10 mm MCPCB

[BrianMc]

I have built electronic kits back in the day, and built and re-built my wheels and other bike stiff and car repair when they weren't all computerized, but I feel a bit uneasy when some things are not according to Hoyle. So rather than gain experience the hard way, maybe some advice would be a good idea?

Issue 1:

Need to mount XP-G R5 10 mm MCPCB to a CPU heat sink on a crude light bench I cobbled together for lens testing. I want them mounted reversibly so they can go into a second light eventually. I plan to lap them, and bolt them using threaded holes, using AA to the lapped heat sink to help thermal amnagement and allow easy removal.

I read in some thread that 4-40 screws after resizing their heads would work, but likely on the larger MCPCB forms. I don't own a micrometer and my best measurement with dividers and ruler said they would be a tight fit. Now the MCPCBs are here, I find the fit too tight. The mounting holes are about the size of the thread valleys of a 4-40 bolt, so the start up threads fit.

It seems I have two options to use bolts:

1. Resize the current bolt holes to use the 4-40 bolts but keeping the larger hole no closer to the LED to help the heads clear the LED and lens.

2. Find a tap somewhere for the smaller screws I have that are a size smaller. Or new screws and tap. Any on-lline resources recommended in US? A trip east or west to a hobby store is $20 gas round trip. USPS is much less.

Issue 2:
I want one of my lights running while I mess around with aspherical lenses and experience with I hoped a narrowish but now, a medium lens.

The LISA2's RS XPG (Real Spot) once labelled 11 degree FWHM are RNS (Really Not a Spot) 26 degree FWHM and did not arrive with the R5's. Good.

Next up: the Khatod 10 mm 10417 is better at 17 degrees FWHM though maybe a bit too floody for the road and I have three of them. They fit well on the square 10 mm MCPCB. They seem to want to hang three legs off, and one leg on the round 10 mm with the lens resting on the LED dome. I assume this is NOT a good arrangement? If not, then is the best solution to take a little off the inside of whatever legs to let the lens center above the LED dome, and make mounting pads for all four legs to the heat sink beside the MCPCB and keep the lens off the dome. Agree? other ideas?

 

[Capo_au]

Im having a little trouble following your plan so fill me in if I miss something.

So from what I can gather your trying to work our how your going to mount your PCB's for testing and then finally how to mount them to the heat sink for the finished light?

For testing I generally use any old thermal compound and a small clamp to hold it in place. Once i'm ready to mount I have been using a thermal glue from DX that has worked great. The bond is strong enough that unless your careful you will destroy the MCPCB if you try to remove it from a heatsink once set. This is a great alternative to screws as there is no risk of shorting to the pads on the board. Its thermal conductivity might not be as high as some of the compounds for CPU's, but its more than enough for this application.

Im not sure about the optics as I havent played with any 10mm size ones as yet, but im sure someone else might know a thing or two.

A quick rephrase of what your after exactly will help get you find an answer.

Best of luck

 

[BrianMc]

Try this:

Issue 1:

I have these: http://www.cutter.com.au/prodimages/10mmxpedim.gif

With XP-G R5's

I need to mount them removably and can't fit clamps.
I was planning on studs through the holes.

1. Is there any reason I can't make the mounting holes a tad bigger to fit 4-40 studs if I keep the stud heads small and clear of the LED and lens?

2. Is the thermal pad neutral? Because there is some solder on the sides of the LED near the holes on a couple of these. I think it is a non issue. Is it?

3. If resizing the MCPCB holes is a bad idea, I need smaller studs. Our hardware store can't go smaller that 4-40. Who would use 3-48 and have a tap? I don't want to wait for them. Who do I call? Gunsmiths? RC modelers? Ghostbusters?

Issue 2:

I have these: http://www.cutter.com.au/proddetail.php?prod=cut837

#10417 They want to drop three legs off the round MCPCB.

1. Anyone dealt with this match up before? What did you do, how well did it work, and would you do it differently?

I am thinking pads on the sides of the MCPCB to the same height as the MCPCB to allow the legs to be where they need be to align the lens on the LED.

2. Any comments?

 

[panicmechanic]

Good morning!:tired:

You could resize the holes, as long as you don't touch any copper paths. If you come close, a plastic or cardboard washer should help (to be found in a computer store if you can't cut some from waste plastic).
Or, forego the holes and place the screws near the pcb. You might even go to the .75" spacing for the usual star pcb, and use large washers. These would also hold the pads that support your lenses. Careful with lenses pressing onto the dome

 

[BrianMc]

The boards break off the substrate at the mounting holes. So you have flattish spots there.

Looks like the copper layer has only the thermal pad close to the holes.
I can easily stay clear of it if I enlarge the holes more to the circumference.

That brings up a related issue. Two have small spheres of solder at the sides of the LED facing the holes. I assume I should carefully prod the solder spheres to see if they are easily loosened and remove them?

I could use wider spaced stud holes for the light, but the clip mounts on the light bench won't allow it. I like the idea of rectangular washers with rounded outer corners to clear the inside curve of the light. They would spread the studs' clamping force keeping the MCPCB flatter and provide mounting for the lens.

 

[panicmechanic]

The small spheres will come off easily. That's why it is better to remove them now
The thermal path is electrically neutral, so shorting anything is not an issue with the spheres.

 

[BrianMc]

Thanks for the pic showing the need for some care.

It made sense to remove the solder balls (with care). I thought the thermal pads were neutral. I couldn't see why enlarging the holes would hurt IF the stud heads are in the clear. Just wanted to be sure.

It was a long wait for them to arrive. Don't want to wait again because I did something everyone here knew not to do.

 

[BrianMc]

In case anyone else is looking for very small screws, I found some #1 metal screws (smaller than # 2's they stocked anyway) in the spare parts bin at an RC hobby store. Fit holes in 10 mm MCPCB perfectly with just a touch of play. The heads need a flange ground off but no biggie. $1 plus a 50 mile round trip.

Discovered that with a little less caffeine, I can get about 33% of all four legs of the 10417 on the edge of the 10 mm MCPCB. Riding helps.

One XP-G R5 is test mounted with scews on the light bench heat sink.

Now I can finish Version 1.0 of the first light to get more light on my rides and test the asperherical for V 1.1. Thanks for the support :twothumbs.

 

[BrianMc]

Three 10 mm MCPCB XP-G R5 are now installed on the copper heat sink using screws and AA. Wiring required leg shortening on two of the 10417's.

Note to anyone fitting the square Carclo optics to these: if lens is aligned, you can see the die color in the whole lens. When lit, brightest in center and equal concentrically. You get about 10% of each leg on.

Have to get film for the 35 mm SLR and some distance signs to do the beamshots tomorrow night or not for a week given the forecast.