Description

Solid-copper heat-spreader moves heat much faster than mounting a TO-3P directly to the heatsink.

The best way to replace an MT-200 transistor! Equally outstanding thermal performance. Adapt your choice of TO-3P or TO-247 transistor to an MT-200-sized solid-copper heat-spreader.

Adapters are solid copper, 1.6mm thick. Transistor is soldered to the adapter, SMD-style. (You’ll need a hot-air soldering station. It’s easy!)

MT-200 transistors are becoming unobtanium, which is unfortunate, because they have superior thermal performance compared to any available modern device. An MT-200 has nearly three times the heat-spreader surface area of a TO-3P, and nearly twice that of a TO-264!

Why is MT-200 even a thing? Surface area.

A transistor’s ability to quickly move heat into the aluminum heatsink depends on how much surface area it is in contact with. Copper conducts heat nearly twice as well as aluminum. The more the heat is spread out before it gets to the thermal pad, and then the aluminum heatsink, the better the heat transfer, and the cooler the silicon die. The cooler the die, the more power you can dissipate. This is why MT-200 devices were invented in the 1970’s. Transistor dies were more expensive then, so squeezing an extra 50W out of a them with a large copper heat-spreader made a lot of sense. The expectation these days, is that designers will opt for using multiple smaller TO-3P devices in parallel. (There are technical advantages to both approaches.)

Surface Areas of interest:

I measured and calculated the area of the heat-spreaders of output transistors in TO-3P, TO-264 and MT-200 packages, as well as the Hoppe’s Brain MT-200 Adapters. The contact patches are not perfect rectangles; I had to account for the plastic area around the mounting holes, the beveled corners, and all the odd-shaped cutouts. These figures are pretty accurate.

TO-3P – 207mm² (NJW1302)
TO-264 – 313mm² (MJL1302)
MT-200 – 592mm² (2SA1169)
TO-3 – 620mm² (MJ21193)
Hoppe’s Brain MT-200 Adapter – 732mm²

Hoppe’s Brain  MT-200 adapters actually have a 140mm² larger surface area than MT-200—24% larger—because the copper goes all the way to the edges, while MT-200 has a thin plastic border around the outside that subtracts from its total surface area. I think this explains the slightly better experimental results I was getting. I don’t mean to make too much of this, there are diminishing returns on the size of the heat spreader; 24% more area does not mean 24% better cooling. But it’s awesome that it’s clearly better not worse!

Some Interesting Thermal Conductivity stats:

• Monocrystalline Synthetic Diamond – 3320 W/mK (The ultimate thermally conductive dialectric! A little expensive.)
• Naturally occurring Diamond – 2200 W/mK
• Elemental Copper – 401 W/mK
• Electrolytic Tough Pitch “ETP” Copper – 390 W/mK (Most common copper in electronics)
• Copper PCB substrate – 380 W/mK (These adapters)
• Elemental Aluminum – 237 W/mK
• 6063 Aluminum alloy heatsink – 201 W/mK
• 6060 Aluminum alloy heatsink – 166 W/mK
• Steel – 8-66W/mK, depending on alloying elements
• Tin/Lead 63/37 Eutectic solder – 50 W/mK
• Mica – 0.7 W/mK
• SilPad – 0.9 to 5 W/mK (Higher performing SilPads get expensive real quick.)
• Thermal Paste, Electrically insulating, Silicone/Metal Oxide – 0.6-9W/mK

Thermal paste, Silpads and Mica do not actually conduct heat very well! They are down in the single digits. What can ya do? It’s just inconvenient physics; Materials that don’t conduct electricity well, usually don’t conduct heat well. Metals conduct heat via free electrons, the same phenomena that transports electron currents. Diamond conducts heat hugely better than any metal, via phonon waves, rather than free electrons.

What’s inside an MT-200?

The highest-power MT-200 devices are usually constructed of a TO-3P heat-spreader soldered to an MT-200 backplate. It’s a heat-spreader on top of a heat-spreader. A Hoppe’s Brain adapter plate with a TO-3P transistor soldered to it, is basically the same exact same thing, and so naturally its performance is the same. (*Even slightly better.)

Assembly:

Tools required:

• Hot-air soldering station
  *A temperature-controlled heat-gun should also work well enough. But you can get a basic hot-air soldering wand on Aliexpress for like $30. ¯\_(ツ)_/¯
• SMD solder paste, Eutectic 63/37 tin-lead recommended
• Two alligator clips, (Included)
• Helping Hands or similar work-holding apparatus

First, inspect the pad surface: To prevent corrosion, the adapters have a thin, transparent coating of OSP “Organic Solder Preservative”. OSP is an excellent surface finish for things with large SMD pads, like power transistors and LEDs. It’s a super thin coating at less than 0.5µM, so it leaves little residue behind and doesn’t contaminate the solder beneath the pad.

