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 _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
