Component placement when mixing SMT and THT - Page 1
Forum section: electronics / general

author: TwinScroll
author_rank: Contributor
timestamp: April 01, 2021, 07:14:54 pm
content: Hi,I am about to start the layout of a very basic power distribution PCB that will be using THT connectors and SMT Polyfuses.From an assembly perspective, What do people think would be the best/preferred side of the PCB to have the SMT polyfuses ?Would it be the same side of the board as the connectors - or on the (bottom) side that the through hole leads of the connectors would be soldered?Best RegardsTS

author: fcb
author_rank: Super Contributor
timestamp: April 01, 2021, 07:50:09 pm
content: I've used both THT on the same and opposite side of the board. Really only makes a difference if your production process uses wave-solder.The majority of the time (on my boards) the THT is soldered on the opposite side of the SMT - that said we have a product entering production that has the THT on the 'underside', just make sure that there is plenty of space to get an iron in.

author: penfold
author_rank: Frequent Contributor
timestamp: April 01, 2021, 08:26:01 pm
content: One thing to note is that polyfuses are highly temperature-sensitive, so if mounting on either side of the board gives less favourable thermal conditions, it is worth considering how that affects their de-rating before considering manufacturing constraints.From a manufacturing perspective, the decision probably comes down to one of volume and CEM capabilities so well worth asking them the question directly.

author: SMTech
author_rank: Frequent Contributor
timestamp: April 01, 2021, 09:42:19 pm
content: Keeping both parts one one side is the simplest, it means no tooling is required for the THT part as any old wave solder process will work. A very high volume manufacturer might wave solder the the THT and SMT parts together (no idea if your fuses would like it) but if you have seen boards produced that way you will know they look nothing like those made more conventionally. At the end of the day its a balance of physical requirements, tooling & volume, you can use pallets, plates, dip soldering, selective solderers, robots or people to mix sides and technology if you need to. As you do so your product will either get more expensive or more difficult to outsource.

author: TwinScroll
author_rank: Contributor
timestamp: April 02, 2021, 08:08:55 am
content: Thanks for the input everyone. I will keep the design such that SMT is on same side as the THT components - and leave the underside nice and clear for soldering.

author: Feynman
author_rank: Regular Contributor
timestamp: April 02, 2021, 01:55:31 pm
content: It always a good idea to ask your assembler directly. Depending on the geometries of the connector it might even be possible that your assembler will solder a THT connector with a reflow SMT process (at the same time with the other SMT components).

author: Mangozac
author_rank: Frequent Contributor
timestamp: April 04, 2021, 12:45:08 am
content: Quote from: SMTech on April 01, 2021, 09:42:19 pmNot quite - there will still be either a stencil or CNC setup to apply glue dots if you want to wave solder SMT parts on the underside.Quote from: Feynman on April 02, 2021, 01:55:31 pmThis is absolutely the best advice - the best method will absolutely depend on which way your assembler prefers to do things!

author: Mangozac
author_rank: Frequent Contributor
timestamp: April 04, 2021, 12:49:30 am
content: Sorry, double post

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 04, 2021, 01:43:21 am
content: For reflow only: both top side preferred. This way, only one reflow step is needed (one side SMT), and ordinary wave soldering can be done on the bottom -- no solder shield, no selective or hand soldering needed.For wave only: SMT bottom. SMT parts are glued in place, and soldered at the same time as the THT components. Typically, footprints and layout are tuned specifically for this, to control the amount of solder left on SMT pads as the wave recedes, and to minimize shorts. (Example: DILs placed lengthwise with wave direction, QFPs placed diagonally, "thieving" pads placed towards the end side of a row of pins to collect excess solder; etc.)For more demanding designs, some mixture of these methods might be used. SMT components should be kept away from THT pins, on the solder side, to allow for selective solder (large clearance), solder shield (small clearance, usually in a conical area around the pin -- small components can be closer than tall ones) or both, or hand soldering as a final resort, for pins that cannot be done by automated means.Tim