double sided part(s) - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: October 13, 2020, 07:56:15 am
content: I'm fantasizing about a design that allows me to plug/stack PCB's so that I can have a common bus. I can do this in a compact way if I use small pitched surface mount parts back to back of the PCB, via next to the pins and the shortest of trace to the pads. But this means a "double sided" load for one connector.Is this outright crazy manufacturing wise? what about the home IR oven? if I do the lone connector on the back turn it over and do the other parts will that connector stay on during re-flow of the main side that is not hit with IR? I believe glue is used in manufacturing?

author: jduncan
author_rank: Regular Contributor
timestamp: October 13, 2020, 12:49:02 pm
content: I do double sided boards all the time in my home oven setup, it's no big deal. Certainly not a problem for a real fab, prototyping companies at least don't seem to charge any extra for that.Most things won't have a problem durign reflow, but if I'm worried I use some of this:https://www.digikey.com/en/products/detail/chip-quik-inc/AD1-10S/9681971

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: October 13, 2020, 03:27:43 pm
content: Generally, you put your small passives (or small transistors, etc.) on the bottom of the board, reflow that first, then put the large stuff on the top of the PCB, then reflow that.if you are talking SOT23 or 1206 and smaller parts on the bottom side, just reflow with the same profile. The surface tension of the solder will keep the components in place during the second reflow. I do this daily.If you have "heavy" components on top AND bottom, then yes, you need assembly adhesive or mechanical fixing to the PCB. Many connectors have mechanical pcb attachments just for assembly.

author: Simon
author_rank: Unknown
timestamp: October 13, 2020, 03:51:04 pm
content: These connectors have location pegs only, no clips although it means that for a "bus" design I can literally put them back to back as I don't think the peg goes in more than 0.8mm but am yet to check. I should probably look at samtec too as i hate the near monopoly TE have.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: October 13, 2020, 05:04:16 pm
content: Push comes to shove, you can do high temp and low temp solder. Run the high temp first, assemble other side (still, preferably with your ICs and such) using the low temp solder.This way, the first high temp solder never reflows.

author: asmi
author_rank: Super Contributor
timestamp: October 13, 2020, 05:45:57 pm
content: I do this at home all the time. For bottom-mounted SMT connectors (typically Samtec high-speed connector like Q-pairs) I use thermosetting glue during original reflow not just to hold them during subsequent reflow of the top side, but to also increase mechanical strength of connector-to-board bonding to decrease the chance of connector being ripped off during connection/disconnection. Specifically I usethis gluebecause it allows to cure the glue and do the reflow in a single pass.

author: Gribo
author_rank: Frequent Contributor
timestamp: October 13, 2020, 07:21:33 pm
content: Depending on your stack-up and mechanical design, you might want to consider single piece connectors (spring loaded leaf to PCB pads) This way, you avoid double sided parts.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: October 13, 2020, 11:08:17 pm
content: How many boards are you wanting to stack? If it's just a couple then you could get away with "bottom entry" SMD connectors (see picture)Also like other have said, with the double sided approach if your bottom components are heavy enough they will need glue. Some advice about glue, if your connector is high density type (for example has a BGA/LGA footprint), then you need to let the solder reflow first before applying the glue (as in reflow bot side, apply glue, reflow top side). If you do it the other way around then you risk the connector being lifted up (by the now solid glue) while solder is being reflowed, which may cause some broken connectionsEDIT:Then there is the PC/104 approach (using press-fit connectors)

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 14, 2020, 02:14:59 am
content: A simple version of this is the Arduino and shields system. I have worked with more complex designs that have more than a dozen stacked boards so it is certainly possible. But nothing is ever free. Most connectors are designed to have one side floating. Dimensional tolerances become more critical when stacking. I don't know how big your bus is, but as pin count rises the insertion force assembling and dis-assembling your boards gets to be a problem. In some cases a mechanical fixture may be needed. Your power dissipation is probably roughly proportional to volume, but first order the surface area of a stacked set goes to the 2/3 power of that volume. Heat can be a real issue in these designs, forcing such fun things as thermal layers in the PC and edge clamping heat sinks. Finally, Murphy always rules. When a fault occurs it will be in one of the middle boards of a stacked set, where troubleshooting access sucks.

author: Simon
author_rank: Unknown
timestamp: October 14, 2020, 06:23:29 am
content: I wanted something a bit denser than 2.54mm, I found some nice 0.8mm double row connectors from TE. I found that with 2.54mm throughhole the sweetspot is with 3 rows but with a "motherboard" that is a lot of routing and non ideal connectors and I used every 6 pins as a ground. The 0.8mm surface mount would be denser, have less track for each signal to run and make the overall thing smaller.

