF] lead free terminations
> >
> > There have already been a number of excellent responses, so I want to turn
> > this around and consider the use of conductive polymeric terminations on
> > passive devices. How can they be attached to OSP copper board with out
> > oxidation of the filler metal (usually silver ) and the circuit board
> > metal
> > usually copper. It seems to me that each supplier is using what is
> > necessary to make his product perform in usual application. Therefore any
> > new application method or material will possibly affect the total assembly
> > process.
> >
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