by Jeff Johnson
Why would I want to concentrate on designing a circuit
board that is a piece of cake to build? Isn’t that for the
manufacturer to worry about? When they have a list of
things to do, they just make the next board in line, right?
Well, yes and no. You see, there are small, behind the
scenes decisions made every day. Some boards are put at
the top of the “to do” list and some are put at the bottom.
Even if you aren’t making a microwave communications
array for NASA’s next exploratory rover, someone else is.
While your board might not cause problems in the shop,
you still want to make sure your project stays on schedule
while there is a redo on someone else’s.
In this article, I’ll talk about a typical two-sided board
and some of the things you can do to make it easier to
manufacture. Some like to call it DFM — Designed For
Manufacturing. I like to call it designing a “big, dumb
board”: big holes, big traces, big pads — any dummy could
make it. While I won’t be discussing any specifics about a
multi-layer board (they already have a complex design by
default), many of the same principles apply.
When reading the design rules specified by your manufacturer, remember that these are the maximums, not
guides. Just as you wouldn’t test the 60-0 MPH stop time on
your car at every light, don’t push the manufacturer
to the limit with your design if you don’t have to.
You have direct control over three steps of the
circuit board making process: 1) drilling, 2)
imaging, and 3) screening. In drilling,
we’ll discuss just that — drilling — more
specifically, hole sizes. Imaging will
include resist imaging, plating,
and etching. Screening is the white
character screen that makes your
board look so professional.
Drilling is pretty self-explanatory. You
are poking holes in copper. What many
people probably don’t know is just what the
capabilities and limitations are regarding
typical circuit board drills. The drill spindles
are very unique and come in two basic
styles: ball bearing and air bearing. The type
that your manufacturer has won’t matter to
you. Simply understand that these spindles
have high end speed ranges. Their lowest
speed is either 14K or 20K RPM — depending on whether
they are air bearing or ball bearing — and the upper range
is from 80K to 120K. To put that into perspective, your
car’s engine probably red lines at about 6,000 RPM.
Don’t specify too small or too large of a hole.
To drill a .250” hole, the ideal speed would need to be
about 7,500 RPM. This is not possible, so the minimum
speed is set and drill bits dull fast. Large bits are good for
about 1/4 the number of holes as smaller sizes. At the
other end of the scale, you probably don’t want to specify
a hole that is smaller than about .028” or so. Holes as small
as .001” are possible in some of the more advanced shops,
but .028” seems to be a “magic” number. Any smaller than
this and anything that can go wrong, will. Namely, small
bits break more often — much more often. Specifying a
.008 inch hole is a sure fire way to slow things down. Try to
keep component hole sizes in the .030’s and .040’s and
mounting holes around .125”. Most manufacturers would
probably prefer to keep the hole size .250” or less.
Don’t make 14 different hole sizes.
While we are talking about holes, you are
the one who specifies the hole