author: asmi
author_rank: Super Contributor
timestamp: October 14, 2020, 01:30:20 pm
content: Quote from: Simon on October 14, 2020, 06:23:29 amIf you want to have the same pair of boards connected with multiple connectors, you might want to use connectors with floating contacts as they allow certain misalignment. If you don't need speeds above 5.5-6 GHz, take a look at these connectors:https://www.samtec.com/connectors/high-speed-board-to-board/rugged-high-speed-strips/floatingIf you only have a single connector per each pair of boards, I would consider usingQ-strips(for single ended signals) orQ-pairs(for differential pairs). I use them a lot for high-speed interconnects, and they also send free samples to everyone

author: Simon
author_rank: Unknown
timestamp: October 14, 2020, 05:00:20 pm
content: I was trying to find something that would allow a variable height. The TE ones come in different heights so I can space boards to suit the components but not make every set too spaced out.

author: asmi
author_rank: Super Contributor
timestamp: October 14, 2020, 05:30:33 pm
content: Quote from: Simon on October 14, 2020, 05:00:20 pmSo are the parts I've mentioned above. I think it's a very typical to have these connectors with different stacking height.

author: Simon
author_rank: Unknown
timestamp: October 14, 2020, 05:51:54 pm
content: i can't remember my log in for samtec to check prices but as they are qualified at 14 Gbps i bet they are not cheap. Really nice by not what I can afford to use.

author: asmi
author_rank: Super Contributor
timestamp: October 14, 2020, 06:09:49 pm
content: Quote from: Simon on October 14, 2020, 05:51:54 pmDK stocks a quite a number of Samtec's parts, so you can look them up there (use part configurator on Samtec's website to set up the part you need and then just pull the part number out of it and search for it on DK/Mouser). Or just request a dozen or so free samples from Samtec directly if you only need a few

author: Simon
author_rank: Unknown
timestamp: October 14, 2020, 06:44:49 pm
content: i don't think the more costly parts are so freely given out by samtec. Actually because of the nature of these connectors they are harder to get because there are so many combinations that it's just impossible. This is why samtec sell direct and build to order.I went round in circles with the samtec card edge connectors. The stockists only stocked certain ones because evidently there was a demand like one big customer or something standard used them but as soon as you wanted something different it was a no go.

author: asmi
author_rank: Super Contributor
timestamp: October 14, 2020, 07:34:11 pm
content: Quote from: Simon on October 14, 2020, 06:44:49 pmThey sent me ten 400 pin HPC FMC connector pairs (both male and female connectors) with no questions asked, and they are quite pricey.Quote from: Simon on October 14, 2020, 06:44:49 pmThe only downside of ordering direct is that you will have to pay for shipping, as I remember Samtec's online store doesn't have MOQs for most items. So far I haven't actually ordered anything direct as I was able to source production parts via DK/Mouser, but I wouldn't avoid designing in connector just because it's only available direct - I would email them though and ask if there are plans to EOL my part of choice any time soon. I'm extremely happy with their parts, and I especially appreciate that they give free samples to small clients like me, and every time I had some questions about their parts, they always responded promptly and to the point. And they are not concerned by me using gmail email account while talking to them and not some corporate one (I couldn't be bothered to set up a corporate email service for my company).I once tried to request a sample parts from TE - they showed a bigass form with a million of fields for me to fill out, by field #10 I said "f*ck it!" and just bought samples myself. So you will immediately feel the difference in customer service.

author: Simon
author_rank: Unknown
timestamp: October 14, 2020, 07:54:08 pm
content: I had a samtec rep see me at work. He was very good and i certainly rate them but I know those connectors could cost a lot more than a simpler solution. I'll have another look another day

author: asmi
author_rank: Super Contributor
timestamp: October 14, 2020, 07:58:13 pm
content: Quote from: Simon on October 14, 2020, 07:54:08 pmI guess I'm starting to sound like Samtec's sales repI apologize. I'm simply a very happy customer of theirs, and it's not very often in this business that you get that kind of service as a small company or a one-man-band.

author: Simon
author_rank: Unknown
timestamp: October 14, 2020, 08:40:42 pm
content: The funny thing was this rep had just left Wurth and we talked almost as much about Wurth products as Samtec. He was a really good guy that was just passionate about solutions which made me all the more happy to use samtec. Wurth have some connector stuff but it's nothing really to the giants of connectors.

author: SMTech
author_rank: Frequent Contributor
timestamp: October 14, 2020, 10:05:39 pm
content: Ha, well I hear being a rep for the connectors side of Wurth is no fun whatsoever, they typically don't last long, but there is plenty in the range, they typically have equivalent drop in for most of the common stuff.For Samtec tho' Toby can usually sort you out samples of most of it, and if you have the volume perhaps even a generic alternative...

author: Simon
author_rank: Unknown
timestamp: October 15, 2020, 06:12:25 am
content: Hm no sorry, the wurth range is hopeless. The rep keeps trying to get me to use stuff and with 2.54mm pitch headers I don't mind giving it a go but even then they struggle. Jut comparue their 2.54mm stuff to samtec..... oh.... you can't they just don't come near them.They don't have specific connector reps anymore. When I first started seeing a rep he was all components but connectors and claimed that it was too much for one rep to do both. He brought along his connector colleague to one meeting. Then they seemed to have some sort of restructuring which is when the guy now working for samtec "left" and my component rep became connectors as well and his connector colleague moved up to dealing with the larger customers.