The OSP coating is fragile and easy to scratch. Scratches or nicks can cause corrosion of the copper. Time can cause corrosion. Inspect the component pad on your adapter, and if you see any corrosion, just buff it bright and shiny with steel wool or scotch-brite.

It’s normal for the coating to look a little spotted or cloudy. There may be scratches in the copper that have been coated at the factory with OSP. That’s fine as long as it’s not brown and corroded.

Next, inspect the transistor itself. If you are re-purposing old transistors, you might find there is corrosion on the heat-spreader. Hone the surface flat and shiny using a large flat file, a diamond hone, or wet sandpaper placed on a flat surface like glass.

Spread a thin—emphasis on thin—layer of solder paste on the back side of the transistor. Don’t intentionally put any on the plastic parts but it’s OK if there’s a little.

Use two alligator clips to hold the transistor perfectly in position.

The alligator clips serve an important second function;

As the solder melts, the spring pressure from the alligator clips will squish the transistor down flat against the plate, pushing out excess solder, and pushing out voids. This will make the solder layer as thin as possible while still being contiguous.
Don’t let the transistor just float as one normally does in SMD soldering, that will make the layer of solder too thick and reduce heat transfer efficiency. Solder conducts heat really well, but it’s only about 1/4 as good as copper.  But if the solder layer is thin it contributes very little thermal resistance.

Soldering:

Set hot-air station to 350C, high airspeed. No tip.

Clamp the device in such a way that you can heat it from above and below. I use helping-hands with alligator clips.

We’re going to heat mostly from below. But first, pre-heat the top of the transistor for just 10 seconds or so. The prevents causing a large temperature difference between opposite sides, and reduces thermal expansion stress on the plastic body of the transistor.

Then, heat the adapter from below until the solder melts. The copper substrate will naturally heat extremely evenly, which makes it go nice and easy.

Eventually the solder will melt and go all shiny, and the transistor will sink down tight against the plate. Just a little solder should squish out the edges. Remove heat about two seconds after this happens, and let cool. Be careful not to overheat, or the transistor die’s bond to the case could be damaged. Most transistors specify a maximum die to case temperature of 260C for 5 seconds. Eutectic solder melts at 183C. That’s your window, no problem.

Really satisfying.

You should now have a super thin layer of solder holding the plates together with no voids.

I destroyed this example to verify I wasn’t getting any voids. I filed it down at a shallow angle, then polished it to reveal a wide cross-section of the seam. You can barely even tell where the solder layer is, it’s so thin! I cut it in a bunch of spots, and couldn’t find any voids. It’s like one piece of metal. (Click to enlarge.)

Extra credit:

For best thermal performance, the back side of the adapter may be honed flat, using a large flat file, a diamond hone, or wet sandpaper placed on a flat surface like glass. The adapters tend to warp just slightly concave as they cool down from soldering. So do real MT-200’s. Here I’ve lightly sanded down an adapter and a Sanken MT-200 to show how both are just a wee bit warped.

The thermal pad will take care of this unevenness, but honing them flat makes for more more even pressure against the heatsink, and squeezes a little more thermal performance out of them.

The copper is soft so this goes pretty quick and easy.

Hone until the surface is all evenly sanded. It doesn’t need to be polished. Thermal paste or Silpads will conform microscopically to the scratches.

Flat AF.

Installation:

The copper substrate of the adapter is connected to the transistor’s collector. Use a thermal insulating pad as usual. (Mica or Silpad) The mounting holes are metal and are electrically connected to the collector. This is different from the original transistor, which has plastic insulation around the mounting holes. For this reason, electrically insulating nylon shoulder-washers must be used, and are included. They are compatible with M3 or #4 bolts.

Thermal pads:

• • Mica pads actually work pretty good and you can re-use them if they are in good shape. Thermal conductivity around 0.7W/mK. Check for de-lamination or cracks. Clean the old thermal grease off with naptha or lighter fluid, it dissolves like magic.
  • MT-200 Sil-Pads are not readily available from reputable sellers. I suggest cutting your own from a sheet.  I like Parker CHO-THERM T441. It has better performance than mica (1.1W/mK), awesome tear resistance, and is reasonably priced. Cut pads to 40x25mm. Use a leather punch to make 3mm bolt holes spaced at 24.4mm.

Here’s where the nylon shoulder washers go, these are essential so the collector doesn’t short to the heatsink.

Install bolts and fasten the transistor to the heatsink.

Tighten both bolts lightly at first, then tighten them down alternately until you’re at final torque.

Check for zero continuity between the heatsink and the collector.

Got Metal Brackets?

No problem! you just need some spacers.

Some amps or receivers will use a metal bracket for various mechanical purposes. This one is holding a connector in place. This causes mounting issues with the adapters because the top surface of an MT-200 is flat like a table, and the adapters are not. There’s a gap, and if you were to tighten the screws, it would bend the metal bracket.

