-
Notifications
You must be signed in to change notification settings - Fork 176
Molex KK 254 series drill size too big (conn_molex_kk_254_tht_top.py) #465
Molex KK 254 series drill size too big (conn_molex_kk_254_tht_top.py) #465
Comments
A 0.64 mm square pin has a diagonal of sqrt(2*(0.64 mm)²) = 0.91 mm. |
Ah yes good call. It's a square pin indeed.
Yes exactly, looking at the following Molex KK254 connector drawing in the bottom left corner, they also recommend 1.02±0.05 which should be enough clearance for a pin of this size. |
Here they recommend (1.19±0.05) mm. |
So far the drawing consistency, here is another one with a different recommendation being 1.14±0.05 :") I'm thinking about measuring the pin itself tomorrow to see what the pin-size really is, since some of the drawings mention that the 0.64mm is already the max diameter of the pin (as seen in D11 of this drawing; note the ø0.64mm). |
TlDr: We need to indeed fix the drill size. But we need to fix it to the datasheet suggested value of 1.19mm! (Rounding here is definitely not ok as it will increase the error if somebody orders at a board house that uses imperial dirlls) Both datasheet you linked are for a different part number so they can be discarded (the other part numbers are expected to have different tolerance ranges). You measured one device that can be anywhere in the tolerance range. You would need to measure a huge set of devices from different production runs to get any idea of the tolerance range. Plus your own measurement tolerances would also need to be taken into account (and i guess would need to be improved as the tolerances we want to measure are likely to be smaller than your measurement tolerances). So lets stay with the datasheet values. The datasheet tells us the pins are 0.64 square (but does not state where in the tolerance range it is) which results in a diagonal of 0.91mm leading to at least a drill size of 1.11mm when assuming that 0.64 is indeed the maximum size for such pins and that there is no tolerance in pin placement. The 1.19mm size is therefore not really far off from what i would expect if there are even moderate tolerances here. Especially if the pin size is given as nominal (which i expect as normally in a drawing everything is nominal unless otherwise specified). Remember the KLC definition comes from IPC which is not intended for a tight fit between the part and its leads. There is meant to be ample space for solder to properly fill up the hole without voids. If your process requires tighter holes for alignment reasons then this guideline will not apply to you. This is ok. Technology is full of tradeoffs! In your case i suspect that you had bad luck. The parts you got will likely be on the low end of the pin size tolerance range. And the drill might be on the large end (compounded by the fact that we already started with a 0.01mm increased hole size assuming your manufacturer even has a 1.2mm drill in stock. If you ordered at an imperial board house than the target hole size might be even larger than the 1.2mm requested by the footprint) |
Don't forget that these are plated holes, so there's more to it than just whether they happen to have a 1.2mm or 3/64" drill. |
With drill i mean "target final hole size". I was just lazy when writing the above. |
This fixes pointhi/kicad-footprint-generator#465 We discovered that the hole size was wrongly rounded. This now uses the suggested size as given in the datasheet
This fixes pointhi/kicad-footprint-generator#465 We discovered that the hole size was wrongly rounded. This now uses the suggested size as given in the datasheet
@poeschlr Now that I'm looking at this, I see where the problem is why I'm having so much difference in drill size compared with the datasheet @chschlue showed before. I'll come to that in a moment.
That is not entirely true, I did measure multiple of the connector series (and also from 2 different orders), but it's hard to tell if it's still the same batch. If measurements are off way more than the 5% of the datasheet, there must be something else wrong. I'm ok with staying close to the datsheet measures, because of course only the manufacturer can know of the tolerances and sizes (also because of the imperial measures as stated before).
Well I can assure you that 0.91mm with hole sizes of 1.19mm is really overzealous, that's actually why I posted this issue with close-up pictures in the first place. I even ordered a second batch of PCB's with drill size ~1mm (same as the default 2,54mm pin headers in Kicad) and they were perfect, not overzealous or too small at all.
I'm perfectly aware of that, I don't even prefer tight fits, but when using connectors, it shouldn't be rotating on the board in all kind of directions like they do now.
I thought so as well until a few batches (PCB's and connectors) still had the same problem. Ok and here comes the part where I must apologise for the confusion. I have misinterpreted the product KK254 as the series naming of the product I've been using, which is not the same as the script implemented series (just 6410). The ones I've been using is the product KK254 6373 series, which have a drill size of 1.02mm specified in the datasheet here. So again, sorry for the confusion that was definitely my fault and thanks for the help 👍 |
@HendryKaak |
OK, will do. Thanks |
Hello again,
|
Hello,
I've been using multiple pin variants of the molex KK 254 serie connectors on one of my PCB's, but when I ordered the PCB and fitted the connectors in it was possible to rotate the connectors in place quite much. Looking into the code here I found out that the drill size is oversized compared to the KLC 7.6, which describes a hole size of the maximum pin diameter + 0.2mm in size. I would expect the size to be set to ~0.84mm AFAIK since using a bigger hole size adds up an excessive amount of solder necessary for creating a proper connection and the rotation problem as well.
Also I couldn't find any reference about this "minimum 1.1mm drill size" (as mentioned in the comment of the previous code link) in the manual (maybe something of old times?), so is there another reason for making the hole diameter this big?
The text was updated successfully, but these errors were encountered: