Designed to Wear Out / Designed to Last – Pt. 2

Designed to Last

There are many examples of technologies which are designed to function for decades into multiple centuries.

What goes into an item that can make it last decades or even centuries ? Here at LCM, a lot of the original logic we are running in our collection, was not rated as long life when originally used in our machines. Expected lifetime data had the best of them lasting for a few decades, maximum.

This has generated a set of rules we abide by: If it looks like stone or amber, it can last a long time. If it looks like plastic (or is plastic), it is not going to last. One particular exception is epoxy packages. Certain epoxy resins are similar in structure to tree resins, which so far, hold the record ( millions of years ) for preserving ancient insects, pollen, and plants. )

  1. Semiconductors and Integrated Circuits – Life data in databooks for IC expected lifetimes from the era in which they were created, typically have these devices failing more than a decade ago. Yet here, they are still performing their function. Staying functional tends to favor ceramic packages (stone) and specific epoxy packages (amber)(See “Notes on Epoxy Packages for Semiconductors” below.
  2. How Much Heat – Systems designed to have a higher internal ambient operating temperature, tended to have a much higher failure rate. Datasheets at the time ( and today ) show a direct correlation between ambient operating temperature and operating life.
  3. Cooling Fans – These sit right in the middle of the life curve. Although the bearing has a low wear rate, 30 years seem to be the upper limit. ( It would be cool if someone could come up with a frictionless magnetic bearing.)
  4. A Special Note About Fans and Heat: A lot of our power supplies have an intimate relationship with fans, meaning, if the fan fails, the power supply will fail, as well. When we re-engineer a power supply to replace an older or failed unit, we specify that the new supply can keep operating even though the fan has failed. This is accomplished by the fact that the replacement power supply components have a higher efficiency ( thus generating less heat performing their function ) and can tolerate heat better than the old power supply components. ( This has the added advantage of lower air conditioning costs )

Winners In The Longevity Game

The absolute winners we have found and utilized in our systems are what are known as “bricks”. These are power supply modules which are fully integrated. They come in various sizes which determine their power ratings. Full bricks top out around a kilowatt. Half-Bricks are around 500 watts. Quarter-Bricks around 250 watts.

Lifetimes (MTBF) at full load and temperature for “bricks” go from around 40 years to around an astounding 500 years. ( That is not a typo. The part in question is a Murata UHE-5/5000-Q12-C. The whole UHE series has this rating. Price $61.90) These devices, as you may have already guessed, are epoxy encapsulated.

Designed-In Longevity

This refers to what we have encountered upon restoring the machines in our collection. There is a definite intent at work when one examines the component choices. For example, DECs PDP-10 KL series power supplies have filter capacitors at four times the necessary capacitance. The amount of capacitance declines with age pretty linearly till the end of lifetime (around 14 years). This means these particular components will still allow power supply function 4 times longer. That’s at least 3 times beyond the machine’s commercial life rating ( 5 -7 years). We got these machines 15 to 20 years after their last turn-on, and they ran for most of a year before we had cascading failures of the filter capacitors.

Notes on Epoxy Packages for Semiconductors

Epoxy packaging for semiconductors became popular in the mid-1960’s. It replaced ceramic, as it was less expensive. Epoxy, unfortunately, can be made with different resins and other ingredients that give it different material characteristics. There is a correlation between cost and moisture intrusion. Lower cost, more moisture. This gave epoxy a bad name as it was used to make ICs’ more competitive in the market. This led to a number of market loss moments for certain manufacturers, as the moisture intrusion occurred at a predictable rate depending on ambient humidity for a particular region of the country.

It was a multi-faceted problem. The moisture intrusion occurred where the IC lead connects with the package. Moisture intrusion into the epoxy and poor metal quality ( tin alloy ) of the lead frame causes corrosion of the lead, which allows moisture into the IC cavity and changes the bulk resistivity of the IC die. The electrical specs go off a cliff and the IC fails.

( Note: If the lead frame had been made from a different alloy or the epoxy was a higher grade, this failure had little or no chance of occurring. I find it hard to justify the cost differential given the ultimate cost to the end users and the manufacturer )

(I was a field engineer in the mid to late 1970’s and spent many an hour replacing ICs with this problem.)

The only epoxy packages that have made it to the present day used better materials and thus are still functional. There are thousands functioning ICs on circuit boards in the Living Computers collection heading toward their 50th anniversary and a spares inventory of thousands.

Mechanical Switches

Whether toggle, pushbutton, slide, micro, or rotary, mechanical switches are all over the lifetime map. In the commercial world, switches are rated at the maximum number of actuations at a specified current. For the most part, the switches I have encountered meet or exceed the actuation specification. There is a limitation, though.

Aging of Beryllium Copper

If the internal mechanical design use a beryllium copper flat or coil spring, it has almost surely failed by the time we at the museum have encountered this type of switch. Beryllium copper goes from supple and springy to brittle after 30 years or so.

The result, as you may have guessed, is a non-functioning machine due to switches that operate intermittently or not at all. ( We had a whole line of memory cabinets, with hundreds of bright shiny toggle switches for memory mapping, that wouldn’t function till we replaced the switches.

Circuit Breakers

These fall into the beryllium copper spring family, so they are pretty much failed, or in addition to not closing, their trip point has typically shifted so you get a premature trip or a trip well above the trip point ( sometimes no trip and the protected circuit burns up ).

Slide Switches

We’ve had one surprising winner in the switch longevity department, and that is slide switches. With few exceptions ( usually due to mechanical damage to the sliding element ), an intact slide switch can be quickly resurrected with cleaning and a little light oil.

Rotary Switches

The runner up in the longevity game is the rotary switch. Typically all a wonky rotary switch needs is a spray from an alcohol cleaner and contact lubricant, and it functions, no matter what the age. ( we have hardware going back to the 1920’s whose rotary switches are still functional ) You can consider the switch failed if the contact wafer is cracked or broken. My guess as to longevity involves the phenolic wafer getting significant mechanical strength by being riveted.

Relays

These devices are essentially an electrically actuated switch. Their most common configuration is some kind of leaf spring. If the spring is beryllium copper based, you of course, have a failed relay 30 years hence. Rotary relays tend to be fairly reliable, but require a little more maintenance to keep them going. Mercury wetted relays are fairly reliable ( we still have some running in a couple of pieces of hardware ), but are not recommended because of their mercury content along with the mercury being contained in a fragile glass envelope.