Looking for feedback on a TQFP socket design - Page 1
Forum section: electronics / projects

author: ataradov
author_rank: Super Contributor
timestamp: September 17, 2020, 10:46:11 pm
content: I'm working on something where I need a TQFP-100 socket, but the regular sockets, even higher end ones have very high parasitics, especially inductance.So I was thinking about ways to solve that and after looking at how regular sockets are designed, I came up with this idea for a socket without any leads.You use a regular footprint, but you place some frame on the board for mechanical alignment of the chip to the pads. Then instead of having spring contacts, you use natural springiness of the pins to contact the pads directly. The pins would be pushed by some rubber-like material (or even individual springs like in normal sockets, but they will be there just for mechanical reasons).Am I crazy? Are there any obvious reasons this won't work? Are there existing solutions like this?I'm aware of elastomer sockets, but my understanding is that they are not used for TQFP, only for BGA.

author: thm_w
author_rank: Super Contributor
timestamp: September 18, 2020, 12:04:17 am
content: How high is the inductance and will it actually cause a problem with your device?Seems like it should work, I might press directly on the flat portion of the pins and not on the chip itself, to avoid deforming the leads. Hard rubber as you say.Use a backplate to support the PCB from the bottom if necessary.Gold plate PCB would be preferable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2020, 12:46:30 am
content: Hmm, sounds reasonable. A firm rubber should be able to deliver even pressure, without smooshing between the leads. And even if the plating (of board and lead) aren't very friendly, this pressure can be increased almost arbitrarily (using ever-higher durometer push-pads, up to the limit of PCB,or even leaddeformation -- this could get very tense indeed with a ceramic PCB and carefully fitted clamps!). Yeah, you'll want a bracket under the footprint to support this clamping force, and hard gold is probably the best plating option, even if not ideal against the most common (tin) plating used on components.The sheer area taken up by the clamp, and the amount of deformation used, may again limit performance though. You can't place components there. You might not even be able to place vias there: the compression and squishing of the PCB itself may fatigue and crack them. Ditto for ceramic chip passives.If vias are reliable, you do at least get the benefit that a multilayer board can be used, immediately underneath the footprint; you eliminate the long looping connections of a spring type socket, giving good GND/VCC at least, if not as great GPIO signal quality. Planes provide good bypass, without having to rely on chip caps placed right beside or under the part. (Ah, assuming you don't mind that the socket is customized to each particular device it's being used on..!) And signal quality can be dealt with, to some extent, at modest distances from the pins; give or take what kind of device we're talking here. (A cm or two stub length is perfectly fine for an MCU, and probably fine for a typical LVDS rx/tx pair, but wouldn't be so great for modern DDR, or PCIe and other high speed differential channels, or precision RF circuitry.)Probably, much less clamping force is necessary than where all these various mechanical limits manifest at, and it'll be reliable after all, or at least useful (hundreds of cycles, maybe thousands?).These are somewhat unusual questions to ask of a PCB, i.e., the particulars of strength, elasticity, deformation and fatigue -- so it would be nice to find some technical papers discussing them.I wonder what kind of centering and latch mechanism would be best. Some kind of over-center clamp or cam would be typical I suppose. A chain of levers and right-angle links might be used to synchronize them. The clamps do need to move out of the way of the chip for insertion/removal, then something either sitting at the bottom (a plastic film cutout?) or grabbing the corners (but it has to move out of the way of the clamps..?) keeps it centered while unclamped. There might not be room to get tweezers down in there (reliably anyway, seeing as most parts have a tapered body making it difficult to lift from above the parting line), necessitating gravity (turn it upside down..) or suction to insert/remove parts.Oh, also you could turn this kind of on its face. Going back to the spring style sockets, what if instead of a long loop, simple leaf springs are used? Probably a PITA to retain leaf springs of such fine pitch; but the loop area could at least be kept short, and the springs can simply slide in horizontally, lapping over the leads as they do (which also causes contact wiping, and pushes the part into the center -- if the slides are synchronized precisely enough, that is). The PCB footprint would simply be a regular TQFP with the pads pushed ponderously far from the body; the part itself rests on bare PCB and the springs bridge from the lead top surface to the nearby pads. Possibly, less clamping force would be needed (granted, a hundred springs working together is still quite a mechanical advantage), so that the mechanism can stand up from the board a bit perhaps, giving more clearance for nearby passives.Tim

author: ataradov
author_rank: Super Contributor
timestamp: September 18, 2020, 02:37:44 am
content: Existing sockets do cause issues. This is proven in practice. They are big and do not allow you to place decoupling capacitors close enough.For the alignment part I was thinking something like thick capton tape just propping the pins from the sides. I just tried with just 4 square pieces of the tape and they hold the chip aligned good enough to apply any sort of clamping force. The tape is also thinner than the pins, so there is no issue with interference to the rubber clamps. The clamp now can just be a square with a hole for the part plastic on the inside.Now I need to figure out some home made way to make the clamp for a prototype.It is a shame there are no readily available sockets like this.This way clamp does not interfere with component placement at all. May be some of the back side will be reserved if back bracket will indeed be necessary. I don't know that it will be. I only need it up to 100 pins, the side is pretty short.

author: TomS_
author_rank: Frequent Contributor
timestamp: September 18, 2020, 11:40:12 am
content: Is there a particular reason the part cant be soldered to the board?

author: ataradov
author_rank: Super Contributor
timestamp: September 18, 2020, 03:46:27 pm
content: Yes, there is a good reason. It is for testing things and the same chip needs to be swapped between multiple boards.

author: thm_w
author_rank: Super Contributor
timestamp: September 18, 2020, 09:19:45 pm
content: Quote from: ataradov on September 18, 2020, 02:37:44 am3D printer or mill?Yes probably don't need to reserve much space if any, could use soft foam or cutout support block that fits around any components. But again, only if the board flexes excessively.

author: ale500
author_rank: Frequent Contributor
timestamp: September 19, 2020, 06:28:34 am
content: In the old days, HP sometimes used chips packaged in ceramic without leads, like a LGA, these chips would rest on the board where a hole was cut and the pins would rest in a kind of elastomer (like for an LCD) but with thin copper traces. now that I think of it it looked more like a flexible PCB with very thin traces. It made contact on the board and on the chip. The chip was held in place with a bolted down metal heat spreader.At work we sandwich chips in two boards that have a hole cut for the package and traces without solder mask so the pins make contact. Both boards are bolted together. I have seen boards for SOIC-8 and TSSOP16 (0.65 or 0.5 mm). Such an assembly is used to test chips in the wide temperature range of -45 to 170 °C, works pretty well. The problems are almost never at the contact of the chip with the board. This approach may work for you too.

author: ataradov
author_rank: Super Contributor
timestamp: September 19, 2020, 06:33:29 am
content: There are modern elastomer sockets, but they don't make them for QFP for some reason.At the same time I learned that there are sockets that have tiny double sided pogo pins. One side touches the pad on the board, the other touches the pin. This way layout on the board does not change compared to the solder-in version.Those pogo pins still have inductance, but it is way lower compared to the standard ones.I can't even find those sockets, but I've been told they exist. I'll get more info on them.