I'm sending a board for assembly with a through-hole DIP-8 package with a standard horizontal lead spacing of 7.62mm. However, at their ends the pins are wide with a pin spacing of ~8.47mm. Should I modify my footprint to accommodate this wider width or leave the footprint at its default width of 7.62mm? I'm not sure how the fab assembles the DIP packages.
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2\$\begingroup\$ Is this "as bought"? DIP ICs come with the leads slightly spread outward, but the correct spacing on the PCB is 7.62 mm (0.3 in). For manual assembly, you can buy a tool that straightens them. \$\endgroup\$– awjloganAug 9, 2021 at 13:24
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4\$\begingroup\$ "I'm not sure how the fab assembles the DIP packages" Questions like this should be sent to the assembler, not us. \$\endgroup\$– DKNguyenAug 9, 2021 at 13:31
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2\$\begingroup\$ Is there any particular reason you're choosing DIP parts? You're costing yourself money, both in parts and assembly, especially if you have any surface mounts at all on your board. \$\endgroup\$– Scott SeidmanAug 9, 2021 at 15:47
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2\$\begingroup\$ @ScottSeidman Parts shortage 😕 the original version had an SOIC8 package. However, I couldn’t find it with any of my usual suppliers. And I’m in the corner of the world where getting stuff from Digikey, Mouser and Arrow is very expensive. Ergo the DIP-8 package. \$\endgroup\$– user3418620Aug 9, 2021 at 15:51
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3\$\begingroup\$ @ScottSeidman: Alternatively, it may be possible to form the DIP leads into a gull-wing pattern and treat it as an over-sized surface-mount part. Bending the leads under could also work, but I think I've seen gull-wing form more often. \$\endgroup\$– supercatAug 9, 2021 at 22:10
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5 Answers
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It is normal for a new unused DIP package to have its pins flared outward. The assembly house will run the chips through a lead former to bend them inward to the nominal width.
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Use the standard DIP footprint with 0.300" (7.62mm) spacing. The PCBA house will compress the DIP to fit. Holes around 0.8mm.
When I assemble a few by hand, I just push the side of DIP against the ESD mat, one side after the other, to roughly get the leads the right distance. Assembly machines pick the DIP package up by the outside of the leads from the tube and compress the leads to the right spacing. Hand production assembly lines probably use tools to space the leads.
In any case, you should use the standard spacing on your PCB footprint.
answered Aug 9, 2021 at 13:55
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No changes are required. DIPS are spread to allow straightening either manually or in auto-insertion machines.
Here’s one clever tool that keeps the IC’s in their ESD protected tube.
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4\$\begingroup\$ Tony - Hi, Similar to my recent comment on another answer, that image has come from somewhere else, so this site rule needs to be followed. Please can you edit the answer to add the required link to the original source? Thanks. \$\endgroup\$– SamGibson ♦Aug 9, 2021 at 20:52
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tl, dr: Use the PCB lead spacing that is specified for the package in the datasheet. For an 8-DIP this will be 0.300" (7.62mm).
Why the weird lead shape, then? The leads are purposely splayed that way to hold the IC in place during wave or hand soldering.
During automated assembly, the auto-insertion machine puts just enough side pressure on the leads to get the pins into the holes. Once the IC is inserted, the machine releases the pressure and the leads spring back. This holds the IC in place.
For hand insertion you can mimic this process using a tool like this: https://www.jensentools.com/jonard-tools-mos-2428-dip-insertion-tool-for-24-to-28-pin-wide-chips/p/606wi753 It does the same thing as the insertion machine: puts pressure on the leads so that you can get them in the board, then releases them so that the IC stays in place.
That said, for prototypes most folks just bend the leads straight to get them into the board and tack the IC in with solder to keep it from falling out when you flip the board over. Lay the IC on its side and bend the leads straight.
answered Aug 9, 2021 at 15:32
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1\$\begingroup\$ It may be worth noting that if all of the leads are spayed outward slightly, it won't matter if some legs aren't bent out as far as others, and chip-handling equipment would only need to apply inward pressure. If instead the chip makers had tried to make the legs vertical, any legs that were bent in too far would need to be pulled outward by the chip handling equipment, which would be more difficult than simply applying pressure from the outside. \$\endgroup\$– supercatAug 9, 2021 at 22:14
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\$\begingroup\$ Assembly houses often use lead straighteners to ensure a consistent bend prior to loading the DIPs into the machine. That said, there are some DIP packages that don't have the splay, like side-brazed ceramic DIPs that were / are used for EPROMs, mircoprocessors and other high-value chips. \$\endgroup\$ Aug 9, 2021 at 22:20
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\$\begingroup\$ Worth noting too that IC sockets are not splayed. Nor are some 8-pin DIP switches, but others are. \$\endgroup\$ Aug 10, 2021 at 4:29
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You need to modify your footprint to accommodate the actual package and pin dimensions. The fab house, depending on how good they are, will either 1) tell that there's a problem with the hole pattern, or 2) just try to force the pins, by bending them slightly, into the holes.
You may be able to get by with the latter, particularly if this is a one-off or hobbyist project. But I would not recommend that for any type of production run.
Correction
I thought the pins were spaced differently along the long axis of the package. But now I see that the spacing being talked about is across the package (pin 1 to pin 14 in a 14-pin DIP). Like others have said, there is fair amount of tolerance in this axis as the pins have a good deal of compliance in this direction. So you should be OK with your standard footprint.
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1\$\begingroup\$ I probably should have been a little clearer. The pins are flared out at an angle as with standard DIP packages. That’s where the width is 8.47mm. I was reading another answer here which asked why the pins are flared in the first place. One of the answers said that the PNP machine picks them up alongside the pins ans squeezes it to get to the correct width of 7.62mm or thereabouts and then drops the chip to fit the hole. \$\endgroup\$ Aug 9, 2021 at 13:25
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\$\begingroup\$ Ahhh, I misunderstood. I thought the pins were spaced differently along the long axis of the package. \$\endgroup\$– SteveShAug 9, 2021 at 14:07
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\$\begingroup\$ @SteveSh That's how I read it at first. A DIP-8 is the only DIP where we could make that mistake, (since it's 0.3" in either direction). \$\endgroup\$– TheodoreAug 9, 2021 at 14:34