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I'm building an LCD display test harness on a perf board. I'm looking for a solution for connecting an 14 pin FPC/FFC cable to a 2.54mm perf board. Unfortunately, even with a digital caliper, it's very hard to measure the precise pitch of the cable. A quick search lead me to an article titled How to know the Pitch of FPC /FFC Ribbon Cable?, which explains an easier method for finding the pitch.

I measured the width of the cable several times with two slightly different results, 18.8mm and 18.7mm. I divided each by 15 (14 conductors + 1), giving me 1.2466mm (18.7mm width) and 1.25333mm (18.8mm width). Presumably, the pitch is 1.25mm since the article mentions it's a standard FFC/FPC pitch.

However, I'm thoroughly confused when trying to find a suitable connector. Using a pin header as a gauge, the existing connector's PCB footprint is 2.54mm with the second row of holes shifted over by what looks to be half the pitch or 1.27mm, not necessarily 1.25mm I calculated for the cable. I searched for more information and found a DigiKey article. The formula for calculating the pitch doesn't add 1 to the number of conductors which results in around 1.31 which is way off.

Is the pitch of the PCB connector different from the pitch of the cable? Am I dealing with an odd ball connection, or am I measuring incorrectly? How do I determine the pitch of the cable and the connector's footprint? Moreover, is it even possible to adapt a FPC to an IDC connector?

ASIDE: As a software engineer, I'm a bit puzzled by this experience. I always worshipped real engineers for their precision, consistency, and clarity when it comes to specifications and documentation.

Bottom of side connector with ribbon cable

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  • \$\begingroup\$ "I divided each by 15 (14 conductors + 1)". Should that not be "I divided each by 13 (14 conductors - 1)"? 14 conductors have 13 gaps, don't they? \$\endgroup\$
    – Transistor
    Commented Jun 7 at 21:23
  • \$\begingroup\$ I got the formula from the first link, which I suspect to be wrong. N-1 doesn't seem to be correct either as the result isn't anywhere near 1.25mm nor 1.27mm. \$\endgroup\$
    – user148298
    Commented Jun 8 at 1:45
  • \$\begingroup\$ Where are the datasheet link for your parts? There should be a dimensioned and toleranced drawing. Why would you want to use a part without specifications? \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jun 8 at 3:48
  • \$\begingroup\$ It's an old STN 14-pin display. The datasheet doesn't mention anything about the connector. \$\endgroup\$
    – user148298
    Commented Jun 11 at 1:48

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Measure exactly the center-to-center* distance between the first and last contact and divide by the number of contacts minus 1 (because there are N-1 spaces between N conductors).

(* Or, equivalently, the edge to edge distance on the same side of the contact.)

I measured the width of the cable

That's not helpful because there is an unknown additional width on either side of of the cable.

On a side note: This is one of those few cases where the PCB pitch and the mating pitch are different. Specifically, the PCB pitch is twice the mating pitch because the PCB termination has two rows but the mating face has only one row. I.e., the PCB pitch is 2.5 or 2.54 mm, while the mating pitch is 1.25 or 1.27 mm.

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