Create a shortcircuit between two PCB pads - Page 1
Forum section: electronics / general

author: jm_sanz
author_rank: Contributor
timestamp: August 23, 2022, 09:44:52 am
content: Good morning everybody,I am trying to shortcircuit the drain to source pads of a mosfet (attached) trying not to have any influence on the thermal model of the system.For this, I have tried using some solder tip but pads do not join together. I have found this copper tape that might to the job but for sure not as good as soldering as it includes some thickness.https://www.3m.com/3M/en_US/p/d/b00041333/Any tip for joining both pads?Thank you

author: jpanhalt
author_rank: Super Contributor
timestamp: August 23, 2022, 09:59:39 am
content: Suggestions: Short pieces of wire or "zero" ohm resistors. If you want copper strip, get copper flashing at a hardware store. Hobby shops in the US also sell thin copper sheet. You can also buy copper foil without adhesive. The adhesive is no advantage. I would certainly not rely on it for electrical contact. It can be removed with some organic solvent. I would use MEK.

author: Feynman
author_rank: Regular Contributor
timestamp: August 23, 2022, 10:51:16 am
content: Not sure, if I understand your intentions correctly, but you want to "bypass" the power dissipation of the MOSFET, right? The resistance of zero ohm resistors is usually higher than typical Rds(on) values of MOSFETs. So maybe that is not what you want in this case.Most likely solder mask is preventing you from joining the pads with solder. Maybe you can cover the solder mask between the pads with copper tape (or thin aluminum foil) andthentry to make a decent low resistance connection with solder (on top of the copper tape)?

author: jm_sanz
author_rank: Contributor
timestamp: August 23, 2022, 11:53:12 am
content: Yes, this was actually what I already tried and got a 1mohm resistance in comparison with the 3mohm of the MOSFET. This is good but not enough for my application. I think this might be due to a poor soldering from the pad to the copper foil. It is kind of tricky as it forms sort of a sandwhich between the pad, the copper foil and the MOSFET.

author: jm_sanz
author_rank: Contributor
timestamp: August 23, 2022, 12:52:17 pm
content: I have found this one which seems very suitable for the application, the thiner the better.https://basiccopper.com/6x61milcofo1.htmlThanks!

author: Kean
author_rank: Unknown
timestamp: August 23, 2022, 01:00:57 pm
content: Quote from: jm_sanz on August 23, 2022, 12:52:17 pmThinner copper sheet is probably going to give worse results.A quick calculation based on a 4x6.5mm section of 35um copper (approx area between S & D pads) at 25C gives 800uohm, so not much better than the 1mohm you achieved.The same calculation with 1 mil (25um) copper sheet is 1.1mohm.You will need thicker copper to get a lower resistance, but that will be harder to solder and may affect thermals.Calculator used:https://www.omnicalculator.com/other/pcb-trace-resistance

author: free_electron
author_rank: Super Contributor
timestamp: August 23, 2022, 02:53:28 pm
content: There are preforms available that solder directly on that footprint to short source and drainLaird makes those i believe

author: jm_sanz
author_rank: Contributor
timestamp: August 24, 2022, 09:26:51 am
content: Do you have the name or the reference to look into their website? they have too many products.Thank you.