Order spacer kit here.

The spacer kit includes 4mm thick aluminum spacers, and 0.5mm thick brass washers. These are stacked on top of the nylon shoulder washer, to come level with the top of the transistor. Then the metal bracket can be installed as usual.

The nylon shoulder washer is 0.8mm thick, plus the 4mm spacer equals 4.8mm, which is just shy of the 5mm thickness of most transistors. Most of the time you don’t need to add the brass washer. It should come level, or close to level with the top of the transistor. It’s OK if the bracket squeezes the transistor a little bit, the metal is thin and will bend just a smidge. In this example, I have have a typical 5mm thick transistor. I’m stacking the nylon shoulder washer, the 4mm washer and I don’t need the 0.5mm washer.

Now the metal bracket can be tightened down just like it was with the MT-200 it replaces.

Alas, you might find your bolts are now too short, because the adapter+transistor is slightly thicker than an MT-200. Unfortunately, long transistor bolts with captive washers and spring-loaded lock-washer are hard to find. I suggest building them with individual parts from McMaster-Carr.

1. Socket-Head black-oxide Bolt, M3, 2.5mm Hex-drive. Pick the length you need, at least 1.6mm longer than original.
2. M3 Split-ring lockwasher High-collar type specifically made for narrow head of socket-head bolt.
3. Flat washer
4. M3 star locknut
   These are just suggestions, it depends on your equipment.

Uninstallation:

If you need to replace a transistor, the adapter can easily be re-used. Clamp the device in a vertical position. Heat at 350C from the back side. When the solder melts, the transistor will just slide right off, perhaps requiring a gentle tug. When the adapter cools, clean the surface with alcohol. You can now solder a new transistor to the adapter. You won’t need as much solder paste as the first time because there is now a layer of solder already present. It would be a good idea to put some extra flux on the pad before soldering.

Compatibility beyond MT-200:

There’s a few different transistor packages with 24.4mm mounting bolt spacing, but smaller sized. They should be compatible with the adapters, as long as your heatsink has room, and the packages don’t physically interfere with each other.

There’s so much confusion about these package names! Here’s what I’ve deduced. LMK if you have corrections.

Apparently “MT-200” is a Sanken proprietary name, and the industry-standard name is actually “TB-35”. Nobody calls it that, but it’s interesting. I found this in an old ECG catalog, and it agrees with Toshiba datasheets.

RM-60 and TB-34:

Often incorrectly referred to as “MT-100”, maybe because it looks like a smaller version of MT-200. Nope! MT-100 is Sanken’s name for TO-3P.

RM-60 and TB-34 are basically identical to each other; just a slightly different shape. They’re made by NEC, Fujitsu, Toshiba and others. They have the same 24.4mm bolt spacing as MT-200, but are slightly smaller, thinner, and have exposed metal tabs on the sides. They have lower thermal performance than MT-200, due to less contact area, and a smaller copper mass overall. They don’t have the double-thick layer of copper in the middle, so the MT-200 adapters are actually thermally superior to RM60 or TB-34.

Make sure your heatsink has enough space for the slightly larger MT-200 adapter outline. You might need to get creative with the mounting. If you really need the adapter to be smaller, use a belt or disc sander to shave them down to size.  (Or a file and some patience.)

You might need new thermal insulators, as the originals may not be large enough.

An RM-60 adapter is in the works for applications where it really needs to be smaller than the MT-200. Let me know if you’re interested in this, I’m not sure how much demand there would be.

Hitachi HPAK:

Smaller still! Same 24.4mm bolt spacing. Hitachi HPAK is used in Pioneer, Sony, and Hitachi amps and receivers, among others. Hitachi also made power MOSFETs in this package. The adapters should work for the Pioneer SX-1080. Send me pictures if you succeed! (If you cannot make it work for the SX-1080, I will refund.)

XM-20:

This designation is a mystery to me… It seems to be either very similar or identical to RM-60 and TB-34, or it’s a name for BatWings, which brings us to…

Bat Wings:

These adapters are not compatible with “Bat Wing” transistors. An adapter for Bat Wings is currently in testing.

Bat Wings are much smaller, with a bolt spacing of 17.5mm. They resemble TO-220 devices on a larger heat-spreader. (260mm²)

Bat Wings don’t seem to have an official package name, at least that I can find on the internet. If you know, or can find it in an old book, let me know!

Installed examples:

From Arif, AKA K-Amps on diyaudio.com. Yamaha M-80 heatsink with four adapters and four original MT-200 transistors.

Yamaha M-80 installed

From Dan, AKA Saabracer23 on AudioKarma. MT-200 Adapter plates replace original Hitachi HPAK output transistors on a Sony STR-V4.