[quote=“Michel, post:9, topic:143”]
I personally prefer to adopt a design style that is constant over time. I do not want to make my design PCB manufacturer specific, as you never knows where the job may end up, although I use the same PCB shops.
Drill size is no different and I go with the part manufacturer specification assuming it is the finished hole size . . .
[/quote] These days (2015), it seems to be almost universal practice for board designers, and board fabricators, to speak in terms of the FINISHED hole size (i.e., after plating, cleaning, etc - ready to be put in a box and shipped). At least that’s how it has worked with the half dozen or more fabricators I’ve worked with over the last dozen years or so. As a practical matter, that’s the most reasonable way to do it: the board designer has no insight, and even less control, over the manufacturer’s processes. The only common interface where they can interact is the finished hole size. If the board designer orders a hole of, say, 0.035" diameter, then the fabricator is obligated to deliver a hole of 0.035" diameter.
In case there’s any doubt . . . every PCB fab drawing I recall seeing for the last 20 years or so, both drawings I created and those created by others, included a note to the effect that “Hole sizes are specified after any required plating, finishing, etc.”. (Yeah, I know, not everybody bothers to read the drawing Notes, but that’s a different topic.)
So, as a first cut, if the component manufacturer suggests a 0.035" hole, then you put a 0.035" hole in your footprint. But there are two additional factors to consider. The first is plain old tolerances. Despite his best efforts, a fabricator who intends to create a 0.035" hole may actually produce 0.034" diameter. Or 0.037". Some vendors publish guaranteed hole-size tolerances on their web pages, while others need their arms severely twisted before they’ll reveal the information. Component manufacturers make allowances for practical fabrication tolerances when they suggest a hole size. If you work with a micrometer in hand, calling out hole sizes based on measurements from sample components, and expect line-on-line fits between components and holes, you’re going to be disappointed.
The second factor is drill availability. ESPECIALLY for prototype and low-rate (up to a few hundred boards) orders, the board fabricator is going to map your specified hole sizes into the tooling he typically uses. For low-volume orders, unless you throw a tantrum and pay some hefty surcharges, your boards will be fabricated with his “standard tool rack” - typically a couple dozen drills producing the most commonly requested hole sizes. You can usually find these sizes listed on the fabricator’s web site. (And no two fabricators have exactly the same list.) If you call out a hole size that matches the fabricator’s tooling, everything is fine - he’ll deliver what you asked for. More commonly, at some point in your design the hole size will NOT match his drills. E.g., you asked for 0.035" but his standard sizes are 0.033" and 0.036". Some fabricators will round up to the next larger size (0.036" in this example); others may round down to the next smaller size (0.033"); and others will select the nearest size (0.036" again for this case). In most cases the fabricator’s mapping of hole sizes produces usable results but in a few critical cases (or if you use automated insertion machines) it’s important to know how the “non-standard” hole sizes will be treated.
(I once worked with a board fabricator who insisted that the hole sizes specified on my drawing had to match the drill sizes in his standard rack before he would make the board. That put the responsibility entirely on me to specify suitable hole sizes.)
Dale
