By Mark Thompson | Published on: December 01, 2016
Expressing VIA tolerances on a drill drawing
I get this phone call at least once a week.
"Hey Mark, what is the smallest mechanical VIA that can be done by your company?"
This is followed on my end by, "What will the tolerance for the VIA's in question be?"
If they say, "Oh your standard +/- .003 tolerances", I am obligated to tell them the main VIA would be around .0078 with a signal pad of at least .014 and an anti-pad of at least .018. At which point I hear a lot of choking and coughing and they say, "But I am egressing from a .4mm pitch BGA, I don't have that kind of room!"
This is where we talk TOLERANCE. If they are true VIA's where the finished size is NOT of any consequence we say, "Why not call them out as +.003 MINUS the ENTIRE hole size?"
At that point we can now tell them we can drill smaller and need less signal pad and anti-pad size which open the customer up for some routing for these fine pitch parts. While we are at it let's talk 'Pitch'.
Many times we are approached by customers talking about a specific 'Pitch' between mounts or pads like .4mm or .5mm. Understand that without a design to physically LOOK AT via image data it is difficult for a fabricator to properly understand what PITCH means to them.
The 'Pitch' is the distance between the CENTER of a given entity to the CENTER of an adjacent entity. This can mean different things based on pad/mount size.
Let me give you an example as an SMT pad:
.5mm 'Pitch' means .0197 between the centers of two BGA pads or surface mounts. This actually seems like a very reasonable distance given today's circuit board geometries. However, what if the designed surface mount pad width is .015 wide? This would mean a .5mm 'Pitch' would leave only .0047 edge to edge between the mounts. This gives you little or NO room to rout a trace between mounts at that width and pitch.
Clearly as the 'Pitch' between parts decreases based on today's smaller and smaller chip footprints the associated surface mount or BGA pad widths need to decrease as well to be able to rout even a .003 or .004 trace between them.
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