Some SMT terminations such as those used on ceramic capacitors have to be
metallurgically and process compatible with the component's internal
materials of construction in order to enable the mechanical and electrical
performance of the device. Thus, metals are required but pure copper is not
one of the options. For many years silver has been the dominant termination
component for this requirement. For SMT, the termination system is normally
beefed up either by addition of a co-deposit, usually palladium, or
over-plating, nickel covered by tin or tin/lead, to provide more customer-
process robustness than is required when these components are soldered
(leaded) by the component manufacturer. Both palladium and nickel preserve
the integrity of the termination by preventing dissolution of silver into
the solder. Palladium requires no further protection but nickel must be
protected from oxidation. Theoretically, OSP might do this job, but since
the oxide of nickel is very durable compared to those of copper the benefit
of developing OSP as an option is not immediately attractive. This can be
restated that if the OSP fails for any reason, the device is scrapped.
OSP/copper is reworkable. So called "nickel barrier" termination is by far
the most widely used alternative today.
The same arguments apply to SMT resistors.
Ann Bastin
Sr M&P Eng
ELDEC
(425) 743-8492
----------
From: Chada Srinivas-ESC005 [SMTP:[log in to unmask]]
Sent: Wednesday, July 12, 2000 10:44 AM
To: [log in to unmask]
Subject: Re: [LF] lead free terminations
OSP coating is over a metal, usually Cu. It preserves the
solderability by
inhibiting oxidation of metal; hence the name 'organic solder
preservative'.
The bond that forms during soldering is still metal to metal and
intermetallic in nature. Rob Schetty's answer to the query clears
the air.
Srinivas Chada, Ph.D.
Senior Materials Scientist
Motorola
Plantation, FL 33322
(954) 723-5293 (phone)
(954) 723-5584 (fax)
1-888-695-9053 (pager)
-----Original Message-----
From: Kirk D Mueller [mailto:[log in to unmask]]
Sent: Wednesday, July 12, 2000 1:28 PM
To: [log in to unmask]
Subject: Re: [LF] lead free terminations
The short answer is that metal to metal bonds, through
intermetallic
formation, are stronger, usually more reliable and more electrically
conductive than just about any other type. For the long answer
you'll need
to do some reading. I suggest starting with these:
"Solders and Soldering" by Howard H. Manko, published by
McGraw-Hill Book
Co., Copyright 1979, ISBN 0-07-039897-6
"Surface Mount Technology - Principles and Practice" edited by Ray
P.
Prasad and published by Chapman & Hall, now known as Kluwer Academic
Publishers.
Kirk Mueller
Raytheon Systems Co.
El Segundo, CA
Chris Hunt <[log in to unmask]>@IPC.ORG> on 07/12/2000 09:46:51 AM
Please respond to "Leadfree Electronics Assembly E-Mail Forum."
<[log in to unmask]>; Please respond to Chris Hunt
<[log in to unmask]>
Sent by: Leadfree <[log in to unmask]>
To: [log in to unmask]
cc:
Subject: [LF] lead free terminations
I am interested to learn why component terminations are all metal
finishes
and haven't taken a leaf out of the PCB industry and used an organic
coating
as a solderability preservative.
Any offers?
Chris Hunt
NPL
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