2
\$\begingroup\$

If I bolt a transistor to aluminum for an electrical and thermal connection, are there any oxidation concerns for the electrical contact?

\$\endgroup\$
  • \$\begingroup\$ only if the aluminum is grounded and the exposed conductor is not, then you need a mica or 3m insulator with conductive grease. Also co-planarity, surface roughness and conductive thermal grease to gap fill is critical for low effective Rth \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 8 '16 at 17:48
  • \$\begingroup\$ What kind of package? Isn't there almost always another pin to make "good" electrical contact? \$\endgroup\$ – George Herold Nov 8 '16 at 18:42
  • \$\begingroup\$ It's just a TO-220, but we have to use the tab as the electrical connection in this particular case due to where we are sending the current. \$\endgroup\$ – radix07 Nov 8 '16 at 19:00
  • \$\begingroup\$ In that case, use a toothed (star) washer on the tab screw, and the electrical path will be heatsink-screw-washer-tab. The threads will make a fairly reliable connection with the heatsink. \$\endgroup\$ – WhatRoughBeast Nov 9 '16 at 5:05
7
\$\begingroup\$

Yes, potentially.

Galvanic Corrosion is something that you should be aware of when mechanical aspects of electronics design come into play.

Find out what metals are involved in your assembly and select screws, nuts, washers and heatsink materials that are most suitable.

Also, it is occasionally difficult to make a good electrical contact with aluminium parts (heatsinks or enclosures) due to natural or anodized oxide layers. One way to break through is to use a "spring" or "locking" washer to gouge into the soft aluminium and penetrate the oxide layer to form a gas-tight seal. The materials involved are still a concern in safety critical applications.

Aggravating factors can be operation in a moist or salty environment or where harsh, prolonged vibration can cause fixings to become loose over time and weaken electrical connections.

Conformal coatings are another option available for PCB-level ingress protection. The PCB assembly company can add a sprayed-on layer of sealant that provides moisture and particulate shielding. These can be customised for your application. It is a common process and shouldn't be very expensive. However it may cause some issues when diagnosing faults in post-production testing.

EDIT: (The OP indicates that this is a vibration sensitive vehicular application)...

Vibration can be mitigated by mounting the entire assembly on rubber shock-absorbing washers where the outside of the enclosure meets the vehicle chassis.

Internally, individual components can be secured by other means such as using cable ties (aka "zip-ties" or other names) or silicone based flexible adhesives to absorb, dissipate or attenuate shocks to the system.

Depending on your application it may be appropriate to use a solid potting compound to encapsulate the entirety of your internal circuitry to prevent moisture ingress or shock from damaging your circuit. Engine ECU/EMU systems are often fully encapsulated in this manner as they are safety critical systems.

\$\endgroup\$
  • \$\begingroup\$ Very useful information. Although if vibration is a concern, I am not sure this is terribly feasible for what I would like to do. \$\endgroup\$ – radix07 Nov 8 '16 at 18:31
  • \$\begingroup\$ @radix07, can you elaborate on the nature of your application? By "vibration" I mean things like harsh environment, perhaps in moving vehicles or aircraft. What kind of device are you designing? \$\endgroup\$ – Wossname Nov 8 '16 at 18:35
  • \$\begingroup\$ This could be on a moving vehicle driving a motor, typically near a gas engine. So there are many sources of vibration that I would be concerned about shaking loose a seal that could oxidize. \$\endgroup\$ – radix07 Nov 8 '16 at 18:58
  • \$\begingroup\$ Thank you for the updated answer! Those are all things I have dealt with before, but never to maintain a gas seal to prevent oxidation... Will certainly give it a test run and see how it goes, but may also have to try a copper/soldered interface of some sort... \$\endgroup\$ – radix07 Nov 8 '16 at 19:17
  • 2
    \$\begingroup\$ @radix07, no worries. Beware of using solder in a mechanical joint, though. A damaged soldered joint is worse than a loosened pressure joint (because it is harder to repair or detect). Have a look into why "press fit connectors" are preferred in MIL-SPEC equipment. A good press fit joint is always preferred over a soldered joint for vibration environments. You will be forgiven for finding this counter-intuitive to begin with :) \$\endgroup\$ – Wossname Nov 8 '16 at 19:21
1
\$\begingroup\$

Yes. Aluminum grows an insulating oxide immediately on exposure to air. There are insulating "washers" for TO220, etc. packages that are made from anodized aluminum. If you want a reliable connection to aluminum, use a bolt/screw with a toothed lock-washer.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.