Multiple-pass reflowing?

[Deku Tree]

I recently made a board that has a lot of components on it, many of which seem to be spec'd to reflow at different temperatures. For example, there is an OEM XTend radio module that suggests a temperature profile that maxes out at 250C. This connector is on the board as well: http://www.jst-mfg.com/product/pdf/eng/eSH.pdf

I set the oven's temperature profile to the suggested profile for the radio, but when the board came out the plastic connector had turned brown and had obviously gotten too hot. My question is; is the proper way to address this to do multiple passes at different temperatures to avoid damaging components? Or should I stick to hand-soldering these plastic connectors?

[helius]

Plastic connectors may get hotter than other components, because they absorb IR energy and have lower specific heat than metal.
Vapor-phase reflow is one way around this.

[SMTech]

The standard method is to find a profile that broadly meets the requirements of every part on the board, so melts your solder, stays beneath the peak temp of of parts and keeps the times at key temperatures to specification. A radio module might require a bit more energy but a peak reflow of 250C is in fact 10C lower than you will see for many generic parts. On complex designs you might shield delicate parts to protect them somewhat from the heat required to reflow other parts of the board, flex circuits are an area this is commonly required.

Conventional convection based ovens go to great lengths to reduce the amount of heat transfer via IR, by contrast small desktop ovens are often entirely IR, it works fine for some stuff and is terrible for others. If you put white plastic connectors in an IR oven, they will go at least a little brown for the reasons Deku Tree mentions. However if you can create a profile that peaks far lower (235-240 should be plenty good enough) but still solders your radio module on, it might go less brown.

The smaller your oven is the more difficult it becomes to reflow larger items in the 4-6 minute window specified by your paste, that can mean you need quite high oven temps to get the energy transfer required. With IR ovens that can end up being pretty fierce,push buttons, connectors and leds will be the first to show the signs.

[DerekG]

I set the oven's temperature profile to the suggested profile for the radio, but when the board came out the plastic connector had turned brown and had obviously gotten too hot.

If you can't get the temperatures right to suit all the components & you don't want to buy a vapour phase oven, then the simplest solution may be to use through hole connectors & either wave solder them or hand solder them. Through hole connectors are often cheaper to buy than SMD connectors.

[Deku Tree]

I don't think I'll be able to buy a different oven, and you guys were right that I'm using a small IR oven (cheap one from Ebay). I'm going to experiment a little bit with a lower temp to see how the connectors handle it, I do think that the browning occurred only near the peak temperature.

[AlanS]

I think DerekG is on the money here. Depending on volume I think splitting the mounting technologies on the board make more sense - the best of both worlds.

[DerekG]

Using mixed technologies can reduce component costs more than you think.

Take electrolytic capacitors. The SMD versions are generally quite a lot more expensive as the manufacturer has to design them to survive SMD oven temperatures. Through hole electros don't have to be designed to withstand such high temperatures.

Also larger electros can suffer a lot of shadow effects in the SMD oven meaning that a vapour phase is the only reliable oven to use.

[Styno]

It is true that SMD connectors and caps are quite a bit more expensive than the through hole variants. But you have to ask yourself the question how the savings of reduced manual labor and increased productivity compares to the added material cost.

The answer may be different for everyone and even between different projects, so I can't give you advice what to choose.

But I can give you my experience: for us specialty/niche hardware is a necessity to run our software so we make it ourselves but we don't really have a floor full of people dedicated to hardware production, it's something we do besides our usual tasks (projects/software development/administrative work). No-one here looks forward to a full day hand soldering connector pins or caps and it distracts from our more lucrative work. So when I can choose between paying triple the price for something as simple as an 8-pin smd header with pick and place tool then I gladly will and it will probably be cheaper in the end (labor + missed income of not working on a project) and certainly more enjoyable.

Edit:
I must add that we have the luxury of a big drawer type IR/Convective oven (Essemtec) and a vapor phase (Asscon). A sizeable project would be 30 panels finished in two weeks, to give an idea of the size of our operation.

[Peabody]

You might consider trying a hotplate instead.  They should have no IR issues at all.  I posted recently about a $20 version for DIY using leaded paste, but I think there are relatively inexpensive commercial hotplates - around $120 or so - including the "946C" on Ebay.

And if permitted, you wouldn't need to get anywhere near 250°C with leaded paste.

[madison.chodikov]

Hi everyone, I have been having a lot of issues with the Vision Capture Board on the Charmhigh Pick and Place Machine (CHMT36VA) and I was wondering if anyone has replaced the vision capture board before? I have a new board and was wondering if there are any words of wisdom or tips to replacing the board. Let me know!