I've designed a LED board 8x8 where LEDs are assembled in reverse. So, there is a through hole at every LED position, LEDs assembled at the bottom, and light goes through the PCB hole and comes out the top layer.

The issue is that I notice some portion of light going through the PCB prepreg to the adjacent LED which is lit up a bit.

The first batches were OK because the prepeg had a darker color and now with a new supplier it is more transparent. What I would need is to spec the type of prepreg and core dielectrics correctly.

Which type of prepreg do I have to use to avoid LED light transmission through the PCB substrate?

This is a comparison for your consideration:

enter image description here

  • 6
    \$\begingroup\$ Can you convert those light-transmitting through holes to plated-through holes? The plating should block the light. \$\endgroup\$ Commented Jul 2, 2022 at 14:39
  • \$\begingroup\$ How about using a row of vias as a light blocker between each pair of LEDs? \$\endgroup\$ Commented Jul 2, 2022 at 14:39
  • 1
    \$\begingroup\$ Both ideas seems very interesting and good but the topic is that i've got in prodcution the PCBs. the first batchs were OK because the prepeg had a darker color and now with a new supplier it is more transparent. What i would need is to specy correctly the typr of prepeg and core dielectrics and I don't know much about it \$\endgroup\$
    – user317023
    Commented Jul 2, 2022 at 14:44
  • \$\begingroup\$ From my point of view is how to specify prepreg an core to avoid such an issue \$\endgroup\$
    – user317023
    Commented Jul 2, 2022 at 15:23
  • 1
    \$\begingroup\$ use plated holes and black* solder mask. (or some colour that looks black under the LED illumination) \$\endgroup\$ Commented Jul 3, 2022 at 12:31

1 Answer 1


Your deleted photos are unclear about the LEDs but this transparency would have been corrected by specifying a black solder mask on both sides. enter image description here

  1. optical crosstalk demands careful design specs. to avoid unintended effects.

  2. FR4 is made from epoxy resin with fibreglass and sheets are normally opaque, soldermask can vary widely.

  3. tinted FR4 requires a special request from supplier and not common stock so soldermask is used for tinting

  4. Bending leads of thru-hole LEDs must be done per the datasheet to avoid common faults as sheered gold-wire bonds are common otherwise.

  5. The clear epoxy in LEDs is softer at solder temps. and lead stress/ bends must never be made < 5mm from the base and manual solder time = 3 s, and more can induce thermal stress from thermal gradients.

  6. Handling with ESD safe practice and if open bagged, may need oven drying depending on solder method.

  7. absorbed moisture can have thermal popcorn effect micro stress on micron thin gold wire bond as clear epoxy is more prone to moisture ingress compared to black epoxy.

I have found the best way to eliminate optical crosstalk and reduce beamwidth is to use a black heat shrink. I choose the desired length over the lens body. This can also reduce beam spread if desired.

BIVAR also has many solutions.

  • \$\begingroup\$ OP didn’t ask about 4-7. \$\endgroup\$
    – winny
    Commented Jul 3, 2022 at 8:55
  • 2
    \$\begingroup\$ epoxy is usually traslucent, not opaque. \$\endgroup\$ Commented Jul 3, 2022 at 12:27
  • \$\begingroup\$ FR4 has fiberglass that makes the epoxy opaque \$\endgroup\$ Commented Jul 3, 2022 at 12:37
  • \$\begingroup\$ I think Tony that epoxy is traslucent, that coul d could be the key of the issue \$\endgroup\$
    – user317023
    Commented Jul 3, 2022 at 13:39
  • \$\begingroup\$ It`s hard to say without examining all the dimensions of the lens and FR4 and variations in LEDs, insertion depth, and tangential leakage of reflector cup with focal angle. Who can guess without all this detail/< \$\endgroup\$ Commented Jul 3, 2022 at 14:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.