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Feature Articles

RoHS, Machine Vision, and Lessons Learned

by Winn Hardin, Contributing Editor - AIA

 

July of 2006 has come and gone, and Europe’s Restrictions on Hazardous Substances (RoHS) has joined WEEE, and Japan’s J-MOSS. The good news: The world is still here. Electronics manufacturers haven’t had to resort to shipping the latest Braun beard trimmer with their lead-free products to keep ‘tin whiskering’ under control.

At first look, the great RoHS revolution seems to have come and gone with little significant impact. At least, that’s what the consumer sees and should see. Behind the scenes, machine vision component suppliers have had to work diligently for the past few years and spend considerable money to make sure the RoHS deadline was only remarkable for its anonymity.

Here, we’ll look at those behind the scenes efforts and lessons learned. Issues related to high-temperature soldering and the impact on microlense arrays, corrosion and the PCB, changing self-alignment properties of tin solders versus lead solders, dealing with contamination in mixed-production environments, and the occasional bug-a-boo with component selection.

Price of Knowledge
If by some happy miracle, you’re not familiar with RoHS, and what constitutes a hazardous substance under RoHS and similar regulations around the world, be sure to check out AIA’s articles, ‘‘RoHS Part I: Machine Vision Tackles RoHS Operational, Business Challenges‘‘ and part two of the series on RoHS technical challenges.

Unlike the articles from last year, machine vision component suppliers have had time to evaluate not only pilot runs of RoHS-compliant products, but production runs as well.

‘‘As we went from a leaded to lead-free [solder] process, people were rightly apprehensive about what would happen,’‘ explains Daniel Vuk, Matrox Imaging 's (Dorval, Quebec, Canada) Engineering Hardware Manager. ‘‘It turns out that this was actually a good thing. It caused us to question everything, and examine the process under a microscope to make sure we made the right choices. After each pilot run, we shipped the units off for cross sectioning, x-ray analysis, and sheer strength testing on BGA balls, for instance – the works.’‘

Like Matrox, Imperx spent years preparing for RoHS and essentially, ‘‘starting from the ground up,’‘ according Petko Dinev, CEO of machine vision camera and board manufacturer, Imperx [link in new window to Buyer’s Guide listing]. ‘‘Although RoHS is little more than a year old, we’ve been working on this for about twice that time. For the first six months, we went through a substantial learning curve, working very closely with engineers at our manufacturing facility. We did a root cause analysis for each failure. The result has been a much better understanding of the soldering and manufacturing process and yields in excess of 99%.’‘

RoHS and Cameras
Although there are several hazardous substances with acceptable levels outlined in RoHS regulations, lead has generated the greatest concern because of the need to go to a new solder material, namely SAC (tin-silver-copper). This solder has a peak temperature of approximately 250° Celsius compared to 220° C for leaded solders.

Electronics manufacturers were faced with the decision to refit wave soldering machines to accommodate the higher temperatures, or move to intrusive soldering systems that allowed soldering of both surface mount (SMT) and through-hole components in a single step (wave soldering systems require SMT components to be glued first before soldering, while intrusive applies a solder paste to the pad or hole, then the components, and then heats the board to the solder’s melting point).

The silicon in a CCD camera is impervious to these temperatures, according to Imperx’s Dinev, but other elements are not. ‘‘The last thing a CCD wants to see is higher temperatures, not because of the silicon, but because of the plastic microlense array on the chip. Different lenses can handle different peak temperatures for varying amount of times, but if you exceed it, the plastic lens turns brown, and there goes the transmissivity. If it’s a monochrome camera, you lose light; if it’s a color camera, you lose color.’‘

Most visible camera manufactures now use sockets instead of soldering the chip directly to the board. ‘‘If you’re selling a VGA camera that costs a few hundred dollars, a couple of dollar sockets adds an unacceptable amount to the product, but if your camera costs a few thousand dollars, a few dollars on a socket is no big deal.’‘

The use of sockets can also help standardize the optical alignment for a family of cameras. ‘‘More companies are using multiple cameras in a single piece of equipment, so they want every camera to be identical to the others so they can plug in a replacement and not worry about realigning the optics,’‘ Dinev says. ‘‘Sockets can help through the use of the guiding pins on the CCD, socket and heat sink.’‘

Cohesion and Components
In addition to a higher melting point, lead-free solder also exhibits a weaker surface tension. In the past, if a pick-and-place machine misaligned a ball grid array (BGA), the surface tension of the melted solder would pull the IC into proper alignment.

‘‘That’s not true for lead-free solders,’‘ Imperx’s Dinev explains. ‘‘If you put the chip on an angle, it stays on an angle, and you have a problem. The same can be valid for discreet components. If you have a capacitor that’s a 0201, or 0402, the higher temperatures can cause the component to twist a little, and the lead solder isn’t there to help self-correct the problem.

Other changes included moving from FR4 printed circuit board (PCB) material to FR406, which can sustain higher temperatures. Higher soldering temperatures also aid metal corrosion, which can be further accelerated by contact with oils – even fingerprints, Dinev adds. PCBs are shipped to Imperx in vacuum-sealed bags and extra care is used in the loading the boards into the production line.

While the stress of trying to find RoHS compliant components diminished as demand increased for these products, engineers have had to consider how these compliant components may affect existing designs as cameras and boards segue into lead-free varieties.

‘‘An RoHS replacement part is supposed to work the same way the lead-free version did, but if you’re working with a fast camera that switches in nanoseconds, you may find that not all components are equal,’‘ advises Imperx’s Dinev.

Matrox Imaging’s Fabio Perelli, Product Manager for embedded and security products, qualifies Dinev’s concerns by adding that components from a single source tend to perform the same way. However, if you buy the same component from multiple vendors, it’s a good idea to qualify each component individually.

‘‘As you move from Supplier A to Supplier B, you may have some small differences in the electrical response,’‘ Perelli explains. ‘‘That’s one benefit of qualifying all our RoHS compliant products from scratch. You don’t take anything for granted.’‘

Smart companies have even looked at components that don’t contain RoHS hazardous materials. ‘‘While we haven’t had a single support issue that can be traced back solely to the RoHS transition, we did learn early on that we needed to be careful about our selection of connectors. Our older connectors were molded plastic that couldn’t take the higher soldering temperatures. So even though the older connectors complied with RoHS, we had to switch to new connectors with higher-melting point plastic,’‘ added Matrox’s Vuk.
 
Like most embedded electronics used in industrial application, machine vision components are expected to last at least 5 to 7 years, and vision companies promise to support them during that life cycle. Although RoHS has only been around for 18 months or so, concerns about early product failures because of tin whiskers shorting out circuits have – so far – proved groundless. But as all of our experts agree, time will prove the point.

 

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