Description

Solid-copper heat-spreader moves heat much faster than mounting a TO-220 directly to the heatsink, enhancing its power handling capability.

The best way to replace an BatWing transistor! Equal or better thermal performance than an original BatWing transistor. (Test results are from an MT-200 device, but the concept is the same.) Adapt your choice of TO-220 transistor to a BatWing-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!)

Why did BatWing transistors even exist? 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.

The BatWing package has more than double the copper heat-spreader contact area of TO-220.

Transistor dies were more expensive to manufacture back in the 70s and 80s, so squeezing extra power-handling out of a them with a large copper heat-spreader made a lot of sense. These days, designers will opt for using multiple TO-220 devices in parallel, or simply go up to a larger package like TO-3P.

Surface Areas of interest:

I measured and calculated the area of the heat-spreaders of output transistors in TO-220. BatWing, TO-3P, TO-264, MT-200, and TO-3 packages, as well as the Hoppe’s Brain BatWing 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. Picture shows the back of an NEC 2SB616 next to an adapter, and an OnSemi MJE15030.

TO-220 – 104mm² (MJE15030
TO-3P – 207mm² (NJW1302)
BatWing – 260mm² (NEC 2SB616)
Hoppe’s Brain BatWing to TO-220 Adapter – 286mm²
TO-264 – 313mm² (MJL1302)
MT-200 – 592mm² (2SA1169)
TO-3 – 620mm² (MJ21193)

Hoppe’s Brain Batwing adapters have slightly more contact surface area than an NEC BatWing, because the entire back surface is copper. The BatWing has some plastic areas on the back that subtract from its total surface area. Additionally, the transistor/adapter combination has two layers of copper substrate, and thus more copper mass overall. The TO220/Adapter combination should move heat slightly better than an original BatWing transistor.  I will be doing more science on this in the future, but I believe this is a valid assumption based on the physical geometry of the copper mass, and the results obtained in the testing of the MT-200 version.

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 heat-spreaders)
• 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
• Solder, Tin/Lead 63/37 Eutectic – 50 W/mK
• Mica – 0.7 W/mK
• SilPad – 0.9 to 5 W/mK (Higher performing SilPads get expensive real quick.)

Silpads and Mica 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.

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. You can use a screw to spread the paste at a consistent depth. Like a notched spreader for spackle.

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 MT-200 adapter 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.)

Installation:

*MT-200 devices shown here but procedure is the same for BatWing.

The copper substrate of the adapter is connected to the transistor’s collector. Use thermal insulating pads and shoulder washers as original.

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.
  • Batwing 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 15x26mm. Use a leather punch to make 3mm bolt holes spaced at 17.5mm.

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.

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:

Coming soon! Send me your success pictures.