||2011-06-27 10:59:52, Read : 1,010
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Lead-free solder brings many benefits
Author: Terry Costlow, IPC online editor
Tighter line spacings and better moisture specs are among the benefits gained by eliminating lead.
There's been no shortage of criticism for lead-free solder over the past decade. But the shift away from lead brings some significant benefits that can help designers and manufacturers.
One of the biggest question marks surrounding lead-free solders is their reliability. But Ronald C. Lasky says that it's difficult to support that perception.
"A lot of people claim that lead free solder is not as reliable as lead-based solder," said Lasky, an instructional professor at Dartmouth College's Thayer School of Engineering. "We're now five years into the implementation of RoHS and we've made $3 trillion worth of electronics. We haven't really noticed any big change in reliability. However, it is clear that we have not established long term reliability, as most of these products have not been deployed that long."
In fact, lead-free solder has made it simpler to continue the dramatic size reductions that are enabling the great progress and growth in handheld devices. Today's solders are easier to mass produce when pitches are tight. When high density semiconductors are packaged in small form factors, using lead-free solder can result in boards with fewer problems.
"Lead free solder doesn't wet well so you can get better lead spacing," Lasky said. He cited a study by Motorola where designers built identical boards with lead-based and lead-free solder. It showed that leads printed close together shorted out when lead solder flowed during melting.
The high temperatures needed to melt lead-free solder have created some problems.
"Moisture sensitivity is a big issue that's often overlooked. Much of the moisture research was done for lead-based temperatures, not the higher temperatures of lead-free processes. If you have a moisture rating of four for lead, it might be five or six at lead-free temperatures," Lasky said. This new situation can cause problems, such as "popcorning" and in addition board warpage.
He'll be discussing many of the facets that go into designing and processing boards using lead-free technologies during IPC Midwest, which will run Sept. 21-22 in Schaumburg, Ill.
Lasky's half-day presentation will provide a feast for those who like to see images. He'll be presenting 257 slides during his talk. The presentation will examine issues and techniques for both small boards made in high volumes and large, low volume boards.
Techniques for avoiding problems such as head on pillow defects are among the topics he will discuss. He'll also examine graping, a concept that will be familiar to those who have seen microphotographs of solder. Some solder has microscopic balls that resemble grape clusters. If solder is not designed to minimize this graping, it won't melt correctly, which has serious repercussions for reliability.
Lasky will also discuss some of the non-technical aspects of the regulatory environment that plays such a dominant role in printed board processing. One of those is the Waste Electrical and Electronic Equipment Directive (WEEE), which dictates that 85 percent of what companies sell must be recyclable. "Companies are going to have to submit documentation in the language of the countries they sell into. That means there's a lot to do for many manufacturers," he said.
Lasky will open the workshop with a brief overview of the WEEE and RoHS initiatives and discuss the status of lead-free assembly worldwide.
The professor will also point out some misconceptions about these European Union Laws. "RoHS wasn't written to prevent hazardous materials from going into landfills, it was developed to make it easier to recycle products," Lasky said.
For more information on the show including free exhibition passes, visit www.ipcmidwestshow.org/register.