Monday, May 2, 2016

Use a Dremel Router to Prototype Circuits with SMD Parts

(Little pads for a little part)

My projects are almost always "one-off" projects.  That is, I build one and that's it.

And, although I've overseen the layout of many PCBs for my designs over my career as an engineer, I never make a PCB for my own personal projects.  I find that it is much faster for me to just build the circuitry on a piece of "vector-board" (i.e. a board with or without plated-through holes, usually on 0.1" centers) or "dead-bug" style on a piece of unetched copper-clad PCB material.

(Another construction technique is known as "Manhattan-style", which requires gluing (or soldering) small pieces of PCB to an underlying copper-clad PCB base.  I don't use this technique because, to me, it has always seemed to be a bit too much work, and I have also heard that, if gluing the pcb "pads" to the base, heat from a soldering iron can sometimes cause the glue to soften and the pad to shift.)

For me, my "vector-board" and "dead-bug" techniques work great for most of my projects, unless those projects use surface-mount devices.

SMD resistors and caps aren't usually a problem, as they can be easily soldered between two pads on a vector board.  But there can be issues when trying to incorporate multi-pinned SMD ICs.  I find that the tiny pin-to-pin spacing is difficult to work with, especially when trying to solder wires or components to them.

But SMD has much to offer the circuit builder, not least of which includes smaller circuits and better EMI and Susceptibility performance.

I've tried creating pads in the PCB copper using an Xacto knife to cut through the copper and then removing the unwanted metal with the hot tip of a soldering iron.  But this is time consuming -- I need to make several passes with the knife before I've cut deep enough into the copper to allow its removal with the soldering iron's tip.

But there's a better way...

Recently I was thumbing through a catalog of luthier tools from Stewart MacDonald and I came across a Precision Router Base for a Dremel tool:

With the additional mass and stability it would add to a Dremel tool, this base looked to me to be perfect for cutting SMD pads on PCB material.

(Click on image to enlarge)

So I ordered one, gave it a try, and was happily surprised at the results.

Here's what I do...

The Procedure:

1.  I first draw the outlines of the pads (or traces) on the PCB stock:

2.  Then, before routing, I verify that the pads (or traces) have the correct locations and sizes:

3.  After mounting the bit in the Dremel tool and adjusting the router-base height so that the bit just barely protrudes below the base, I then carefully route along the outlines I've drawn (in the example shown below I'm using a 0.1mm, 30 degree Engraving Bit with the Dremel tool).  Note there is some wiggle on some of the lines -- they aren't perfect.  But these cuts are much straighter than if I had tried to "freehand" the routing without using this base.  (And they could probably be improved upon if I used a straight-edge to guide my router-base):

4.  I can then mount the part (i.e. solder it to the pads).  However, before soldering, first verify with an ohmmeter that all pads are isolated from one another:

That's all there is to it.  Pretty simple! 

What bit to use?

I experimented with four different bits:
  1. 1/32" Diameter Straight Router Bit (Stewart MacDonald item 1187)
  2. Tapered Router Bit (Stewart MacDonald item 1180)
  3. 30 degree, 0.1mm Tip Diameter Conical Engravers Cutter V-bit.  (Note: you can find these on Amazon and eBay for less than the Endpoint version identified in the link above).
  4. Dremel 105 Engraving Cutter Bit (1/32").
Of these four bits, the "30 degree 0.1mm" tip gave me the finest engraved lines, making it best suited for cutting SMD pads.

The Dremel 105 "Engraving Cutter" bit (which has a cutting-ball at its tip end) also works well, but it results in a broader line -- useful if you need more pad-to-pad isolation (i.e. if creepage and clearance distances are issues).  Note, too, that there are other "Engraving Cutter" bits with larger diameter balls at their ends, should you desire even greater isolation between pads or traces.

The two Stewart MacDonald bits were less than satisfactory.  The first reason is that they require a 3/32" Dremel collet (which I discovered I did not have, and so I needed to purchase one).  These two bits result in rougher edges along the cut (in fact, I took two passes for the 1180 bit route in the photo below -- it was much rougher with only one pass).  Plus, being flat-bottomed, a cut is a little bit harder to start with the Stew-Mac router bits, compared to the 105 (ball-tip) bit or the 30 degree, 0.1mm tip.

(Click on image to enlarge)

Other accessories:

I also purchased a companion Mini Air Pump from Stewart MacDonald to blow away the dust as I was cutting the pads.  It helps, somewhat, but a significant amount of debris is still left on the board and it can obscure what I'm trying to cut, so the pump's usefulness is limited.  A more powerful air pump would probably help.

That concludes this post.  If this construction technique interests you, I would recommend experimenting with different tips and Dremel motor-speeds to see which combination works best for you.

Other Notes:

This method works well with SOIC pin spacing.   But I personally find TSSOP pin spacing to be a bit too tight to use this method.

Standard Caveat:

As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations.  If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.

Also, when using a Dremel tool, be sure to always wear protective eye-wear.  This caution is especially important when using some of the fine-tips I mention above, as it is possible to break them if one isn't careful, and you do not want a piece of a tip flung into your eye at high speed.

And finally, I should add -- this design and any associated information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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