We may get  tombstoning of our capacitor and resistor? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: November 03, 2019, 09:07:50 pm
content: Hi,Please advise if we will get “tombstoning” if we remove the thermal reliefs to the pads of the following components..The component ringed in yellow is an 1812 ceramic capacitor.The component ringed in red is a 2010 chip resistor.We want to simply merge the left hand pads of these components completely into the surrounding thermal copper pour. (for better cooling of the components). However, as you can see, the right hand pads of these components have less copper attached to them. If we merge the left hand pads completely into the thermal copper pours, then will the right hand pads heat up more during the SMT soldering process, resulting in the components tombstoning?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 03, 2019, 11:25:32 pm
content: Nah. Components that big don't lift off.Head-on-pillow or wetting failure is possible though.More to the point, if you measured the temperature of the junction both ways, you'll notice maybe a 1°C difference. Better to have the improved assembly.Tim

author: SMTech
author_rank: Frequent Contributor
timestamp: November 04, 2019, 12:07:23 am
content: Well in my experience,it makes it more likely, but still not very.Tombstones are more easy to cause with excess solder during printing, or a poor footprint compared to the device. We used to have a consistent tombstone issue with an 1812 Fuse as the client had used a footprint the was very similar to the physical size of the device , much more paste beneath the device rather than to the pad and providing a fillet up the sides. Two solutions worked quite well, "home plate" on the stencil for that device & switching the fuse model to the version with a notch in the end of each pad.However as with T3sl4co1l I'm not convinced you will achieve much, if they are getting warmer than you want, use different parts.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: November 04, 2019, 07:38:04 am
content: Most tombstones happen due to too large pad size which result in more solder paste on the pad. If the pad size is correct, it should never happen. I used to face this when the pads are larger and problem goes away after pad size is reduced significantly.

author: IconicPCB
author_rank: Super Contributor
timestamp: November 06, 2019, 08:34:40 am
content: A change of the solder alloy to one whihc has a broader range of melting point ( ie solder will melt over a temperature range of say 6C ) may prevent the occurrence of tomb stoning.

author: AndyC_772
author_rank: Super Contributor
timestamp: November 06, 2019, 08:40:01 am
content: Why would removing the thermal relief reduce the temperature of a ceramic capacitor?Ceramic caps have very low ESR and don't dissipate significant power. Its temperature will be the same as the temperature of the PCB, whether you have thermal relief on the pad or not.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 06, 2019, 10:44:57 am
content: Quote from: AndyC_772 on November 06, 2019, 08:40:01 amYou haven't seen what I sometimes to do ceramic capacitors then.Although I somewhat doubt treez is using capacitors quite that aggressively.But anyway, for point of interest -- ceramic caps, the body is basically solid ceramic with a negligible amount of metal inside. The ceramic isn't very good, mechanically speaking: it's weak and not very (thermally) conductive. Like 1/4 the conductivity (or less, I forget?) of the alumina ceramic used for resistors. Unlike resistors, the heat is generated in the bulk as well, so the hotspot in the center can be quite a bit higher than for an equivalent size resistor.For this reason, I shy away from running capacitors at high power dissipation -- I try to keep it maybe half that of the same size resistor. So, 62mW for a 1206, say.You do occasionally see current ratings in datasheets; follow those for best results.Tim