Description

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

For Pioneer SX-980, SX-1280, and SX-1980 receivers, Phase Linear, Luxman, MCS, and others.

Replace those weird NEC 2SD746 / 2SB706 transistors without compromising thermal performance! Adapt your choice of TO-3P or TO-247 transistor to a solid-copper heat-spreader. Outstanding thermal performance. (Link to thermal testing of similar-sized MT-200 adapters. Testing of the 2SD746 is forthcoming.)

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!

Why the big transistors? 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 large-package transistors like the 2SD746 and MT-200 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 various packages, as well as the Hoppe’s Brain 2SD746 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)
NEC 2SD746 / 2SB706 – 467mm²
MT-200 – 592mm² (2SA1169)
TO-3 – 620mm² (MJ21193)
Hoppe’s Brain 2SD746 Adapter – 686mm²

Hoppe’s Brain adapters actually have 219mm² more surface area than 2SD746—46% larger—because the copper goes all the way to the edges, while 2SD746 has a plastic border around the outside that subtracts from its total surface area.

I don’t mean to make too much of this, there are diminishing returns on the size of the heat spreader; 46% more area does not mean 46% better cooling. But it’s awesome that it’s clearly better!

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, 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. This is kind of mind-blowing; Diamond is so hard that it conducts sound extremely quickly, and the heat-vibration of molecules is dispersed as sound waves!

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
• Powerful soldering iron with large chisel tip (≥70W
• 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.)

Collector leads:

The 2SD746 has two collector leads on the outside, instead of one in the center. (Why, NEC? WHY??)

The collector is connected to the copper substrate, so we’ll solder two collector leads to the outside.

Add a blob of solder to the adapter. You’ll need a powerful soldering iron with a large tip.

Melt the collector lead in place. I’m using a locking hemostat to hold the lead while I solder.

Cut off the center collector lead, and bend the base and emitter leads like so:

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 regular transistors. 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) Mounting holes are metal and are electrically connected to the collector. This is different from the original transistor, which has an insulated plastic body. For this reason, electrically insulating nylon shoulder-washers must be used, and are included. There are two sizes included; for M3 or #4 bolts, and for M3.5 or #6 bolts. The M3 types might fit tightly

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.
  • 2SD746 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 30mm.

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 might 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 a 2SD746 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 M3 / #4 spacer kit here.
Order M3.5 / #6 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.

Alas, you might find your bolts are now too short, because the adapter+transistor is slightly thicker than the original transistor. 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.

Pioneer 80-series receiver owners: The metal transistor brackets should not be used, and are unnecessary. (The bracket could touch the metal body of the adapter, shorting it out.) There is a little metal fork-tine thing in the middle of the bracket, and on some receivers, there is a small bypass capacitor soldered to the fork-tine, and the other end is connected to the the B+ or B-. The fork-tine connects the capacitor to the heatsink and chassis. In place of the connection made by the fork-tine, simply install a ring-terminal onto one of the transistor bolts, and solder the capacitor to it. Here’s a ring terminal that fits M3 bolts.
Also, you will need shorter bolts, 12mm, so they do not bottom out in the holes tapped in the heatsink.

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.

Installed examples:

(More soon.)

Pioneer SX-1280 with TO-247 On Semi MJW1302/3281 on adapters: