Lawmate 2.4GHz TX Tuning instructions - Additional suggestions

[rdibley]

In the Builds, Tuning & Modifications section, there are two different approaches to improving the cooling of the Lawmate VTx, one simple, one more challenging. I did the more challenging approach. I discovered a few things along the way that might help out.

First, don't remove the SMA connector. It's a pain to solder it back on the case. I discovered that the center pin of the connector and the plastic around it can be pushed out of the SMA housing from the back side. I didn't get to try this, but it is probably a simple matter of heating the solder on the center pin while pushing on the plastic to move it off the PC board.

The second thing I noticed is that, in the TBS instructions, there is a copper heatsink glued to the bottom of the PCB. In my case, there was no heatsink at all! It was just a blob of solder. I'm guessing they've changed their manufacturing process? I cleaned it off and cut out a piece of copper from an old video card heat sink that fit perfectly. Soldered it to the PCB and applied Arctic Silver, as instructed.

When it came time to close up the case, I thought about the alternate "simple" mod on the TBS web page, which was to use heat transfer glue to attach a copper heat sink to the top of the chip. Instead of doing that, I cut out another piece of copper (it just happened to be the right thickness) and soldered it to the top of the case. Instead of having to mess with glue, I just had to apply more Arctic Silver to the top of the chip and I was done. One thing to be careful of is to leave a little bit of a gap where the copper meets the side flange of the top of the case. (see the attached picture) That gives clearance when you attach it to the bottom of the case. So now, the chip is sandwiched between two pieces of copper connecting it thermally to the top and bottom of the case. I'm planning to move the external heatsink to the bottom of the case, as with the "challenging" approach, since there is more surface contact with the case, and I suspect the PCB was designed for the heat to transfer off the chip through the bottom.

Hope this helps!

[Scotttu]

I like this option since I have an unlocked (oops) 2.4

My only concern, following the instructions, the copper I got leaves about a 1mm gap.

I put arctic silver thermal compound on it - but I understand that's conductive.

if it heats up and changes viscocity will it short out the RF chip?

[rdibley]

I think you have a good point about using too much arctic silver. I'm pretty sure it is conductive, and based on my past experiences using it on CPUs, I'm pretty sure I remember seeing it flow a little (not very much) if some of it extruded outside the heat sink.

The method I suggested is an adaptation of the "Quick Lawmate 2.4GHz TX tuning" on the TBS website. In that example, the author also had some copper that was too thin to reach the case. The solution was to mix thermal adhesive with the arctic silver to glue it onto the chip and to bridge the gap.

You could either take that approach, or find a thicker piece of copper and use my approach. The advantages of my method are that you don't need to buy any thermal adhesive, and you can still open up the case at a later time.

[Scotttu]

I got it close enough and put the silver right on top and mushed it together.

For the top heatsink I cannot find the thermal adhesive so I put arctiuc Silver in the middle and directly over where the RF was, and epoxy everwhere else and mushed those together with a 1lb weight - it's not going anywhere and has better thermals...

All the same when I test this I'll use a mini fan just to keep it cool...

[Airborne]

Thermal compound works best when you use the least amount possible. It is designed to fill in the imperfections between two flat surfaces that are in contact with each other, not to fill gaps.

Arctic silver is supposedly non-conductive, but it will still cause major problems if it bridges traces or parts inside your transmitter.

[ofir]

Hey

what kind of Arctic Silver Thermal did tou used?
ASTA-7G or AATA-5G

[rdibley]

Hey

what kind of Arctic Silver Thermal did tou used?
ASTA-7G or AATA-5G

I'm not sure if you were asking me, but...

I soldered the copper pieces directly to the case. I then used Arctic Silver 5 between the copper pieces and the PC board and chip. I don't know if Arctic Silver adhesive would have been better than soldering?

I did use AATA-5G to glue the external heatsink to the other side of the case.

[ofir]

Actually I don't know what the difference between them, and about the solder I don't know how well it heat transfer.
But when I'll find out I'll write it here

[rdibley]

I couldn't find anything on the Arctic Silver web page, but found other pages that say that both the Arctic Silver grease and the adhesive are around 8W/mK. According to this page:

http://www.electronics-cooling.com/2...ty-of-solders/

the thermal conductivity of solder is much higher, but by how much depends on the type of solder used. They list a lead-tin solder at the bottom that has a conductivity of 50W/mK.

[ofir]

You sold me! then soldering it is :-)

I'm considering to solder the heat sink(of lawmate) to the back of the case....
what do you think?