Posts Tagged ‘PC boards’

‘Tin Whiskers’

I've just recently read of a phenomenon that I had never heard of before this week ... "Tin Whiskers". Tin whiskers are hairlike crystalline structures that can grow from tin surfaces and have the potential to wreak havoc on modern electronics.

Ever since the European RoHS (Restriction of Hazardous Materials) implementation, most manufacturers have been forced to switch to lead-free solder. It seems that lead, when combined with tin, inhibits the growth of tin crystals. Although tin whiskers have been around for many years, it is just since the switch to lead-free solder that their impact on circuit boards (particularly with SMD sizes) has been garnering a lot of attention. Even the military, which has always demanded a higher standard for its components and circuit fabrication, is worried. With almost everything, from alarm clocks to aircraft, reliant on electronic circuit boards, are we awash in a sea of electronic ticking time-bombs? Some experts believe that we are.

The whiskers themselves can grow from a few millimeters to over 10mm in length. Growth rates as much as 9mm per year have been observed. One might think that these tiny hairlike structures would not be capable of carrying sufficient current to cause damage and would act more like a fuse when shorting with a nearby pc trace or component lead but such is not always the case. Tin whiskers have been known to carry as much as 30ma of current ... more than enough to cause catastrophic failure in delicate circuitry ... not a pleasant scenario if you are cruising along at 36,000' in a 10-year old aircraft.

An article in Aviation Today reports:

It’s rational to ask, after a few years of RoHS policy in force, does this effect ever really cause any problems? As it happens, yes, and some examples are so spectacular that it’s amazing they have not reversed the policy for RoHS. In Europe, the most impressive example was $1 billion recall of Swatch watches from Switzerland, as the use of lead-free solder caused a roughly 5 percent watch failure rate in 2006. The “solution” to this problem was yet another rule exemption, and lead was again used in Swatch construction. A real RoHS policy triumph there.

John Keller, Editor in Chief of Military & Aerospace Electronics magazine reported in a 2005 article that:

Reports indicate that six satellites sustained partial or complete loss due to tin whiskers. These involved Galaxy-3, Solidaridad 1, Direct TV3, and HS 601 satellites built between 1998 and 2002. Problems also have been reported with the F-15 jet fighter radar, the Patriot missile, and the Airborne Warning and Control System (AWACS) aircraft.

One prominent manufacturer, Apple, has been using lead-free solders since 2004, reportedly, without any issues. As noted by Kurt Jacobsen, author of "Within A Whisker of Failure" (published in The Guardian):

Perhaps manufacturers haven't developed an "experience base", or perhaps it isn't registering as a problem. Many customers will probably chalk failed devices off to their own isolated tough luck, when the cause might really have been microscopic whiskers inside their machines.

And Bob Willis, Technical Director for the Surface Mount and Related Technology Group in the UK mused:

Overall, was it sensible to go lead-free? "I would say no," ... Earlier obsolescence means more discarded devices. Critics argue that substitutes are more toxic and energy-wasteful than the lead they replace - and that lead doesn't leach from circuit boards, because it doesn't migrate as lead in paint or petrol does.

Well worth reading and perhaps one of the best summaries of the dilemma can be found in "Death By Tin Whisker" by Walter Shawlee:

This is clearly a problem that has to be addressed for the avionics industry sooner rather than later. Either a clearly worded exemption from lead use is required, or the frankly ineffective removal of lead from electronic assembly construction rule should be abandoned. This misguided policy has introduced a random failure mode into every item now being made under these standards or with RoHS compliant parts, a simply unacceptable risk for all involved. Plus, the ridiculously small (2 percent) targeted lead-use area fails utterly and completely to address the very real concerns of lead toxicity in the environment. Clearly, all of these issues are something worth thinking about, and their impact on our industry and all of us is significant if no useful action is taken. 

Although this is all new to me personally, perhaps this is old-hat by now, since the problem has been around for the past few years. Maybe a viable solution has been found already or will we see the introduction of lead into solder once again? I also wonder about the millions of life-critical circuit boards already in place that are, at this moment, quietly growing 'tin whiskers'.

Drawing PCBs With ‘MS Paint’

I recently had an inquiry about using 'MS Paint' for drawing PC board layouts. Although there are several freeware programs available for designing and drawing PC layouts, the ones I have tried had onerous learning curves. I also found that unless I was designing boards fairly regularly, I would have to go back and re-learn many of the functions each time I used the program. If you are regularly making a lot of boards, then these programs are certainly the way to go as they are packed with every feature you might need.

On the other hand, MS Paint meets all of my requirements and is simple to use ... I like simple. Although some might turn up their noses at MS Paint, I have found it to be a powerful and underestimated software tool. Here are some of the things that you might want to remember if drawing a PC layout with 'Paint'.

1. Take the time to read the built-in HELP and USING files. It doesn't take long to learn all of the functions along with some of the shortcuts.

2. Always draw with the IMAGE ... DRAW OPAQUE function turned off (unchecked). Do this first. This way, lines can go as close as you want without any blocking or overlapping. Try it the other way and you can see what happens.

3. If you are working with IC's, take the time to make a 'master' pattern that can be saved and copied anytime that you need it. A 16-pin layout can by used for 14 and 8 pin IC's by copying and pasting it and then erasing the unwanted pins. I can send you one via email if you contact me.

4. Similarly, once you have established the correct pad spacing for certain capacitors or resistor sizes, copy and save them for future use. Finding the correct spacing will require a few trial printouts so you can measure the exact gap.

5. For detailed work, use the VIEW ... ZOOM...CUSTOM  function to magnify the layout. This also allows you to use a grid if you find this helpful.

6. Never SAVE your layout while in ZOOM mode as you can't go back to the original size when you re-load your plan. I learned this the hard way.

7. For drawing perfectly straight lines, squares or round circles, hold the SHIFT key down when drawing the element.

8. For large areas of copper (groundplanes), outline the area then choose the FILL symbol to create the area.

9. With a little planning, you can almost always avoid jumpers to make a crossover connection, unless your circuit is very complex.

10. When you make a mistake, just use the EDIT ...UNDO command. You can go back several steps with this helpful function.

11. I will often draw a thin line between to pads to see if they are on the same level ... and then UNDO the line once I have checked.

12. Always SAVE your layout as a Monochrome Bitmap with the FILE ... SAVE AS function. Using some of the other modes will create less than solid blacks and some random pixels that can lead to etching problems.

13. If using the 'toner iron-on' method of etching, set your printer options to the highest resolution (mine is 1200 dpi) and then choose the darkest 'print' option. You want as much toner thickness as possible. Note that this method works only with laser / toner printers and not the ink-jet / bubble types.

14. For the iron-on transfer paper, I have had good success with the shiny yellow transfer paper widely available on E-Bay, usually with free shipping. I get the best results when pre-heating the PCB in the toaster oven before ironing-on the pattern ... not too hot to touch however.

If you just build the occasional PC board, you might find using MS Paint worthy of a try. It has been meeting my needs for many many years and once you have done a few, unlike some PC software, it doesn't take long to build a new board without having to learn how to use it all over again.

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