PCB Contract assembly - fitting SMT parts only on mixed technology boards - Page 1
Forum section: electronics / general

author: Pete_UK
author_rank: Contributor
timestamp: September 01, 2016, 10:10:58 pm
content: Hi all,I've hand-built some SMT prototypes in the past, but am now looking at getting my first batch of small boards assembled by a contract manufacturer (probably a China-based one, fronted in the UK). This particular board has a number of 9mm pots that protrude through a Eurorack front panel, and will have 95% SMT devices making up the board electronics. Since the pots will really need to be jigged to ensure that the front panel fits nicely to the completed assembly, I was planning to have the contract assembler just fit all the SMT parts, and to then fit the 9mm pots myself to ensure mechanical alignment. So, the main question is whether there is any problem in having a moderate number (say up to 20) no-fit conventional parts (that I could identify as no-fit on the BOM provided to the contract assembly house) - I guess that this should in theory be no problem for them? Also, does it make any difference at all, from an assembly point of view, whether the SMT devices are on the same side of the PCB as the pots, or the opposite side of the board? I was thinking of doing the latter, since it would seem that this would give better access to the SMT parts if any rework is needed.Very many thanks for any input. Cheers!

author: thm_w
author_rank: Super Contributor
timestamp: September 02, 2016, 10:42:53 pm
content: Having no-fit parts is not an issue. Usually referred to as DNP (do not populate).Having through hole/smd on opposite side doesn't matter, your suggestion of having the SMD available to probe/rework sounds good.If TH pots were done by them they would either solder by hand, or a separate wave soldering step is done. You could provide a jig for alignment at that point.

author: Pete_UK
author_rank: Contributor
timestamp: September 03, 2016, 06:16:20 pm
content: Very many thanks for your reply, good to know that I'm on the right track!

author: langwadt
author_rank: Super Contributor
timestamp: September 03, 2016, 06:26:52 pm
content: Quote from: thm_w on September 02, 2016, 10:42:53 pmif they were going to wave solder the through hole parts it would matter what side it was on

author: SeanB
author_rank: Super Contributor
timestamp: September 03, 2016, 07:24:46 pm
content: You could simply make an assembly jig for the through hole parts, that provides the required alignment, then have all the SMD parts put on with adhesive and pass the whole board through a wave solder bath as a single pass. Makes the board testable in production, and there is only a single heat cycle on the parts.You just have to be careful in placement and with pad size, so that you provide enough solder area and do not shadow components in the bath, and that the IC's have solder thieving pads in the right places to reduce solder bridges. Only issue is that you will not be able to do a double sided board as easy with TH parts both sides ( like connectors) but you can do SMD parts both sides with the top side being solder paste and glue, then the bottom side is simply glued, the whole board is reflowed for the top and then you populate the TH parts, put the alignment jig on, and then solder bath time.In any case, put a few pads next to components for test points, just expect them to be solder coated, and use as needed.

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 03, 2016, 07:30:04 pm
content: Quote from: langwadt on September 03, 2016, 06:26:52 pmAnd for selective as well, by the way.

author: elecman14
author_rank: Unknown
timestamp: September 04, 2016, 01:25:36 pm
content: You may also want to look into peelable mask for the pads you intend to hand solder. I have seen contract manufacturers order stencils incorrectly in the past that paste non-pop parts. If you plan to user multiple assemblers or just want the initial run to go smoothly it might be something to consider. That being said I have not quoted peelable mask in probably ten years so I have no idea on the risk/reward for that.