Reflow oven for prototyping

A decent convection oven should work fine. We just got a really nice B+D for $99.

You could do BGAs too, with these:

There are people who make mylar laser-cut solder paste stencils cheap, too. But it might be less messy to send boards out for assembly.

You don't. It is good practice to put vias in power pads so the amount of solder past isn't critical. If you solder the chip on the top side you can solder the powerpad through the vias using a big soldering iron or a hot-air pencil. I recently bought an Atten 858D hot air station and it works like a charm.

But doesn't that force the part to always be hand soldered?

Google on "skillet reflow" and "toaster oven reflow". Sparkfun electronics sells toaster oven reflow controllers.

Then re-read what Nico has to say. I've had one experience with a part with an unavoidable power pad. I just put one honkin' big thermal via on the bottom that was big enough for the tip of my soldering iron, and soldered the pad on from underneath. If that didn't work, and if hot- airing it didn't work, I think I'd get an electric skillet from Goodwill next, and see if that didn't work.

I got a T962-A a few years back. Some folks complain it's Chinese junk but mine has worked fine. I do prototype and very low volume production on it. I use AMTECH LF-4300 paste and did my own temp curve. I routinely do double sided boards in it.

You can spend more and get a fancier unit or go the other way, as Larkin posted and just use a toaster oven.

If you DYI just remember how much your time is worth.

Also, get a paste dispenser. You can get one from Newegg and others for less than 200 bucks. I just charge up a portable air tank and set it by my bench. I have tried a "caulk gun" type dispenser with minimal success. Quikchip has a manual dispenser but I have not tried it.

Soldering a QFN with hot-air is doable, but

-Lasse

But

I can do that, but I'd really like the fast turnaround time. It's a combination product design and makey-learn for my son Simon. I'll have a squint at the small convection ovens, thanks.

Any suggestions for solder paste types for protos? I notice that you can't get SnPb eutectic with RA flux. Kester claims that R231 is RMA flux, but all the distributors say it's no-clean. Is the "no clean" flux supplied in SnPb paste really rosin?

Cheers

Phil Hobbs

But

This is what we use for leaded/aerospace stuff:

It seems fine for low-leakage apps. We do use a "vapor degreaser" machine to deflux and wash the boards in solvent after reflow. There's a big minimum order for this stuff, but I can send you a jug.

CAUTION: CONTAINS LEAD!

My experience with water-wash and no-clean fluxes is that they tend to be leaky and hygroscopic. Maybe a rosin no-clean is possible; possibly it's just a paste with reduced flux content; don't know.

I tested some deliberately ugly messy hand-soldered rosin flux solder joints with my Keithley electrometer a while back. Pinned the needle on the 1e14 ohm range.

So barring cosmetics, there may be no need to clean after a rosin paste.

I got a large toaster oven from WalMart a couple years ago, brand name is GE. It has two calrod-style heating elements above, and two below, the food tray. it has a convection fan, but I generally don't use it. I got an Omega ramp and soak temperature programmer on eBay for about $70, and a roll of tiny thermocouple wire. I had problems controlling board temperature with the thermocouple in the air, so I then thought to poke it into a through-hole in the board. That works great. I do everything from 0805 up to 25 mm FPGAs with it. I do all the parts on the back side first, then stuff the top and re-reflow for that side. Even a TO-220 regulator stays put on the back when the top is reflowed.

My profile is ramp to 180 C in 2 minutes, hold at 180 C for one minute, then ramp to 230 to 238 C (depending on experience with that particular board) in one minute, hold for one minute and cool down in 5 minutes. This is for lead-free soldering, I go to 225 or so for SnPb.

The soldering oven is really simple, getting the solder stencil apertures right has been the big problem for me, but I'm getting a good handle on the right aperture sizes now. 0805 and SO chips was immediately easy, but the higher density stuff (SSOP, 0.65mm and finer FPGAs) need much tighter control on the amount of solder to avoid bridges.

Jon

Hi Phil,

I did this with a cheap walmart non-convective toaster oven and solder paste (free from a local assembly shop). I had some laser cut stencils made and used a mechanical oven thermometer inside and visually watched that to determine how to adjust the heat. That is about as simple as it gets, and seems to work but I did burn/char some FR4 a bit using that method. You can also visually watch the solder paste, and put a blob of it in there and when it becomes molten/shiny then wait a minute or two and turn off the heat. You can also buy premade cones of solder that will melt at a certain temperature. I think it is simpler to use large vias under power/mounting pads as others stated as this gives the most options for mounting. The whole stencil, solder paste, oven idea works though just good to avoid if possible I think. I did double sided boards in the oven too with not too much problems.

cheers, Jamie

Thanks to all. For present purposes I'll probably go with the oversize thermal via approach (thanks, Nico), maybe with a little help from a heat gun beforehand. I really don't want to be in the manufacturing business, and getting everything dialled in sounds like a big time sink for the amount I'll be using it.

Cheers

Phil

Even the non oversized vias will work like a charm. I presently have one design with five 0.3mm vias (ENIG PCB finish) and it's plain easy to solder/unsolder. I just pre-tin the PCB then wick-suck it to make sure that no surface tension effects (is that the right English term?) prevents the solder to flow under the IC. First solder the pads, then the thermal pad with "enough" solder. That's all.

I believe that. The rest of the board will be hand-soldered for that reason anyway--it's not that complicated.

Thanks

Phil Hobbs

Actually, once you have the paste applied to the blank board and the parts planted in the paste, the oven, itself, is so simple you can't believe it!

Jon

Vias in proximity to a power tab layout pad ENHANCES thermal performance, it does not degrade it.

Hi,

That's only guaranteed if there is a properly soldered power pad on the top side, with via(s) in it, and not just a single via soldered from the back of the PCB.

cheers, Jamie

Even empty, the plated thru vias transfer heat to the backside better.

So, it only works 'better' if the backside (read opposite side) is where you mean to carry the heat away from.

It normally isn't. The ground and power plains are usually inside. On heat pad parts there is also a local top metal area stitched to the ground plane (if ok to connect that nose to ground). A single large via (to be able to reach in) does not necessarily conduct heat as well as half a gazillion small vias.

But, I found the penalty to be small and I never run such stuff at high dissipation.

OK, simple little boards I often do by hand anyway. But, I've done boards up to about letter paper size and 800 parts/side with lots of SSOP-size chips, and I SURE would not want to do those by hand!

The biggest problem I have is getting the amount of solder right on the stencil, but I'm very close to having that perfected. I still get occasional bridges on the highest density parts.

Jon