Electronics > Metrology

New Precision Resistors

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babysitter:
Hi all,

i might have the chance to get stuff thru parylene sometimes in the future, maybe I can give a ride as a secondary payload so you can save setup costs.

@Edwin: Around what time did you start using the misbehaving epoxy resin and when did it phase out?

(No such complaints from my co-worker using those we bought, but the cellar the machine is running at has a quuite stable humidity and tempure.

best regards, Hendrik

plesa:
And what about using e.g 3M/Scotch spray
http://multimedia.3m.com/mws/media/123615O/insulating-sprays-1601-1602-1603.pdf
We were using it in humid environment for low leakage application.

Edwin G. Pettis:
Babysitter:  After acquiring and testing here where the humidity is usually in the range <10% to <35%, two orders ended up being built in August and September of 2015.  Orders in October were back to a previous resin which, while somewhat difficult to work with did not cause the humidity problem.  I might add that the resin performed mostly as presented in the data sheet except for the humidity absorption and a rather curious 'spongy' characteristic.  None of the previous resins had such a characteristic and perhaps this may have had something to do with absorption in high humidity.  After feedback from one of the two customers I simulated a higher humidity environment and found that the resin did indeed absorb humidity to the point of swelling if exposed long enough, while heating the resistors up and drying them out would return them to near original value, that did not stop absorption again under high humidity and apparently these two customers live in significantly higher humidity than I do.  In the mean time, I began looking for a new resin which is quite time consuming, trial and error does not go quickly, while I was filling a few 'special' orders with the previous resin, I did not want to do more orders until I had found a better resin.  Currently the resin I am using is working well but the handling of it has slowed down production, there has been no complaints.

Plesa:  In many cases you do not need to have a 'perfect' barrier against humidity as its effects are usually quite small, there are many dip and spray on coatings which are quite good at holding humidity at bay and are very reasonable in cost.  There should be something about humidity in the data sheet if the product is intended to help block humidity.  Parylene, PTFE and hermetic cans are used where the cost is justified, usually in harsh environments such as in industrial or aerospace.  To get that last tiny bit of humidity reduction can be comparatively expensive, hermetic sealing is labor intensive, I do not know of any automated method, I've done quite a bit of hermetic sealing myself over the years and even the big customers don't use it where it isn't absolutely needed.  As MisterDiodes mentioned, a bag of desiccant is cheap and effective, put in a small enclosed enclosure keeps humidity at bay.

JS:  Thermal tracking is a complex problem and the tighter the tracking requirements the more difficult it gets.  Simply enclosing resistors in an oil filled package does not always get the job done, it certainly helps and even oil can absorb some humidity depending on the type of oil and encasement.  Thermal coupling of resistors is not easy, even if they are on the same substrate (film, foil), in many instances the customer is looking for near perfect coupling which cannot be achieved, fractional PPM TCR tracking can be achieved but with difficulty and expense (nothing good comes for free unfortunately).  Oil is a decent thermal conductive median but there is a fairly long time constant involved to come to equilibrium and if the environment is changing that time constant may be a problem under certain circumstances.  If you need very good tracking but not fractional PPM, copper tape wrapped around the resistors can do a good job at fairly low expense.  There are few options available when going after <1 PPM/°C tracking, oil is one of them, other options are not production friendly.....they involve labor or other special procedures.  Various methods over the years have been tried with variable results, such as the resistor chips in the HP DVMs with limited success.

In short, trying to get to under a couple of PPM in any spec is going to get expensive and difficult, a lot depends on the resistors and the customer, just specifying a resistor ratio with tight tracking without considering the consequences (and some trial and error possibly) will very likely end in difficulties galore and friction between manufacturer and customer.  Every resistor house I'm familiar with has run into those problems with customers, even Vishay has failed with all their resources in some instances.  That magic resistor ratio for boosting output voltage to 10V with very tight tolerances is going to be difficult and fraught with problems, you can't just solder something in and expect near perfect performance, circuitry doesn't work like that.

branadic:

--- Quote ---If it is an epoxy only mechanical removing is possible, which should be avoided. Paint could probably be removed with some solvent, but could also dissolve the glue between ceramic substrate and foil?
--- End quote ---

What ever it is, it seems to withstand IPA and Aceton.


So here are my thoughts:

Hermetic cases are mostly made of ceramic, metal or glas, plated with gold. Because the gold layer used to make it conducting is such thin CTE mismatch is not that dominating problem. However, these are no materials everyone can handle at home.
Brass is, as far as I was told, no good idea, as it is a wrought alloy and therefor not hermetically tight, while it is easier to process.
Because of its high heat capacity aluminium would be a good material. On the other hand, if you want to use feedthrough capacitors it's hard making aluminium solderable. Plasma treatment and PVD could be a possible solution, to deposit a Au layer. But be aware, there are also a lot of aluminium wrought alloys out there, having the same problem like brass.
A case made out of copper is a cheap and good solution, using soldering technics to close and seal it.

Parylene is a bad idea for what I know, because of CTE mismatch the coating will get micro cracks so again water vapour can enter.
There are other coatings that offer hydrophobic properties, but at the end of the day you only change time constant. The same with wax, while it's easy to handle it will not avoid water vapour in any case.
Silicone oil is also not the first choice. Silicone can absorb water and it's still the devil in most production lines.
Galden prevents humidity and because of its good heat conductivity keeps everything inside a case at the same temperature level. There are different types available, depending on your specifications, but you want to make sure it's liquid over the entire temperature range in your application.

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