SMT vs Thru hole: Thermal/vibration application? - Page 1
Forum section: electronics / beginners

author: RogerThat
author_rank: Regular Contributor
timestamp: January 28, 2019, 08:54:01 am
content: Hi,Trying to understand when there is a good time to use thru hole rather than SMT part.When googling the above I find alot of copy-past information 'Use thru hole when there is a lot vibration or thermal cycling'.But, there is no info what 'a lot' is?Seeing that more or less all accelerometers are SMT I guess the vibration argument goes away?Looking at other products like bike computers, smart adventure watches, etc that are subject to both vibration/shocks and thermal cycling it's surface mount all the way. Maybe due to size?The only reason for thru hole that I found is if the component is physically large/heavy (like big inductors/capacitors).If I build a IoT which will be placed outdoors(+60c in summer to -30c in winter, also daily cycling) and that I want to work for years on end, is SMT ok or should I go for thru hole?

author: Jeroen3
author_rank: Super Contributor
timestamp: January 28, 2019, 09:03:40 am
content: Quote from: RogerThat on January 28, 2019, 08:54:01 amAccelerometers still break internally when the forces applied are too high. Not only the duration, also amplitude and frequency matter.Both smart watches and bike computers are not special applications regarding vibrations.Engine mounted sensors en equipment, rockets, airplanes, tanks are heavy vibration environments.The reason to go for through hole is that some components are only available in tht, are cheaper in tht or can do more power in tht.If you want more reliability, look into coating or potting. That keeps moisture out.

author: hsn93
author_rank: Regular Contributor
timestamp: January 28, 2019, 12:03:58 pm
content: i did not read this but two days ago i stepped by it and now i remembered it after seeing your question.https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100029736.pdf

author: mvs
author_rank: Frequent Contributor
timestamp: January 28, 2019, 12:19:46 pm
content: Quote from: RogerThat on January 28, 2019, 08:54:01 amModern automotive electronics has excessive use of SMT and the conditions are even more harsh. If BOM costs are not a major concern for your application, look at automotive ( AEC-Qxxx qualified) or industrial grade parts. They may have some additional features like soft termination (MLCC caps, chip resistors) to prevent cracks.

author: max_torque
author_rank: Super Contributor
timestamp: January 28, 2019, 01:00:53 pm
content: TH vs SMC used to favour TH when the SMC components were basically the TH ones with the leads cut off or bent differently.Today, however, most SMC componentry is optimised for it's package and usually a lot smaller and lighter than the TH stuff, and hence is suited to high vibration environments. The exceptions are high current or high temperature components, where they have to be physically large, and hence heavy, and hence require a stronger mechanical restraint. Increasingly however, that is simply provided with larger, better designed footprints with a larger surface area.In really high vibration environments, the devices will need to be potted, and that brings a whole other load of constraints and potential issues

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2019, 01:58:47 pm
content: Vibration doesn't matter much to an SMT part, if it's rigidly mounted. The problem is more to do with flex of the board (especially if poorly mounted), and repetitive and poorly limited stresses, like on connectors, causing fatigue.THT with long leads, is able to take up the stress in its body mounting, while the resulting strain is dropped along the flexible leads.As long as I'm using the technical terms here; stress is the pressure (force per area) within a body, and strain is the resulting elastic deformation (stretching, bending). Fatigue, is the repetitive stress of a body, which generally causes hardening and cracking to accumulate, until the body eventually fails.Or, THT with short stubby leads, which grab the board very rigidly, transmitting stress into the board and its mounts, keeping strain away from its solder joints.The most vulnerable mechanical interfaces are solder joints, pad adhesion, and brittle ceramic chips. You want to keep both stress and strain away from these locations if possible.And: pad adhesion is copper foil to laminate strength. Shear strength is surprisingly good, tensile is okay, peeling is awful.Pulling on a soldered SMT pin, you'll usually pull the pin off the pad, rather than the pad off the board. At least, with softer SnPb solder, that's been my experience. But bending on an SMT pin, generating a peeling sort of force: that'll rip pads straight off the board, easy.Ceramic chips are vulnerable for two reasons: one, the ceramic itself is brittle and cracks; two, the metallization on the ceramic may not be as strong, and that cracks/peels/tears off. Resistors are usually made on strong alumina ceramic, so are more prone to metallization or solder or pad failure. Capacitors are made from a mechanically inferior ceramic; its only purpose is to have a huge dielectric constant -- it's a fairly soft material otherwise. So, ceramic chip caps tend to crack, and board flex delivers that strain with extra leverage.So, putting all that together:- Avoid SMT connectors where a lot of strain is expected. SMT connectors are bad for external connections, and usually okay for internal connections. Internal connections may not be very reliable if there's a lot of vibration (>= 1G shock and vibe?). Cable restraints and glue can help.- Avoid putting peeling forces on pads. Avoiding SMT connectors, or some types of them anyway, basically covers this.- Avoid fatigue on solder joints. THT does this by surrounding the pin with a near-sized hole, and filling the remaining (modest size) gap with solder. ("Cold solder joint" failures are very common on single-layer, unplated boards where a tiny solder fillet is all that's holding the pin down, but uncommon on multilayer, PTH (plated thru-holes) boards.)- Don't put ceramic parts, especially capacitors, in board areas with high strains. Also, if you must, try to align them perpendicular against the strain, so the strain acts against the width of the part, not its length (this puts less strain on the part, and gives less leverage from the board to the part -- it's the part's "stiff", "thick" direction).- And if you must (or even if you don't have to, but you still need high reliability), consider using capacitors that mitigate these problems. There are types which fail safe (open) by keeping electrodes away from the ends (where cracks are most common), or with multiple internal sections so that a single crack doesn't cause a short, or with different end-metallization that can take up some strain, or at least is weaker than the ceramic itself so the plating acts sacrificially. ("Flex terminations" are usually a conductive epoxy underlayer, as far as I know; this has the teeniest bit of stretch, or it can peel away without ruining the component.)- Do place mounting points near sources of stress (like connectors, and heavy components). This keeps the strain low, by drawing the stress away nearby.- Conversely (or, perhaps seemingly contradictory), when the mounting points themselves tend to be sources of strain, keep some distance between them. Example: you don't usually want to place screw mounts right beside a panel-mount connector, because that transmits panel deflection/vibration right into the board, and heavily stresses it. You're better off keeping some distance between things -- treating the connector itself as a mounting point as such -- and dealing with the resulting (moderate) strain in the board around it.- Pay attention to order of assembly: you don't want to torque down a connector and then discover that, because it wasn't quite aligned in its hole, now all the screws are out of alignment, and forcing them in will do all kinds of nasty to the board. Place screws and connectors loosely, then tighten them down in sequence. This is especially helpful for components mounted to heatsinks: insert them into the board, but don't solder; assemble with the heatsink, using the board to align the components on the heatsink. Then tighten them down, and solder everything in.- Also consider potting, so the board is supported all around by something. Preferably, something not itself rigid, but rubbery. You don't usually do this unless you need the environmental tolerance, but it definitely helps mechanically. A compromise might be filling the space beneath the board with potting, so it's secured, but components are still sticking out the top because you don't care about environmental.Tim