Optimise PCB Assembly costs - Page 1
Forum section: electronics / general

author: Saimoun
author_rank: Frequent Contributor
timestamp: September 26, 2020, 09:46:10 am
content: HiYou guys have been already super helpful, so I have (again) a few questions about cost optimisingAs I understood, it is better to have all SMD components on one side, otherwise the assembly company needs to glue the components on the other side - which I guess will definitely add extra costs?So, let's say all SMD components are on one side - let's call it side A. Then if I also have a few TH components (like buttons and connectors) on side A, when assembling for production these are wave soldered, correct? This will work because the solder is on side B so it will not desolder or damage the SMD components.So far so good - but what if I also have a very few TH components on side B - these cannot be wave soldered since you cannot have hot molten solder on side A, so they will have to be hand soldered, correct?Thank you!Simon

author: opampsmoker
author_rank: Regular Contributor
timestamp: September 26, 2020, 11:27:58 am
content: You can wave solder TH's on side B or A....you just have to keep the solder pad at least 3mm (sometimes 5mm) away from the nearest SMD pad....speak to your board stuffer to ask for the distance.They may just hand solder the few THs...in which case they may allow 1mm distance

author: Mangozac
author_rank: Frequent Contributor
timestamp: September 26, 2020, 11:28:09 am
content: Quote from: simonlasnier on September 26, 2020, 09:46:10 amActually, that's not necessarily true. If only small components (low mass) are placed on the bottom side you could do it as follows:1. Place all bottom side components on solder paste as usual.2. Reflow the board.3. Flip the board and place all top side components on solder paste.4. Reflow again.The surface tension of the solder will hold the bottom side components in place while the board is reflowed a second time. Note that the oven will require a "finger" conveyor rather than a mesh conveyor. And there is obviously additional cost with this as the board has to go through the SMT line twice.It is possible to use larger size SMT parts on the bottom side, have them held in position with glue dots and then run the board over a wave solder with the THT parts. I believe this is pretty uncommon practice these days though as glue dots are a pain.The better option is to do as much as possible in SMT. THT components can then be placed and either soldered by hand or a selective solder machine.Of course, the best cost optimisation will depend on how large a batch size you are running, as certain procedures lend themselves better to high or low volumes.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 26, 2020, 12:47:27 pm
content: One-sided load is a good idea if you can manage it. Large BGA parts tend to require bypass caps on the opposite side, though. If you only have smallish components, gluing is not needed, but double sided assembly is still an extra cost because it requires two reflow passes.Minimize the number of TH components. Zero is a good target, but if you need a few, it's cheaper to manually solder them, than to go through wave solder; in that case, they can be placed top or bottom, anything you like. Note, most assembly houses do manual soldering as well or you can arrange it yourself if the batch is small.Also minimize the number of BOM lines. If you have a a weird 39.2k resistor in a feedback divider, use that value for general-purpose pulldowns instead of, say, 47k, even if the resistors cost $0.01 instead of the usual $0.001 each. Even on a complex design, you can easily combine initial 30 resistor values down to some 10. Sometimes this even means adding part count by putting two resistors in series/parallel somewhere to get that odd value you need there!

author: SMTech
author_rank: Frequent Contributor
timestamp: September 26, 2020, 11:21:47 pm
content: There is isn't a hard and fast set of rules for optimising a design for manufacture. Rather what you have to do balance the costs of physical tooling, labour with component cost and losses that might be caused by a marginal design or something that's difficult to make. Its also about making your design compatible with the assembly house you are using, they come in all shapes and sizes and the work they do/have done will have dictated the equipment and processes they have invested in.So for low volumes, hand soldering a few connectors on one side is not an issue. Setting up 120 different items on the pick and place however could be a significant proportion of the build cost, so you might do what Siwastaja suggests and work on minimising the BOM lines, move to 10K boards tho' and suddenly that extra $0.099 per random value might actually constitute a saving worth looking for by using a more complex BOM. Equally it could be worth paying for the the tooling to build wave solder pallets for the through hole bits.You do still see quite a lot of wave soldered surface mount, but in very specific little niches of highly mass produced product. It takes knowledge and work to get this process right with very specific layout requirements.Double sided SMT isn't really a problem at all on its own, its just makes the THT side of things slightly more complicated, push buttons are easy enough to find cheap SM versions of. The story can get more complicated with connectors, juts because connectors are annoying. Lastly there is Pin in Paste where special THT connectors are reflowed with your SMT.

author: Saimoun
author_rank: Frequent Contributor
timestamp: September 28, 2020, 11:38:44 pm
content: Thanks all that is super helpfulI already knew about BOM optimisation, I saw one of Dave's videos about it, but also I think for me it is a very natural thing - so my BOM is *very* optimisedI also understand the idea to minimize TH - in my case it is just the connectors, display and push buttons. All the passive and small components are SMD.The push buttons could probably could be SMD, but since they are on the opposite side ("side B"), then they will require to do SMD soldering on both sides so I do not think it would help the price :/