Engine Management Units

Sorry, no idea what you're getting at here.

I can't find any pictures that resemble these things on the web. Following a closer look with a stereoscope, there are outbreaks of these growths in at *least* a dozen places on this board. Here's two more sharper pictures with the problem areas outlined:

All the pictures I've seen on the net show whiskers as silver in colour but these are dark, metallic plum. They typically spring from vias in a kind of branching, fern-like formation and certainly reach out to touch other traces on the board.

Anyone seen anything like this before? Ideas on how to proceed?

Cracks?

Choose one of these, try acetone on a Q-tip cotton ear bud to dissolve the coating (try to not disturb the ?thing? that lies below the coating--if, in fact it is below the coating). Get a ?burnishing brush? with fiberglass bristles (used to clean oxidation from copper, etc.) and start gently rubbing the site. I think you?ll quickly discover if it?s a surface growth, or a crack or other phenomenon.

Good luck.

Thanks. They're definitely not cracks, they're fibres of some sort.

This type of caps is known for drying inside with time.

Yes, I know. But these are not faulty. I tested them with one of these meters:

Current most likely possibility is shorting between PCB traces due to growths of some kind:

Absolutely not! I had some Xilinx chips where the leadframe was tinned before the IC was mounted and encapsulated, then the leads were bent. This put strain into the tin plating, and whiskers grew from that. Xilinx' solution was to REALLY FRY the chips when reflowing the board, which I did not think highly of! They seem to have changed their plating to reduce the whisker growth. These were easily seen with a microscope, and were clearly sprouting out of the leads RIGHT at the bends.

Lots of reports indicate tin whiskers on circuit boards, connectors, IC leads, etc. Moisture exposure seems to be an important part of the conditions where they grow.

Yes, they should start out that way. But, oxidized Tin can take on a VARIETY of colors. Most of these I'm more accustomed to see where the Tin has been heated (as when soldering) but I have definitely seen Tin turn all shades of purple. Also, the conformal coating may have chemicals that caused the tin whiskers to change color.

Tin can definitely turn this "plum" shade.

Jon

These do NOT look like the whiskers I have encountered before. But, they could be some sort of conductive stuff that has grown between nets on the board. One possibility is they are fibers from a brush that either cleaned the board or applied the conformal coat. But, they could still be trouble, maybe they start out non-conductive but become conductive when moisture is absorbed. Since you say the sprout from vias, maybe the board had some contaminant left in the vias that slowly creeps out under heat and electrical field. Anyway, not much to lose by trying to remove the conformal coat at these spots and see if the EMU comes back to life. If it does, then you need to get some new conformal coat and re-seal those spots.

Jon

With a square wave that has a fast enough slew rate for the vertical parts of the scope trace to disappear. The square wave comes from a known impedan ce and is high enough frequency so Xc is minimal. Measure the amplitude of the parts you can't see and then it is simple ohm's law.

I use 400 mV 1,000 Hz with a 360 ohm impedance which does not make the numb ers easy but it makes for quick troubleshooting.

In that case I wonder how I would go about measuring the resistance of these things? Maybe I could lift one off with a piece of sticky tape if they're not too fragile. Can't think of a better way!

Thanks for that. Yes, I would have liked to have posted images direct from the stereoscope which is just in its own class for this kind of thing and shows *so* much more detail, but I don't have a suitable adaptor. I'll check out your link tomorrow as it's getting very late here now...

You're going to have to get more localized images of individual sites, using a minimum x10 magnification, and enhanced/varying side-lighting. A 'star' group, or part group should occupy a full image. Manual marking should not interfere and should use line widths

I'm still thinking these were left behind by some kind of brush. But, they could still be trouble after years of exposure to .

Are these on top of the conformal coat or underneath/within it? If they are in the coat, then it will be hard to extract them.

Jon

Early technet response suggests chemical contamination in vias, but would require SEM EDX to confirm. This contamination could difuse in the conformal coating cure stress lines (ie the pucker).

I don't see vias involved in every instance - some sites seem to develop at pure surface mount pad sites.

There's an image of copper dendrites on Steve Zeva's gallery

These tend to be inter-node rather than centrally expanding.

A close examination of tin whiskers should reveal a characteristic three dimensional spike/thread detail. Not likely here.

RL

They seem to emerge from under it and spread along the surface, but I can't confirm that yet as I've resisted disturbing them so far in case they disintegrate and defy further analysis. It's a good question, though, and I expect to be able to report back on this later today. I'll sacrifice one of the patches and find out.

Can you post a url for this site? I searched but got about a million microsoft support links instead.

"SEM EDX"? Is that something to do with electron microscopy? I have an admittedly huge collection of test gear here, but not a single electron microscope.

True, but they're in the minority. Maybe that's relevant; maybe not. The we don't have a large enough sample here to say.

Yes, still no precise matches.

Yes, TW seem to come out at right angles to the plain; these are surface creepers.

Right, basically what I said above is correct. There seems to have been some sort of breakdown of the clear coating which has enabled these things to get a foothold. They then break out and traverse along the board *on top* of the coating. So it's doubtful they could cause any short circuits unless by sheer chance the tip of one of them hits another spot of coating that's broken down above a trace - or a chip pin. When touched with a scalpel blade they break apart readily into shards and can be blown away by an air duster can. The coating has lost its integrity and could probably be scrubbed away with a toothbrush. I personally prefer to use clear varnish for my own boards. It hasn't the depth that this coating has, but at least when it's set, it's set for good. I wonder what the reverse side looks like? Perhaps there's no coating at all, in which case who knows what state it's in.

Thanks, I hadn't spotted that link.

If there is some formal defintion of this term, I don't know it. But my plans with this unit are to spend as little time as possible on it and if it can't be fixed reasonably quickly, to get rid of it. Items like this are way off-beat for me and I have a pile of other stuff more in my line awaiting my attention in due course.

I'll get there. I just have to fit this EMU in between other jobs with higher priorities, plus avoiding the usual divorce threats.

I agree entirely.

Nope. If you could see it through the stereoscope, you'd agree straightaway. I shall have to post to the photo groups for adaptor ideas to make it possible to post much better quality macro shots online than the ones I'm pissing everyone off with at present.

Easier said than done (see my update posted 2.10pm local here).

Technet link was previously posted.

Question is, what is your repair plan?

You don't seem to have any indication of the unit's present state other than sticking it back in the car and seeing if the car starts. No fb on xtal replacement yet.......

The blemishes you're chasing may have nothing to do with actual fault.

The spatter pattern is also suggestive of the impact from an external source, such as a flicked paint brush.

The single metalic entrapment pictured should also flick off with a knife blade. But without some before and after passive probing, you'll never know if this did anything.

RL