JLCPCB Bottom Entry Connector? - Page 1
Forum section: electronics / projects

author: hal9001
author_rank: Regular Contributor
timestamp: May 23, 2021, 09:54:11 am
content: Does any one know if JLCPCB assembly offers a SMT bottom entry connector?Something like in the picture but less pins

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 23, 2021, 01:53:21 pm
content: Found some, although they are all extended parts:C59983, 2mm pitch 10pin (2×5) female headerC124403, 2mm pitch 12 pin (2×6) female headerC135211, 2mm pitch 20pin (2×10) female headerC840484, 2mm pitch 18pin (2×9) female headerNote: C840484 requires two locating holes for plastic pins (1.2mm diameter, 14mm between centers; centered between the four outermost pins at each end); the others do not.You can find these in EasyEda, LCSC, orJLCPCB SMT Parts Libraryby searching for the product code (C124403, for example).If by bottom entry you mean the pin could be inserted from the PCB side, none of the above seem to have anything obstructing that, but I obviously haven't verified this.

author: phil from seattle
author_rank: Super Contributor
timestamp: May 23, 2021, 03:11:22 pm
content: Surface mount 2x pin header receptacles (I think that is the LCSC designation). Nothing special about them, makes me wonder if that is what you are actually looking for. The term "bottom" is a bit confusing here. Are you looking for a cutout in the PCB to insert pins from the other side of the board? Mounting bosses will get in the way of that.I've looked at those before and wondered about alignment of the ones without bosses. SMT alignment can be off by quite a bit and still be "correct" so I think you have to design to accommodate that possible outcome. I have a number of designs that take daughter cards with mounting holes - even slight misalignment becomes a potential problem. So far, I have used TH receptacles/sockets with slightly off center (5 mil typically) holes to create a "snap-in" effect and make for reasonably precise alignment.And as long as I am here, is the PnP alignment with pin 1 or the center of the connector? The drawing/datasheet shows no center - typical of many surface mount connectors. (though it doesn't explicitly show pin 1 ether). Perhaps this should be asked in manufacturing...

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 23, 2021, 04:27:59 pm
content: Quote from: phil from seattle on May 23, 2021, 03:11:22 pmNo, but very few are available for JLCPCB assembly; which right now isfree.If you wonder about the footprints of the abovementioned parts, just click the link, then on "EasyEda Model"; it shows both the schematic symbol and the footprint right there in the same window.The 18-pin one has two locating pins, so perhaps that would be best for through-the-PCB construction; especially if the outermost 8 pins (so only center 10 pins) would be usable through the PCB (as otherwise the locating pin holes may be too close).

author: phil from seattle
author_rank: Super Contributor
timestamp: May 23, 2021, 06:01:05 pm
content: Quote from: Nominal Animal on May 23, 2021, 04:27:59 pmGood point on looking at the EasyEda model. It doesn't look like it is centered - if the grid is any indication. They show a pin 1 indicator in the silkscreen. I will have to play with it to see if they use PnP machines to place.

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 23, 2021, 06:26:15 pm
content: Quote from: phil from seattle on May 23, 2021, 06:01:05 pmIn EasyEda, the footprint is centered, and looking at the actual pad coordinates, the footprint at least is symmetric.If you want to check and have a play around, just openEasyEda Designer(no registration necessary), then Library on the left.When you find the part you're interested in, note the footprint name, choose Types: Footprint, and search for that footprint, then click Edit.I'm lazy, so what I do, is Edit the Symbol itself, then click on the right side on the Footprint field. This opens up a Footprint Manager window. Clicking on the footprint name in the upper right corner selects it, so I can press Ctrl+C to copt the name. Then I cancel the window, and close the tab (inside EasyEda Designer); then click on the Library again, Types: Footprint, and paste the footprint name there, and press Enter. The footprint will be first in the list, highlighted in yellow. I click that, and Edit, and I get the full multi-layer edit window on the footprint open.In EasyEda, mouse scroll wheel zooms, and you pan with the right mouse button held (or with cursor keys). I often modify the existing footprints for my own use, for example to make hand-soldering easier.

