How to protect smd connectors against reflow heat ? - Page 1
Forum section: electronics / general

author: Kjelt
author_rank: Super Contributor
timestamp: August 21, 2018, 07:07:16 am
content: Hi I am building some pcbs for a friend but have trouble with the connectors melting.I could use some tips how to protect these smd connectors against the heat of the reflow process.The connectors are smd type and should withstand reflow according to the datasheet but they don't.It is leaded solder so temps are not going >220oC and I stick to the standard reflow curve.Mounting afterwards is a PITA due to the "wingpads", the mechanically solderpads on the sides.thanks,

author: SMTech
author_rank: Frequent Contributor
timestamp: August 21, 2018, 08:18:09 am
content: Looking at the picture I would be assuming you are using one of those small IR/toaster ovens, where a grill or high power IR lamp is mounted over the PCB. Regardless of what your "profile" may be set to or what the temperature sensor wherever it may be mounted is reading, those connectors are getting very hot. Direct IR energy radiates from the heating element into the plastic casing of that connector, plastic is an insulator so unlike devices connected to copper traces, the energy stays put & the device gets hot.You could try shielding the connector slightly - perhaps polyamide tape and some tin foil or fashion it a small hat from an old stencil/shielding can. I would suggest your profile is probably hotter than you think as well however.Manually soldering these would be easy if you gave yourself a little bit of extra pad to get some heat into with the soldering iron.

author: Kjelt
author_rank: Super Contributor
timestamp: August 21, 2018, 09:31:48 am
content: Quote from: SMTech on August 21, 2018, 08:18:09 amIt is a medium reflow oven with 6 ceramic IR elements.QuoteI measure it twice a year with dual temp probes mounted across the surface and a logging meter, profile is ok better than the factory default ever was,but indeed agreed that the 6 elements do create hotspots.QuoteYes thanks, I thought about Kapton tape, alufoil might be better reflecting the ir more.Do you think that this issue will be non-existent using commercial reflow ovens or does my friend need to rethink his choice?

author: SMTech
author_rank: Frequent Contributor
timestamp: August 21, 2018, 09:48:48 am
content: We have an IR "commerical" 5 zone oven with about 1.6m of active reflow, it does OK with plastic connectors (white ones certainly go a tiny bit browner). However advice and papers you might find out there suggesting IR is better than convection because it is efficient etc, no longer apply to modern electronics. We surface mount things these days that just do not suit, connectors, tall electrolytic capacitors (shadows), big inductors, big boards, component density, all of these things exaggerate IRs weaknesses. Modern convection oven don't have these issues, however the bigger and more expensive ones will handle bigger denser things better, I suspect a benchtop £5-15k convection oven for instance probably still struggles with big inductors on a dense power board or ceramic PCBs.

author: asmi
author_rank: Super Contributor
timestamp: August 21, 2018, 10:15:31 pm
content: I have these issues only with connectors from Molex, other brands (like Samtec) work just fine. Even noname Chinese connectors from Aliexpress mostly work OK. So I just avoid Molex connectors as much as I can, and whenever I can't I solder these manually.

author: chris_leyson
author_rank: Super Contributor
timestamp: August 22, 2018, 12:57:10 am
content: IR and convection reflow use two different heating and cooling mechanisms. In IR reflow the heating and cooling mechanisms are radiative because the air is static, whereas in convection refow the heating and cooling mechanisms are convective and rely on the movement of a heat through a conducting fluid such as air.In IR reflow you have to put radiative heat energy in to heat up metal conductors and that heat is distributed throughout the metal whereas for insulating materials like plastics the same heat energy goes in but stays there because there is no conductive mechasism to get the heat back out, apart from the connector pins. So heat insulators like plastic connectors will get hotter than heat conductors like pcb tracks and components. In convection reflow you have a moving heat conducting fluid, air in this case, that puts heat energy in but also takes the heat out.Not the best scientific explanation, but imagine the air as being a heatsink. IR reflow just puts heat in whereas convection puts heatn in and takes it back out.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: August 22, 2018, 04:27:10 am
content: I have doubts that these connectors are rated accordingly for lead-free (if that is what you are using). I used mostly Taiwanese and Chinese imitation of the Japanease's JST, Hirose, etc. They work all fine with a small 1500W 4-element reflow oven.

author: Kjelt
author_rank: Super Contributor
timestamp: August 22, 2018, 09:43:57 am
content: Quote from: asmi on August 21, 2018, 10:15:31 pmInteresting, these are from Molex and there might be a more temperature resilient version we will try next.

author: Kjelt
author_rank: Super Contributor
timestamp: August 22, 2018, 09:52:07 am
content: Quote from: chris_leyson on August 22, 2018, 12:57:10 amSo with all probability these connectors won't give issues when using a pro P&P firm in production.

author: asmi
author_rank: Super Contributor
timestamp: August 23, 2018, 04:18:38 pm
content: Quote from: Kjelt on August 22, 2018, 09:43:57 amThe datasheet for these connectors says they are good for LF reflow. But apparently not for IR ovens. Here is a telling example:Connector at the bottom is from Molex, while two connectors on the left side are from Samtec, which despite being bigger and taller, survived reflow just fine. So there's definitely something about the plastic used by Molex that makes it very vulnerable to IR reflow.

author: Kjelt
author_rank: Super Contributor
timestamp: August 23, 2018, 10:34:30 pm
content: Tomorrow the next batch Molex's are going to get a tinfoil hat

author: xaxaxa
author_rank: Regular Contributor
timestamp: August 25, 2018, 04:01:59 pm
content: Quote from: asmi on August 23, 2018, 04:18:38 pmMakes me wonder how hot those chips are getting...

author: Kjelt
author_rank: Super Contributor
timestamp: August 25, 2018, 06:47:31 pm
content: Update: the tinfoil hats work fine. It just takes awhile to put them on.There should be fitmade versions.