Do SMD resistors fail open after some time?

[**Rafael**]

Hi there!

This is the first time since I started working with electronics (37+ years) that I've seen passive components (SMD resistors) fail open. Their power rating is not exceeded in any way, but I have seen this piece of equipment, a μDS2000 Moog servo drive come often to my shop due to resistors failing open. Every time it comes is for a different cause. Resistors work as voltage dividers, pull up resistors, op amp input and feedback resistors, etc. At least 25 to 30 resistors have been replaced.

Any idea as to whether these devices have expiration dates for planned obsolescence purposes? This servo drive was manufactured in 2010.

Thanks in advance.

**Rafael**

 

[kellys_eye]

New one on me. Never heard of this as a cause nor come across any examples. It may be a manufacturing issue - to hot a solder reflow or issues with lead-free solder perhaps?

 

[Cannonball]

What kind of environment does it operate in?

 

[Alec_t]

Is it the resistor itself which fails, or an end joint?

 

[**Rafael**]

Resistance values change, for example: 1K changes to 3MOhm. At first I started changing the bad ones, and similar function resistors (as in gate driver optocouplers for 6 IGBTs) values were a bit off. Next time drive came, the ones that were a bit off now exhibited several megohms. I started changing them along with the ones that had not failed to prevent future visits to my shop.

Then several months went by and drive came back with different bad resistors.
Last week unit didn't want to run and it was because an NMI pull-up 10K Resistor was completely open. I'm amazed that only resistors failed and no semiconductor IC has failed.

This drive is used in a sausage vacuum filler machine which works most of the time in a walk-in cooler. Temperature around 17°C. This drive is rated at 400V, 22A and does the twisting to separate the sausage portions. The drive for the filling motor is bigger, same brand and rated at 400V, 92A and hah had only its display replaced.

Well, I'm in agreement with that this may be a manufacturing induced problem. I called the vacuum filler manufacturer in Italy and they said drives sometimes fail and have to be replaced. So far, I've kept it up and running. This me be a one of a kind case.

Thanks for your input on this issue.

**Rafael**

 

[(*steve*)]

I would be curious about whether you've tried measuring the resistors after you've removed them.

If it's a problem with the solder joints fracturing, the resistor may read ok. However it's also possible (maybe) that the fracture has pulled off the contract at the end of the resistor.

I would consider modifying how the board is mounted. Perhaps you can isolate it a little from vibration or stress with some rubber grommets or something.

It sounds to me that you've been fighting a poorly designed product (or at least one with a particular design fault).

 

[**Rafael**]

Resistors read open or several Megohms after being removed. I've replaced resistors in other two servo drives from a similar machine, different location, different customer. That's why I had thought manufacturer had some sort of planned obsolescence.

Solder joints look good, I use a microscope when I solder and desolder them in and out as they measure 1.5 mm long (0603).

There is vibration, but not something that may crack solder joints. When I first replaced one of those tiny resistors, I didn't have any similar replacement so I used a standard 1/4 W Resistor which could have vibrated due to its size. I replaced it when I finally got the right size replacement, after it came back two years later.

Regarding planned obsolescence, some German sausage vacuum filler machines manufacturers recommend replacing drives after 5 years of use. Could it be because of vibration related problems? Go figure.


Thanks Steve.

**Rafael**

 

[kellys_eye]

Potentially, and I mean potentially, there could be thermal flexing of the resistor - enough to cause the 'end caps' to be pulled away over time. It's indicitive when you state that only the OEM parts, not the fitted replacements, suffer these faults. Poor QC, dodgy suppliers etc many b the ultimate reason.

If these are low quantity production units then a 'batch' of dud resistors would go an awful long way.

Other than replacement of parts as they go wrong, replacement of all resistors on the unit seems excessive.

On a 'business' basis you can relish the thought of repeat work from income!

 

[Harald Kapp]

One mode of failure that has led to issues at my workplace is bleeding of the internal solder joint where the endcap contacts the ceramic body carrying the resistive layer. The resistors become highly resistive to open-circuit. If the PCB is operated in an environment containing increased levels of sulfur, special sulfur-resistant resistors should be used. Read this description for more detail.

 

[**Rafael**]

Harald,

It seems you hit the nail on the head. The environment this drive operates in has the PCBs coated with a very thin layer of a white-ish dust, which I've been told is corn starch.

According to this site, sulfur is used for processing corn, if it stays there then it may be the problem.

Thanks for your input and have a nice day.

Rafael
http://www.feingold.org/PF/cornsyrup.html

 

[Harald Kapp]

An option to prevent that effect in the first place is, apart from using sulfur resistant componnets, conformal coating of the PCBs with a protective layer before putting them into operation. If the manufacturer doesn't offer that as an option, you could do it yourself, but you'll probably void any warranty by that manipulation.