author: hal9001
author_rank: Regular Contributor
timestamp: May 23, 2021, 07:36:43 pm
content: Thanks for the useful information and part numbers.Quote from: Nominal Animal on May 23, 2021, 01:53:21 pmI want to have top side SMT assembly and insert pins to this connector from the bottom of the PCB.Quote from: phil from seattle on May 23, 2021, 03:11:22 pmI see the mounting bosses in the spec sheet too but not in some of the pictures for C59983 as example. Possible different versions of the same connector?Quote from: phil from seattle on May 23, 2021, 03:11:22 pmThanks for the tip. Do you think something similar would work with SMT to make a tight fit or SMT alignment makes it less predictable?

author: phil from seattle
author_rank: Super Contributor
timestamp: May 23, 2021, 08:44:02 pm
content: If the connector actually works the way you think, I would try hand assembly first. I'd start with soldering the pin header in place and then mating the two board without the top connector. Then I'd install the connector on the pins, align the boards and solder the connector in place. That gets you good alignment. I surely would not trust reflow alignment for that. It will not take much to have poor alignment. I've had TSSOPs that were visibly misaligned but still within tolerance (and electrically correct).To be honest, I don't fully understand why you want to do it this way. I think I would be inclined to mount the connector on the bottom and use one with bosses to force the correct alignment. One other thing, I don't trust LCSC pictures 100%. I've seen a few clearly incorrect ones. Some times they use representative photos that are not exact. Data sheet trumps a disti's photos.

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 24, 2021, 01:43:12 am
content: Quote from: hal9001 on May 23, 2021, 07:36:43 pmBased on the EasyEDA footprints, only C840484 has locating pins. Of course, it ispossiblethat the footprint is wrong, but the footprint is from the JLCPCB SMT Assembly official package library, which to me means "this here footprint works for this part". (Also, the 1.2mm diameter locating hole in the PCB is from the footprint, not the part datasheet; datasheet says the locating pin or boss is 0.80mm in diameter.)That said, the closest thing to anything like this that I have done, is using the SMT male version on Teensy 3.0/3.1/3.2/4.0 for the bottom pads to expose ten additional pins on 0.1" spacing for carrier boards, soldered by hand. The alignment issue exists there too; I use a piece of perfboard, temporarily, to help align the SMT male header while soldering.

author: phil from seattle
author_rank: Super Contributor
timestamp: May 24, 2021, 03:40:25 am
content: Quote from: Nominal Animal on May 24, 2021, 01:43:12 amI've done exactly the same. With the teensy, I use a row of 1x sockets to align the surface mount strip. The nearby 0402s pop off with barely a whiff of heat. I managed to fix it, though.The datasheet for that part shows the boss hole size on the footprint as 1.2mm. Not unusual that the data sheets often spec much larger holes than necessary - probably a lack of confidence in their manufacturing tolerances. I have several connectors I use on my boards and the holes are huge compared to the pin. From memory, a .9 mm pin and 1.6mm hole. The parts were practically swimming. I reduced the hole down to 1.25mm and they were still pretty loose. Makes soldering a row of 10 with good alignment a complete PITA. After about 5 iterations, I've got it close to snap-in tight so soldering them straight is pretty easy. Hope they don't go changing their process...

author: hal9001
author_rank: Regular Contributor
timestamp: May 24, 2021, 10:22:22 am
content: Thanks "phil from seattle" and "Nominal Animal" for your feed back.The idea is to assembled 2 different PCBs using JLCPCB SMT. The PCBs will be stacked ontop of each other because of space limits and need to share connections with each other.Right now only single sided assembly is offered by JLCPCB so I thought the connections with the 2 PCBs can be made using male pins on the lower PCB and SMT bottom entry receptacles on the upper PCB. But as both of you rightly point out alignment might be a problem.It is ideal to not have hand assembly or soldering of components afterwards. Using flat flex can also workbut not ideal.Are there any other methods used in production to achieve this?Cheers