I used to work for a contract manufacturer building extremely sensitive sensors for a customer. These sensors had a finite life, and they were replaceable in the field. The customer planned to keep spares on hand, so we received a large order for spares.
When we were about 10 percent into delivering the spares order, we started to see field returns coming back. The reported defect was “erratic behavior shortly after installation” and “operating temperature out of spec.” Field returns were covered under warranty unless we could assign the failure to mishandling or some other customer-related fault.
The design of these sensors was such that they would operate correctly only if they were within a fraction of a degree Fahrenheit of the ideal temperature. The sensor was very well insulated, and the operating temperature was controlled in two zones by electrical resistance heaters. The temperature of each zone was sensed by a Wheatstone bridge containing two glass-bead thermistors. Each bridge output was connected to a PID (proportional integral derivative) controller that was in the customer's system, not in the sensor. There was also a monitoring thermistor used as independent temperature readout, for a total of five identical thermistors per sensor.
A glass-bead thermistor is a pretty simple device. It has platinum wires that are stretched close together on a frame, and beads of ceramic slurry are applied between them. The frames are then fired, and the “string” is clipped to form individual thermistor beads with two leads coming out one end. A glass tube is then placed over each bead and melted around it to form a hermetic seal, with the leads sticking out. The high reliability (hi-rel) ones are put through a thermal-shock test, moving them back and forth repeatedly between 400°F oil and liquid nitrogen. Only the strong survive.
The four control thermistors were secured into blind holes in the metal body of the sensor with epoxy, and the monitoring thermistor was surface-mounted to the outside of the sensor body (inside the insulation) with the same epoxy. The epoxy was selected to cure at the same temperature as the operating temperature of the sensor.
Perhaps this one should be titled "The Case of the Squished Leads." A good example of why so much advance testing needs to be done on a product made of multiple materials with different, and sometimes incompatible, characteristics.
A comment made in a Mechatronics column about an 'unnecessary restraint' comes to mind. Yes, the epoxy potting was necessary, but did it have to be rigid? If it must, were all results of the rigid constraint accounted for?
I've done this numerous times, thinking 'more is better' when I've eliminated a choice/freedom that gained me nothing but thought I've trimmed a loose end.
We ran into a similar problem mounting small components on circuitboards for the military back in the '70s. We ended up dipping the components in liquid heat conducting RTV, letting it set, then securing the component by covering it all with a strong heat conducting epoxy. The RTV provided enough give to allow for minor physical changes, and the epoxy held it all securely in place.
Reminds me of a mjor customer of mine on 5mm LEDs who violated the keep away zone of 5mm below the base by bending the leads at the body base and then hand soldering them in series. THe thermal conductance speed is about 1mm/second on the steel leads and the 600deg F soldering iron actually melts the clear epoxy resin which loses its vapor seal around the leads, leading to moisture ingress and also creating thermal sheer forces on the cathode lead to the tiny micron sized gold bond wire to the chip resulting in a sheared wire bond at that time or some time later in the field. Of course series powered LED's fail in a string when one fails in open cct mode which is the dominant faiulure mode. Solutions were proposed and eventually implemented to reduce the soldering time to < 3 seconds and reduce the risk while still violating the application rules. Moisture ingress and thermo-plastic ESD induced failures were also a risk and a Polyurethane protective coating was applied to prvent ESD plastic induced injection molding failures and all the previous failures. Usage 100K/yr failure rate, random but 1% was too high.
I also have an epoxy-potting story from the early 1970. In those days, I was working for Burr-Brown in the function modules group. We made the weird stuff, square and square root, vector adders, log amplifiers and the like. Mostly I worked on computing RMS modules. These designs were all based on monolithic pairs of transistors and the same were used as the input stage of operational amplifiers. They were made by gluing down a pair of transistors in a ceramic cup and bonding gold wires out to six leads. Then a dot of very hard epoxy protected the bonding wires.
