Electronic Design

Bob's Mailbox

Dear Bob: This is a general question about TO-220 packages. I am finding MOSFETs available in TO-220AB packages that claim to have continuous ratings of 120 A. I can believe the die can take it if heatsinked well. But can those skinny 0.044-in.2 wires take that kind of current? (I think they are tin plated copper lead frames unless they are some kind of superconducting alloy.) The copper wires in my house are 14 gauge and are only rated for 15 A, and they are bigger. And what about the die bond wires? Is someone pulling my leg or what? Is this a new form of specsmanship?

Ed Ganshirt (via e-mail)

Pease: Hello, Ed, #14 copper wire is rated to fuse at 160 A (per the CRC Handbook). So there's a safety factor of 10, for surges. But a TO-220AB's lead is just 18 mils thick by 45 mils wide, tapering to 28 mils (min), and the datasheets say it's good for 75 A? I can't believe that! Let's try it! Stand back!

Hi Bob: I am writing to express my concern over what I regard as premature adoption of lead-free soldering in our industry. There seems to be an almost lemming-like quality in the rush to adopt this technology. (Yeah. /rap)

I have yet to see any concrete data regarding manufacturability and reliability, although the consensus appears to be that both are going to get worse. Would you want to rely on a defibrillator that used lead-free solder? (No! /rap)

Ken Neltnor (via e-mail)

Pease: Hello, Ken. NSC's MIL AERO group is keeping the lead-based solder, that is solder-dipped onto the leads of hi-rel parts. Some examples are LM108AH-883 or MIL-38510 parts for military and space applications. Similar industrial parts (LM108AH) will have solder-coated leads without the element lead. It will not be a straight tin solder, as many other IC makers are doing, but we will have a tin-silver-copper coating that is dipped on. And that is apparently better for whisker-free manufacturing. But at any time, if you need the highest reliability you can get—such as for pacemakers or defibrillators (even with inexpensive plastic-packaged parts)—you can use conventional solder with lead. That is, if and when you can get it. That will prevent the tin-whisker problem. Talk to your favorite solder maker, and get his opinion. Okay? Good question, and thanks for asking.

Hey Bob: Yesterday, I took a break from the breadboard of my latest design to graze through the new copy of Electronic Design. Dick Weiner's letter was nice to see (electronic design, June 23, p. 18.). I'm not the only person who wants to actually see his designs work. Makes it a lot easier to defend yourself in the meetings.

What I was taking a break from was installing eight SOT-23 transistors onto a 24-pin Aries SOIC board. They work just fine, if you have the patience to solder them into place. (Actually, a couple of these are gonna be used as diodes. Thanks to you for that.) You can use the transistors, diodes, Zeners, and such that you're actually going to use in production. (True, but the adapter board does add a little bit of stray capacitance and inductance. Gotta watch out for that! /rap)

I design voltage regulators and similar circuits for my company's line of heavy-duty automotive alternators, so I don't have to deal with speeds above the clock on a microcontroller. Therefore, speed is not an issue. These Aries SOIC boards can even be used for five-pin SOT-23 parts. Takes a little imagination and patience, but they work just fine for the little 7101s and 7301s.

Edward Craig(via e-mail)

Pease: Glad you're having fun with the tiny circuits, Edward.

My old friend Tom Decker has some old Philbrick Amplifiers that he wants to be selling soon, such as K2-Ws and USA-3s. You could look on eBay or write to him at New Dimensions, 978 West County Road I, St. Paul, Minn. 55126-1315.

Comments invited! [email protected] —or:
Mail Stop D2597A, National Semiconductor
P.O. Box 58090, Santa Clara, CA 95052-8090

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