Tekmos' Blog

We recently had a requirement to test some parts at 150C.  In order to do that, we use a temperature forcing probe.  This is a washing machine size box with an arm that holds a test chamber.  The machine can force hot and cold air into the test chamber, and allow us to raise the temperature to any desired value.  It is not fast, and can take a minute to reach the final temperature, but it works.

One problem is that the bottom of the temperature chamber is open, and so if we don’t do anything, we will also raise our tester to 150C.  Testers are finicky, and would prefer to be at 25C.  In order to prevent problems, we need a thermal insulator to put around the part.  They sell a rubber sheet that is pretty good, but rather expensive.  And we needed a solution that day.  So we went over to Bed, Bath and Beyond, and bought silicon cooking mats.  These were trimmed to the desired size, and augmented with a lower layer of insulator, and this worked fine.

We have a future requirement for testing parts at 175C.  In this case, we are beginning to exceed the temperature range of our probe, and so we will build an extension cable, and test parts directly in one of our ovens.  The silicon mats won’t be good for us, but the Bed Bath and Beyond oven mitts will definitely be useful.

One of the biggest challenges we face in the replacement of obsolete parts is replacing the obsolete packages.  We can re-create designs and implement them in new technologies, but we don't have the same flexibility for the packages.  And while newer packages are easily available, there may be engineering or regulatory reasons why we cannot change the existing PC board to accommodate a newer package.

If a given package is not offered by one of our main assembly vendors, we first search to see if anyone else in the world offers it.  The initial step is to look at their web sites and see what they offer.  This is harder than it seems.  The web sites may not be in English, and they may not be updated.  We have frequently inquired, only to find that a specific package is no longer offered.  The contact information is also challenging.  And a given vendor may not be interested in what is for them a very small piece of business.

The cost is frequently higher from smaller vendors.  Many have optimized their business around military work, and that can result in an increase of $20 per part to cover the packaging alone.

Some packages are just not obtainable.  In those cases, we have to build an adapter card.  We have used adapter cards for 56-pin SDIPs, 84-pin PGAs, and 84- and 132-pin BQFPs.   

We have a R&D program underway that is looking into two different replacements for PLCC packages.  One involves a special adapter card. The other uses a special LGA (Land Grid Array) package with a footprint created to fit the existing PLCC footprint.  In both cases, our first step is to check the reliability of the cards.  We have created parts with pairs of pins shorted inside of the package.   This allows us to create PC boards that are a huge continuity chain.  We will subject these boards to thermal cycling, and see if any of the cards develop opens.   

A normal gate array prototype cycle takes about 8 weeks.  There is a week to make masks, 3 weeks for wafer processing, another 2 weeks for assembly, a week for test and bake, and a final week that taken by shipping between the various stages.

Sometimes, 8 weeks is too long, particularly if a customer faces a lines-down situation.  In these cases, expedites can help to reduce the total cycle to under 3 weeks.

The first part of an expedite is to pay the wafer fab to turn the lot into a hot lot (or bullet lot).  This will reduce the fab and mask time from 4 weeks to about 10 days.

Expediting the assembly is more complicated.  Most overseas manufacturers are pretty efficient, and an expedite will reduce their cycle time from two weeks to about 1 week.  This is dependent on their capacity at any given time.

 Domestic assemblers are an option, but they must have the tooling for the specific package you need.  With expedites, that can usually turn a package in 2 to 3 days.  And there is an additional time savings to be had from the reduced shipping times.