Cutting a MCPCB

Question

I have a compound LED which is mounted on a metal core PCB. It has 4 different diodes on the package and has 8 leads.

The location I'd like to mount it in does not have enough space, so I need to trim the MCPCB down a few milimeters. I think I will need to cut around the perimeter about halfway into the exposed leads.

I'm wondering if cutting it up with a Dremel will damage it. I am curious as to how these MCPCB's are constructed and how the traces are insulated from the outer metal. I am thinking that if I slice it from the metal side I would be able to prevent a short from being created.

Any thoughts?

Thanks.

Edit: So I got my Dremel replaced today (it's so weird, i owned it for one month -- barely used it for 30 minutes total. Went kaput. I was initially amazed by the engineering and build quality of the tool but all of that confidence was shattered when it just stopped working for no apparent reason because it was running fine, then I unplugged it for three weeks, and then... nothing.) and I got around to trimming one of these MCPCBs and it works great. I sliced at it with the cutting wheel from the sides, because it was the most convenient way to hold it, and i have no shorting issues at all.

Answer 1

Disclaimer: I am not an expert in this, but I have cut MCB with other tools in the past to break tracks and found it very tenacious so I have some idea of what is involved.

If I was attempting your task the first thing I'd try would be a Dremel with a thin cutting wheel (the type that vanish instantaneously if you flex them when cutting :-) ). I have had immense success with these at all sorts of tasks and they seem likely to be as good a tool as any that comes to mind. The high speed and abrasion rather than bladed cut seem like they would be a good fit to the need. (For various reasons I could not us the Dremel for my above task - if I could have I would).

Cutting right through as you are doing should be easier. I would expect that the cut edge may well short between layers but that by cleaning it up with a fine file or diamond board or even an emery board (made for fingernails) that the smoother edge would probably be OK.

I wouldn't have thought that cutting from either side would guarantee immunity to shorting.

One method of construction is a very thin "pre-preg" glass-fibre layer on top of the Al - these are typically 0.25mm thick. I'd expect that they'd use FR4 but no guarantee.

Here is how Cree see it:

That diagram is from this excellent Cree technical paper

• Optimising PCB thermal performance for Cree XLamp LEDs

Answer 2

I've cut MCPCBs with a Dremel and the brownish red cutting wheel (made from emery?). The isolation layer on the boards I've used is some sort of ceramic substrate and it seems to bind the copper slightly less than good quality FR4. For this reason I think it is best to cut it from the copper side.