Tuesday, March 02, 2010

How to waste 2 weeks

Inspired by a class I took from the author of Extreme SMT Instructable, I decided to set up a SMT reflow soldering system.  Although his hotplate technique works well, it isn't as repeatable as I would like.  Recently, he added a follow-up instructable that adds a PID controller to the hotplate system, which should make the process repeatable.  But I didn't see that until I was already committed to the toaster oven system (parts had been ordered). 

And the hotplate technique still bothers me.  The heat is applied to the insulating FR4 fiberglass, so only convection heat is getting to the solder joints.  The bottom of the PCB tends to discolor slightly, so who knows what this means for long term reliability.

Reading several DIY guides, I became convinced that the way to go was to buy a PID temperature controller and a powerful toaster oven.  Why duplicate something you've seen work when you can try to do it "better"?  (hint: This is especially not a good idea when you are under a tight deadline, like I am right now.)

I spent a few days choosing the devices to use, ordering parts, and waiting for everything to arrive.

My materials:
  • a Reflow 3.0 PID controller from The Silicon Horizon, for $65.  This is a PIC-based PID controller with some handy Windows utilities to drive it. No time to build my own - I'm on a schedule here, right?
  • a type-F thermocouple from TSH, $16.
  • a Black and Decker Infrawave toaster oven (recommended by TSH) for $70 from Amazon.  1500W is reported to be sufficient for lead-free work. Also, I know that some industrial reflow soldering systems run on infrared, so I figured it would be a good choice.
  • 35amp Omron SSR from Digikey for $18. (recommended by TSH).  1500W/110V is less than 15 amps, but you want to be overdesigned here so the SSR doesn't overheat.
  • Misc 120VAC parts from HomeDepot and Surplus Gizmos (an electronics surplus store just a short drive away).
Note that this is a fairly high multiple in cost over the Instructable's hotplate method.

Not wanting to risk electrocution or a fire, I put the AC stuff all into an electrical box.  I chose a metal weatherproof  box so the box could act as a heat sink for the SSR.  It turns out that the SSR runs pretty cool in this application.  I don't think the metal box was actually necessary.  The spot where the SSR is bolted to the box with a bit of thermal paste barely warms up under full load.

I added a red indicator lamp so I can see when power is flowing to the outlet through the SSR.  And an on/off switch so I can easily kill power whenever I want.  I leave the switch off when the oven is not in use, for safety.
 The Infrawave oven's controller needs to be bypassed so the SSR has complete control of the heating elements.  I didn't follow The Silicon Horizon's instructions.  They say you need to defeat the door switch, but the door switch just feeds into the toaster's controller. It is not directly interrupting AC, so after bypassing the controller, it has no effect.

Also, they suggest soldering bridge wires around the SSR's on the toaster's controller.  The AC wires coming to the oven and going out to the heating elements are all common 1/4" spade connectors.  I used three spade receptacles, a wire nut, and a small amount of wire to completely bypass the controller.  The unused controller no longer receives power using my method.  Thos connectors are just laying on the green PCB, they are not connected to it.
Right after I took this picture (taken with the toaster upside down), I removed the brown relay board shown at the top of photo completely.  There wasn't room for my wires with that board in there.  But it no longer does anything at all, so why store it in there?

So here it is, all assembled.  Note that I'm not going to give detailed instructions on what I did.  AC is dangerous. If you don't know enough to do this without detailed instructions, IMO you shouldn't be mucking around with AC.

Next, I experimented with some scrap boards and no-lead solder paste.

Some notes on the Reflow 3.0 controller.  It uses a USB connection to the host PC running Windows.  As you change parameters, it seems to reset the controller and its USB connection a lot.  I'm thinking that changing a profile or PID parameter shouldn't require a complete reset.  Probably some optimization can be done there.  The source code for the controller is reportedly available, but I don't have time to look at it right now.

Starting a reflow is hit or miss.  Fairly often, the Windows app will report that the reflow operation has started, but nothing happens.  Just hit stop reflow in the app's Reflow menu.  Check that power is on to the Reflow 3.0 and in the SSR controller box, and then re-start the operation.  It seems to always start the second time. 

The instructions for the Reflow 3.0 say to start with 1-1-1 for PID parameters.  The Infrawave tends to work well at 40-0-120.  That's a long way from 1-1-1.  Other ovens will work best with different PID parameters, but don't expect to be using anything close to 1-1-1.  I wasted an evening finding all that out.  The guy that runs The Silicon Horizon responded to my email first thing the next morning and set me straight.

The Infrawave, for all its 1500W of power, struggles to hit temperatures over 220 C.  It can take over 60 seconds to get from 180 C to 235 C.  The no-lead solder paste I was using was not reflowing at 235 C.
You can see in this microscope photo that the flux has been activated and is mostly gone, but the solder doesn't reflow - it just forms balls.  When the solder reflows properly, surface tension should pull it onto the pads and leads (see below).  Instead, it has just puddled, flowing out from the pad on the 0603 cap on the left, and shorting all the pins on the QFN (lead-less) package on the right.

I've been told this is a symptom of too low a soldering temperature.  I need to get the temperature to 240-250 C for several tens of seconds to properly reflow the no-lead paste.  Eventually, I will try that, but it looks like doing so will exceed the recommended time at solder temperatures for most components.  From past experience, I know this is especially a problem for my SMT LEDs.

For now, I've run out of time, so I have resorted to hand-soldering my boards.  I've also got some leaded paste on order from Macetech.  It should reflow nicely at around 220 C.  That should easily work with my system as-is for doing additional boards. I have over 10 boards I need to solder after debugging the first board of each type - way too much for me to hand solder.

Below is what the joints should look like, under the same magnification as the picture above.  You can see how surface tension has caused the solder to wick onto the metal pads and leads.

Here is the Fail Whale, visiting my attempt to do lead-free soldering in a toaster oven.


Akiba said...

Nice fail whale :)

Gonna try my hand at lead-free very soon. Will probably need to do some experimentation as well. In the worst case, I might set a medium term project to build my own oven. There just happens to be a temperature process control shop in akihabara with lots of fun heating elements.

In any case, I'll be working on it too so I'll let you know if I turn up anything interesting.


Frank said...

I know this is an old blog post, but I just finished setting up an almost identical reflow oven and thought I'd give my impressions for anyone who stumbles upon this. The only real differences in my setup are I'm using a K type thermocouple (as recommended by the TechFX manual), and an SSR I could get at my local electronics store. The oven is the FC150R (1500W Infrawave) that is on Amazon for $68.

I haven't done a board yet (waiting for them to arrive), but my setup has no problem hitting 250C and staying there. That's as high as I've gone but I suspect it will easily get much hotter. I have the profile set to go from 218 to 250 at 2C per second and it easily does that. I'm also using PID settings of 1-0-4. I have a spreadsheet profile and using a readahead of 3, the oven almost perfectly follows the profile. I'm quite please and I suspect my reflows will come out great.