What is the thinnest strip of pcb you can put on a pcb that is reliable?

Discussion in 'General Electronics Chat' started by Gibson486, Oct 2, 2018.

  1. Gibson486

    Thread Starter Active Member

    Jul 20, 2012
    What is the thinnest I can get away with? I have a screw hole that is awfully close to teh edge. I am afraid it may be a little brittle. The hole is about 0.5mm from the edge.

  2. dl324

    AAC Fanatic!

    Mar 30, 2015
    It depends.

    1. What is the board thickness? Material?
    2. How is the hole being made? Drilled or punched?
    3. Is it the only hole?
    4. Purpose of the hole? Support board vertically or horizontally?
    5. Board dimensions?
    I've punched holes in CEM-1, which isn't recommended, and have successfully punched within 1.5mm of board edge. Even in cases where the punch crushed board material, a screw could still hold the board in position.
  3. ebp

    Well-Known Member

    Feb 8, 2018
    [EDIT]: I wasn't paying attention and the following does not answer the original question. D'oh!

    Half a millimetre is a wide trace by today's standards, though around a screw hole it is likely to get "chewed up" which could result in little bits of copper that might cause grief, especially if a lock washer is to be used.

    Unless there is a need for electrical connection via the screw, which can be a bit dubious, I prefer screw holes to be unplated and have no annulus. If electrical connection is required, an annulus that extends a bit beyond the head of the screw was my preference. I see lots of computer mother boards with an annulus with a bunch of small vias all the way around.

    If you are using a "full service" board house you can have holes left unplated, but exactly how that is done depends on the process and the size of the holes. If the pad is smaller than the hole, the hole might get plated and then the plating etched away. I've use that method with prototype boards, but had inconsistent results from the same board house. Some came back with nice clean holes, some (different batch) came back with a more or less intact barrel through the hole. Unplated holes may be drilled after the board is etched, which may cost a bit extra - it depends on whether the board has to go back to a drilling machine or if they can/must do them on the routing machine. Big holes are usually routed since drills all have 1/8" (3.175 mm) shanks and there is a limit to how much bigger the actual drill is relative to the shank. It is always best to consult the board house in advance.

    For general work, "8 and 8" (8 mil tracks, 8 mil spacing; 8 mils = 0.2 mm) is regarded as easy by any reasonable board house unless you are looking for very heavy copper. 4 and 4 is pretty much required for lots of ball grid array parts.
    Last edited: Oct 2, 2018
  4. ebeowulf17

    Distinguished Member

    Aug 12, 2014
    All great info, but I think the thread starter was asking about leaving a thin bit of PCB material between hole and edge, not about copper traces. In other words, I think the question is how much clearance you need between a hole and the board edge.
  5. ebp

    Well-Known Member

    Feb 8, 2018
    "... I think the thread starter was asking about leaving a thin bit of PCB material between hole and edge,"

    Yes, you're right! :oops: I blame my inattention to having had to shovel way too much snow today. Or too many crows hanging around shouting "Polly want an eyeball!" all day.

    Half a millimetre is pretty close, but if you are using an epoxy glass laminate such as FR-4, it is incredibly tough stuff. That looks like a pretty small screw, so I doubt if you'll have any problem. One thing to be aware of is that the tolerance on board routing can be a bit on the sloppy side, so your half millimetre might get further reduced. Even if you crack such laminates, pieces tend to stay attached. Try beating up a scrapped circuit board.

    Phenolic laminates are miserable, brittle and weak, in general. I've seen corners broken off boards at a mounting hole when it was set 2 mm or more in from each edge. You can crack it if you over torque a screw, especially if if the underside of the head isn't nicely formed or a standoff is small.

    If the blue part represents the actual hole, you'll probably have part of the screw head hanging over the edge of the board unless you are using something like socket head cap screws - which are a bit expensive, but can be nice for such circumstances.

    I developed a fondness for stainless steel hardware. I had a supplier whose prices weren't much more than for parts made of soft mushy steelish metal and the forming quality was also better. But I never was involved in consumer stuff where a fraction of a cent extra per screw would get someone worked up about my squanderous ways.
    ebeowulf17 likes this.
  6. Gibson486

    Thread Starter Active Member

    Jul 20, 2012
    standard .062 inch.
    2 holes....the other is about 90 mm away from that hole.
    To screw the board down vertically (note that there are 2 additional screw holes that will not have this issue).
    About 50 mm x 100 mm

    I told the team that we could just make the hole a U, but they did not like that idea.
  7. kubeek


    Sep 20, 2005
    Unless there are some serious lateral forces I would not worry about it.
    Also note that part of the screw head and washer will most likely be outside the edge of the board, so it could collide with something else.
  8. dl324

    AAC Fanatic!

    Mar 30, 2015
    So 4 holes total?
    Unless there are significant lateral forces on the hole, your the main issue will likely be whether the board house design rules allow it. You could consider using a lock washer to increase the friction between the screw head and board.
  9. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    I try to keep the closest edge of a thru-hole at least 0.040" (~1mm) from the edge of a 0.062" thick PCB.

    When constraints do not permit this, I do this: