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The Bare (Board) Truth: Hey, They’re Just Vias—or Are They?

From its traditional use to more unconventional uses, the via has gone through some changes over the years. In this month’s column, I will examine issues such as:

• Expressing tolerances for vias
• Blind and buried vias
• When, where, and why you should fill vias
• Via-in-pad
• Stacked vias
• Vias for thermal applications

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 vias in question be?”

If they say, “Oh, your standard +/-.003” tolerances,” I am obligated to tell them the min via would be around .0078” with a signal pad of at least .014” and an anti-pad of at least .018”. Usually, at this 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 vias 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 I tell them that we can drill smaller and require less signal pad and anti-pad size, which opens the customer up for some routing for these fine-pitch parts. While we are at it, let’s talk about pitch. Many times, we are approached by customers talking about a specific pitch between mounts or pads, such as .4mm or .5mm pitch. Understand that without a design to see the via image data, it is difficult for a fabricator to properly understand what pitch means to the customer.

The pitch is the distance from 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 SMT pad as an example:

A .5mm pitch means .0197” between the centers of two BGA pads or surface mounts. This 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 route a trace between mounts at that width and pitch.

Clearly as the pitch between parts decreases based on today’s shrinking chip footprints, the associated surface mount or BGA pad widths need to decrease as well to be able to route even a .003” or .004” trace between them. 

To read this entire article, which appeared in the November 2016 issue of The PCB Design Magazine, click here.