Re: Rough edges on PCB traces

jeff.birt
 

Not all engraving bits are good for milling PCBs. Some do a really good job, others do a bad job. Whatever bit you use it will need to be carbide or it will not hold up no matter what it is coated with. The PCBs substrates are very abrasive and will quickly ruin a HSS/Cobalt bit. A 10 degree bit would not be my first choice. I mill almost everything with a 60 degree bit s it provides a good compromise between minimum trace resolution and single pass width of cut. The minimum I use is a 45 degree bit, I have not found anything shallower of much benefit.

 

For etching PCB, or anything really, 0.010” of run out is a HUGE amount. With a tiny engraving bit and that much run out you will quickly ruin the bit. The first thing I would is getting a more precise spindle. A Dewalt 611 with a set of PreciseBits collets is a good choice for smaller machines: http://www.soigeneris.com/precisebits_dewalt_611_precision_collets-details.aspx . Then look at the bits you are using. I used PreciseBits PCB bits years before opening up shop and becoming a dealer. This page has the PCB related bits so you can see them all: http://www.soigeneris.com/pcb_making_bits-list.aspx . With these bits a good starting point is to feed 1IPM for thousand spindle RPM.

 

I also found out recently that imperceptible vibrations in your PCB stock during the machining process can cause a poor cut quality. I had someone at the university ask me to machine some ‘Duroid’ PCB stock which is a 0.025” thick dual layer, Teflon core used for HF applications. It is very expensive and a bit like trying to machine a slice of cheese with copper foil on both sides. My first test came out very, very badly. I talked to Ron Reed at PreciseBits and he suggested that the problem might be with the stock vibrating whilst being machined. I took a small piece of the Duroid and used 3M Super 77 spray adhesive to glue it down to a ½” cast acrylic substrate. This not only had the advantage of preventing any vibrations but it held the stock down very, very flat. I was able to use a DOC of only 0.0014” (and 0.001” in one spot) with a 60 degree bit and machine two nice double sided microwave detection antennas (about 1.5”x1” in size). The board shad stapling (vias between the ground planes on both sides) that called for a 0.010” through hole (I had to do 0.011” as that was the closest bit I had on hand.) Even with the 60 degree bit I was able to resolve 0.010” wide traces. To release the glued down board you just need a hair dryer or low power heat gun, the 3M Super 77 releases about 150F and is easily cleaned up with acetone.

 

Jeff Birt

Soigeneirs.com

 

 

 

From: pcb-gcode@... [mailto:pcb-gcode@...] On Behalf Of scott@...
Sent: Monday, May 05, 2014 5:54 PM
To: pcb-gcode@...
Subject: [pcb-gcode] Rough edges on PCB traces

 

 

I could use some advice on why the PCB trace edges are so rough (see attached pic).  I'm new at milling PCBs.  I have a ShapeOko 2 with a DeWalt DW660.  I'm using 10 degree engraving bit (titanium coated). Cutting depth is about 0.005", 15 IPM feedrate, 30,000 RPM.  I have about 0.010" runout.  Most of the edges are very rough, but some edges are really smooth.   Any thought to what my problem is?  Why are some edges nice and smooth while most are not. 

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