Vishay bulk foil / drift after soldering - Page 1
Forum section: electronics / projects

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 16, 2014, 08:25:59 am
content: Hi,I often use VISHAY - Z201 Series bulk foil resistors which are specified to have ± 0,05ppm/°C and are really nice (and expensive).Unfortunately they have one feature that no one wants to share: after soldering (even by hand) they tend to drift about 30...50ppm too high (in my case) and the resistance slowly decreases in 1 or 2 months (!) to a stable level (1ppm). l performed some measurements using ASL AC thermometry bridge to compare DUT resistance against stable Leeds & Northrup resistance standard. My colleague from calibration lab used better equipment and also confirmed that. The results can be worse when using machine soldering when the component is exposed to high temperature for longer time.The first chart shows resistance measurements before soldering and after soldering and cooling down to ambient temperature (first 100seconds).The second one shows measurement over first 24h form the moment of soldering.

author: fcb
author_rank: Super Contributor
timestamp: May 16, 2014, 08:31:44 am
content: Interesting post. Thanks for that.Can't help wondering what is better than those ASL units (F18 etc..)?

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 16, 2014, 08:37:00 am
content: Quote from: fcb on May 16, 2014, 08:31:44 amCheck out this monster.http://www.mintl.com/DC/Products/Resistance/Resistance_Bridges/accubridge

author: jpb
author_rank: Super Contributor
timestamp: May 16, 2014, 08:38:28 am
content: Interesting.Can you reduce the effect by attaching a heat sink to the leads when soldering (like people used to advise for transistors)?

author: quarks
author_rank: Frequent Contributor
timestamp: May 16, 2014, 08:48:07 am
content: when I solder these high precision resistors I recommend and use a heat sink to avoid this problem

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 16, 2014, 08:59:39 am
content: Quote from: jpb on May 16, 2014, 08:38:28 amI developed my own method for minimizing this effect:use THT Vishay resistors instead of SMD.place them about 3...5mm above PCB surfaceprepare small container filled with isopropyl alcohol (you may cool it in the fridge)place the PCB over the container to have the resistor and its leads immersed in the alcoholsolder one lead of the resistorwait for few secondssolder second lead of the resistorSo it uses the alcohol as a good heat sink and cools the leads by evaporation in the moment of soldering.The first thought was to use water but I'm not sure about future consequences of that.The alcohol will evaporate in few seconds from the resistor's and PCB's surface.UPDATE:I forgot to add that for this two solder joints in the whole PCB I use Sn60Pb40 solder and 350C temperature.

author: jpb
author_rank: Super Contributor
timestamp: May 16, 2014, 09:24:47 am
content: Quote from: doktor pyta on May 16, 2014, 08:59:39 ambaths of evaporating IPA sounds a bit of a fire hazard! I hope that you don't smoke whilst doing itHow much does doing this reduce the deviation?

author: quarks
author_rank: Frequent Contributor
timestamp: May 16, 2014, 09:31:07 am
content: over time I used these toolsfor Z-Foils the copper alligator clip works good, alluminium is also okthe stainless steel tools are not really good but better than nothing

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 16, 2014, 09:47:41 am
content: Quote1.To be serious: the container with alcohol is small so there is no danger.Of course I don't breathe the fumes.2. As far as I remember this helps to reduce this effect by about 5 times.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 16, 2014, 10:11:27 am
content: Quote from: doktor pyta on May 16, 2014, 08:25:59 amThis hysteretic effect is well known for this kind of technology, as it is similar to strain gauge resistors.Avoid heating more than 30°C over room temperature by using Cu plated clamp.I have described that in the LTZ blog on VHP201Z type resistors.If this shift arises, do a symmetric thermal cycling, like degaussing.The metal foil resistors are NOT specified 0.05ppm/K, that's an idealistic value only, Vishay call it the typical value, but to the experience of several customers, that is also not true.Most types are specified 2ppm/K max.Typical values are mor between 0.2..1ppm/KFrank

author: tszaboo
author_rank: Super Contributor
timestamp: May 16, 2014, 10:16:31 am
content: It is still within specifiaction, it a MIL-PRF-55182 CHAR J device, which specifies 100ppm maximum difference for soldering it. But in the end, there isn't really anything you can do against mechanical stress. Soldering it in a special way helps a bit, or let it run for the first month, or the famous thermal cycling with reducing the amplitude. Precision equipment has a price for reason.

author: robrenz
author_rank: Super Contributor
timestamp: May 16, 2014, 12:22:54 pm
content: IMO the alcohol bath is not as good as the heavy copper heat sinks on the leads. I don't know the numbers but I don't think the alcohol has the thermal capacity to compete with the copper. Also the amount of surface area available on the lead to transfer the heat to the alcohol before it actually reaches the resistor body is insufficient. Cooling the resistor body itself is fruitless as the thermal influence has already occurred.

author: Vgkid
author_rank: Super Contributor
timestamp: May 16, 2014, 02:38:54 pm
content: If we were discussing heatsinking water would be the best 4190 J/mol*k, with ethanol in second.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 16, 2014, 04:05:40 pm
content: For those who are interested I performed a quick test which allows to estimate temperature of transistor's junction during soldering. So the situation is similar to soldering Vishay resistor. Details in the attachment. As my professor said "a picture is worth a thousand words"Unfortunately I don't have copper tweezers, so You may perform Your own measurements. Instead of copper tweezers I used brass banana plug strongly clamped on transistor's nodes (plug shown on second picture).

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: May 16, 2014, 04:30:51 pm
content: Quote from: robrenz on May 16, 2014, 12:22:54 pmI'm with robrenz on this one.The IPA bath is a good start, but you really need to heat sink the leads.Boiling liquid tends to carry away a good amount of heat (especially for water). However, the vaporization process tends to counteract any gain you get from the phase change heat absorption because the thin layer of gas acts as a high quality insulator.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 16, 2014, 05:51:10 pm
content: Quote from: CaptnYellowShirt on May 16, 2014, 04:30:51 pmThats not the problem here. You aren't boiling the alcohol (unless you are way too hot), and the alcohol is a plenty good thermal conductor for this since it can cool by convection as well as conduction. The problem is that you want to cool the leads directly, not the case. The leads are connected to the resistance element directly, while the case is only connected through an insulator so it is possible to keep the case cool and still heat the film.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: May 16, 2014, 06:38:28 pm
content: Quote from: ejeffrey on May 16, 2014, 05:51:10 pmMaybe so. But IPA's thermal conductivity is around 1/5th of water's conductivity and one around 1/3000th of copper's.And while a liquid offers more surface area contact with the leads, I don't think its 3000 times as much.I'm not sure what % IPA we're talking about here, but the boiling point is probably around 90 C? The solder junction (just a few mm's away) has to be at >180C. My guess is that will boil.But really, this is all just speculation. A test is what is needed.

author: Andreas
author_rank: Super Contributor
timestamp: May 16, 2014, 07:55:11 pm
content: Hello,does anybody know how hermetically resistors (VHP100 or similar) behave under same conditions?With best regardsAndreas