Reverse mount LED cutout


#1

Hi,
Looking at some reverse mount LEDs which seems to be common size and footprint is exactly the same, APTR3216SYCK, PG1111R and LTST-C230KRKT.

Was curious to how PCB manufacturers deal with the small 2.1x2.3mm rectangle hole? Will the corners be so rounded from the drill bit that the square LED won’t fit? Should I rather go for a drill hole with diameter like the square diagonal (taking from solder pad space)?

The square “head” of the LED is 1.6x1.4mm


High Voltage Footprints with PCB Cutouts
#2

Many PCB manufacturers have 2mm minumum milling router in standard. They offer smaller (for ex. 0.5mm) but at extra cost. So I think you have two options: drill hole (red) or slot (gray).

IMO the hole drill is worse than slot in case of automatic assembly (solder paste will fill the hole).


#3

Most probably live with a circular hole. The expensive way would be to punch.


#4

Thanks for answering.
Just to be sure, they don’t route along the edge lines, they route the side of the bit along my edges? So that in the design below, I will have some inner rounded corner to the left. The PCB specs from pcbway says to keep at least 1.6mm between boards when panelizing, so I think their standard routing bit is 1.6mm.


#5

I meant: > So that in the design below, I will have some inner rounded corner to the right.


#6

Yep, the router/milling bit has a >0 diameter, so it’ can’t cut inside sharp corners.
For outward corners that’s not a problem, for inside corners you either have to file them manually or have the bit cut past the edge you want and cut in a bit into either side with 1/2 diameter depth, to get the material removed.

For 1.6 mm thick pcbs I usually use 1 mm milling bit diameter.
For 2 mm boards I make the ‘slots’ 1.5 mm wide.

PS: you can see how they adjusted the pad where the milling bit would potentially cut into the copper (=increased wear) and set it back a little (the PTHs are centered in the original pad).

PPS: that’s what I needed the sharp internal corner for:

One board slipping sideways into another one with no need for mechanical deburring/filing. Just soldering for the mechanical connection and done.


#7

I believe the most common practice (in 2016) is to run the edge of the routing cutter at the center of the line that defines the outline shape. (A few places may still route to the inside edge of the line, or the outside edge, but I believe the center of the line is by far the most common practice. If the line is thin enough - such as 0.15mm/0.006" - the question of center, inside, or outside is irrelevant. The errors due to mechanical wear on the cutter, and positioning accuracy of the CNC system, are greater than the uncertainty of cutting to the center, inside, or outside, of the line.)

So yes, the inside corners will have a radius equal to the size of the routing cutter. The 1.6mm guess for PCBWay’s routing bit may be accurate, or it may be a bit size that is ONLY used for panelized boards. (A larger diameter cutter may be used for routine edge routing of single boards - which helps explain why everybody charges more for panelized designs.) Ask PCBWay what cutter size they use for standard outline routing. The outside corners will be square - and perhaps sharp enough to constitute a hazard to people handling the boards.

Dale


#8

Thank you both for answering me. It is what I expected then and I am soon ready to send the boards to fab.
By the way Joan_Sparky, nice assembly there.


#9

I have done reverse LEDs with an oval-slot as shown above.
They have some allowance in the slot and on their sizes, so I adjusted my slot length down a smidgen as a circle centre on the box-edge is over conservative. Worked great, and the LEDs we used even side-light the PCB too…

In your design as shown, I’d be a little wary of flex across the large common slot ?
Is that a possible risk, or is the PCB always screwed down well ?
If the LED placement is already constrained, you could reduce the RHS overhang to ‘just enough’, to lower the flex in proportion.


#10

Thermal expansion can also catch you when you solder two boards directly like that