Bill,
You are right, at least partially. There is a big risk of tin whiskers
shorting the narrow gaps between the fine pitch lead-free solder
joints, unless the manufacturers know exactly their materials and can
strictly control the process.
There are positive examples since several years when some leading
Japanese manufacturers voluntarily changed over to lead-free assembly
in their consumer electronics. So far no alarming reports.
It is true, higher temperatures put a lot more stress to the material
and components, but that is not the fault of politicians. The industry
itself made wrong decisions when selecting the alloying materials for
the lead-free solders used now generally in the RoHS process. There is
a material and soldering process that would work riskless and even at
much lower temperatures. It is called Transfusion Bonding that is
using bismuth instead of lead for alloying the solder joint.
In this process you tinplate the solderpads and componets and then add
a thin layer of bismuth over tin. Reflow at +180 deg C, bismuth starts
to melt already at 139 deg C, it diffuses quickly into the tin forming
a thin alloy layer. All the time bismuth continues its diffusion into
the tin matrix, thus the molten mix becomes very lean Bi-Sn alloy that
forms reliable bonds. Also, as the ally becomes leaner, its melting
temperature increases. Actually, even after the temperature is lowered
the bismuth diffusion continues until the alloying is uniform across
the whole solder joint.
The remelting temperature of resulting bond is very close to the
melting temperature of pure tin, +132 deg C! This about 1% content of
bismuth in the alloy can relax the internal energy of the crystal
structure and prevent tin whisker formation.
Why this process is not used generally in the industry? The answer is,
it was invented 10 years ago in the wrong place and hurted interests
of big international companies that already invested huge amounts of
dollars, yens, pounds, etc in tin-silver-zinc alloys. Seldom the best
technolgy wins, only big money talks.
Those who are interested, may read more in the publications of the
IEEE. Look for Professor Jorma Kivilahti, Helsinki University of
Technology. Unfortunately those articles are not freely available,
unless you are a subscriber of the IEEE publications. I found only one
free article that shortly mentions this method:
http://www.ept.tkk.fi/Research/Publications/55_Paper.pdf
73, Ahti OH2RZ
On 01/05/06, William Bordy [EMAIL PROTECTED] wrote:
I have been following the RoHS requirements and one issue I see rarely
discussed is the Tin Whiskers issue. For those that are not familiar with
it please see the following WEB site:
http://nepp.nasa.gov/WHISKER/background/index.htm
It appears that with the switch to no-lead that the reliability of the
equipment will be substantially reduced. This appears to be what happens
when politics drive science.
73,
Bill Bordy
NJ1H
-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Lyle Johnson
Sent: Monday, May 01, 2006 10:28 AM
To: Jim Lux
Cc: FlexRadio
Subject: Re: [Flexradio] RoHS was Re: July 1
Bob, I assume you're talking about RoHS, which bans lead (except in
certain
very narrow situations, not applicable here) in electronics.
I don't know much about how Gerald makes the boards for the SDR1000, but I
wouldn't think that changing to no-lead solder is a big issue...
Actually, it is a big issue.
Turns out that no-lead solder manufacturing processes require more heat,
and normal FR-4 PCBs tend to delaminate, so you must use
high-temperature fiberglass. This is available, just more expensive --
20% to 50% higher cost per board.
Fewer facilities are available to manufacture assemblies in a RoHS
compliant way, and willing to certify same, so those costs go up. In
the case of my DSPx, the quotes I have for the raw PCB cost are double
and the assembly costs will more than double what I am currently paying.
The components used in the product must all be RoHS compliant.
And it isn't just about lead. There are six commonly used substances
that are banned or severely proscribed. Normal passivation processes
used for aluminum, for example, contain banned substances, so even the
case may be affected.
73,
Lyle KK7P
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