We sorted out the incoming parts for the ones with the best-matched characteristics for multipliers, so the function modules got the newest parts.
One day a line tech came by and gave me a box of expensive multipliers that had failed after potting. Normal loss was a percent or so, this time about 40% failed. I don't remember exactly why I suspected the dual transistors, but after grinding down half a dozen modules, I found that the leads were open on one or more of the dual transistors. Snapping off the potting plastic let me read the date code on the edge of the cups. Three date codes had failures.
Eventually this was traced to a materials mix up where the hard epoxy and been replaced by a soft epoxy that moved under the stress of potting--enough to shear off the gold leads.
It never occurred to me that I should ask before shutting down production to get the bad parts out of the flow. Months later at a performance review I found this was a strike against me. Not long after that review, I moved on to a job that turned out to be much worse.But that's another story.
It took very good detective work tofind that problem, as well as a very thorough understanding of the product. I wonder if anybody in a current "cost optimized" engineering group would ever find the problem. Hard epoxy has definitely been a guilty party in quite a few encapsulated assembly products, with the cause often being the different rates of thermal expansion. This problem can become even more common with the move to surface mounted assemblies. The challenge is even greater when the design team is spread around the world, and the members are intensely focused in very narrow areas. The result is that a lot more effort is spent to avoid thermal expansion problems, at least in products that need to be reliable and last longer than six months. Of course, the mandate to abandon the old reliable lead-based solders has not made the task any easier. "What were those lead-free monkeys thinking?"
I think we got what doctor ordered. And as is case so often with doctors, someone focuses to one paricular problem and does not see the big picture, the whole person or product. So a doctor might order a drug to lower your blood pressure. Then this lowered pressure might cause dizzyness causing a fall, which might lead to hip fracture leading to a bed ridden patient. Same way some bureaucrat was thinking that "Lead is poisonous, it must be eliminated", but failed to see the big picture that because the lead-free product is more likely to fail, there actually will be more e-trash going to landfills, which is not good. A better approach would probably been to put a recycling system to use. Like we have with lead batteries that are used in cars, because lead-free battery would be much more expensive. Same goes to mercury. It can not be eliminated from fluorecent lamps, so those are recycled.
PPihkala, your comment about lead reminds me of all the hassle immediately post-RoHS that board builders went through trying to find alternatives to lead solder. Many of the candidate materials had very different CTEs from other materials on the board, many weren't strong enough in all the right ways, and others required even higher solder temperatures that damaged other materials on the board.
Several of these comments make it clear that not only is a lot of advance functional testing needed, but that multiple iterations in test need to be done as soon as the designer makes a single change to the design materials.
William, I think your point is also a really good one, about what happens to troubleshooting of designs when the team is spread around the world and consists of specialists. Whose job is it to look at how everything works together? Do we still have project managers of some kind?
It is true that there has been quite a search for a substitute for lead in solder, and also true that an economical substitute has not been found. In fact, I am not aware of an equal substitute at any price. So the result is much shorter product life, and a result of much more electronic waste.
On the item about product problems, it does not really sound like a job for a product manager, but rather fo a product expert, somebody who understands all of the varied technologies that go into each product. Is it possible that Siemens could add something to their PLM product to help with the situation? Perhaps they should consider it. Of course, the requirement for a product expert looks a lot like exactly what ISO9000 and it's derivatives would hope to eliminate: An individual using personal knowledge and judgement.
For the last half of my career I have been making an effort to understand the other disciplines involved in the projects that I worked on. This included learning a lot about hydraulics and pneumatics, and the details and peculiarities of each type of system. THis did help me provide answers when some challenges came up. But that is for a few other postings.
Who would have presumed that the hardness and curing rate of encapsulants was so very critical?
Actually there is an economical replacement for leaded solder, tin. Reliability may be an issue, particularly in stressed environments, but it's not expensive. Tin with a touch of lead has a long history of reliability and while also being economical. I understand gold makes a good lead replacement, but for cost, well, it's not so economical.
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