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Reflow Soldering with Hot Plate and Stencil

This was my 5th attempt to assemble some ROM-inator II boards using reflow soldering, instead of my normal drag-soldering with an iron. After various failures in the previous attempts, this time I used a stainless steel stencil to apply solder paste to the pads, hoping the stencil was the missing ingredient needed for success. I’ll save you the suspense: it didn’t work any better than my earlier efforts. I think I’m cursed.

You can read about the previous reflow attempts here:

1. Reflow with hot air
2. Switched to a hot plate
3. With an aluminum pie pan
4. New solder paste and aluminum sheet

The ROM-inator II PCB has two chips with 0.5mm pin spacing, which makes home reflow particularly challenging. If my boards only had larger components, SOICs, and the like, my attempts would probably have gone much better. With the 0.5mm chips, no matter what method I tried, I ended up with numerous solder bridges between adjacent pins. I was able to fix up the solder bridges manually with an iron, so all the boards worked in the end, but reflow totally failed as a faster and easier replacement for drag-soldering. For that, I’d need boards to come out of reflow with zero solder bridges at least 80-90% of the time, and I was never anywhere close to that level of success.

 
Stencil Process

I ordered a 4 mil stainless steel stencil from OSH Stencils, and paid a little over $20 for the stencil and a couple of acrylic framing pieces. Ordering was easy, but I did run into a a few problems with my paste layer generated by EAGLE. The paste layer also contained my board outline, which I clearly didn’t want cut into the stencil. It also contained openings for all the edge contact pads on the PCB, which are not meant to be soldered. It required some extended futzing around in EAGLE in order to suppress the unwanted outline and pads, and generate a new paste file.

I taped the acrylic framing pieces to my desk, along with an extra blank PCB, to create a secure frame. Next, I placed a ROM-inator II PCB in the frame, and taped the stencil over it. Aligning the stencil correctly with the pads underneath was easier than I expected, even for the sub-millimeter sized pads. I then squirted some solder paste from a syringe onto the stencil, and used a plastic card to scrape the paste across the stencil, filling the holes.

Framed area for PCB:

Blank PCB in frame:

Stencil positioned on top of PCB:

Solder paste applied to the stencil:

After scraping paste across the stencil:

The PCB after removing the stencil:

Scraping the paste into the stencil openings was more difficult than I’d expected. I’d thought I would be able to make a single pass and fill all the holes, but some holes only partially filled with paste, even when there was plenty of paste scraped over them. Something to do with the thickness of the stencil, the size of the openings, or the viscosity of paste maybe? I had to make repeated passes from different directions with the plastic scraper before all the stencil openings were filled evenly.

One problem I hadn’t anticipated was the thickness of the acrylic framing pieces. They’re about 1.5mm thick, but the ROM-inator II PCB is only 1.2mm thick, resulting in the stencil floating 0.3mm above the PCB. 0.3mm may not sound like a lot, but it’s about 3x the thickness of stencil. In practice, the stencil flexed enough that it mostly sat flush on top of the PCB, but it clearly wasn’t ideal.

The final photo above shows the mixed results of solder paste application. For larger things like the C1 capacitor, it was great. And for many of the individual pads on chip IC1, there was a nice clear line of paste right on the pad. But for many other pads, the solder paste was smeared between several adjacent pads. Not good.

The final difficulty was placing the chip on the pads. Even where I had clean lines of paste, it was difficult to position the chip accurately without smearing the paste around in the process. I would drop the chip down with tweezers, close to where it needed to go, but “close” wasn’t close enough. Even a tiny sub-millimeter nudge to line up the chip properly was enough to smear paste from one pad to the next, possibly negating the whole benefit of using a stencil in the first place. Here’s what things looked like after placing the chip, but before reflow:

 
Results

The result was similar to all my previous attempts: lots of solder bridges. Not surprisingly, most of the bridges showed up in those areas where the solder paste was smeared before reflow, but some bridges appeared even in areas where the paste had looked OK. Here’s how the same chip looked, after reflow with the hot plate:

I tried a second PCB, this time attempting to use less solder paste overall, but the results were the same. Then I tried a third PCB, and scraped the solder paste across the stencil left-to-right, parallel to the orientation of the pads, instead of perpendicular. That resulted in some weirdly uneven application, and more of the same solder bridges after reflow.

I could never get the solder paste application to come out cleanly, and it always seemed like too much.

 
Conclusions

At this point I’ve run out of blank ROM-inator II PCBs, so my experiments will have to stop for a while. I hate to end a series on a down note, but I’m a little frustrated and disappointed with this whole effort. Maybe I would get better results with thinner frame pieces, or a toaster oven instead of a hot plate, or yet another brand of solder paste, or a different paste applicator, or more flux, or a pick-and-place, or… but at some point I have to admit this just isn’t working. Once I get more blank PCBs to play with, I’ll consider whether to try further experiments.

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Bootstrapping Apple //c with Floppy Emu

Thanks to Floppy Emu fan Andru Luvisi for contributing a great trick for bootstrapping an Apple //c with Floppy Emu. The Apple II family computers can normally only boot from Disk 1, but when Floppy Emu is connected externally to a //c and configured in 5.25 inch emulation mode, it becomes a non-bootable Disk 2. Until now, the options for making Floppy Emu bootable on a //c were:

  • Switch the Emu’s emulation mode to Smartport hard disk, which is bootable
  • Boot from a real 5.25 floppy in Drive 1, and then access the Emu as Drive 2
  • Connect the Emu internally, in place of the real 5.25 inch floppy drive
  • Use an A/B switch cable to connect the Emu and the real 5.25 inch floppy drive internally

Andru has devised a method for booting the //c from the Floppy Emu while it’s connected externally and configured in 5.25 inch emulation mode. In other words, it’s a method for booting from Disk 2 – something that’s normally impossible. This is great for the scenario where you want to make a bootable ProDOS floppy, and you’ve got a Floppy Emu, but no real floppies with a bootable DOS. Now it’s possible to boot from the Emu externally, then put a blank floppy in the //c’s internal drive and copy ProDOS to it.

  1. Connect Floppy Emu to the //c’s external disk port, and turn on the computer.
  2. The //c will display a CHECK DISK DRIVE error.
  3. Select ProDOS v1.9 from the Floppy Emu’s disk selection menu.
  4. Press CTRL+RESET on the //c keyboard to get a BASIC prompt
  5. At the ] prompt, type CALL -151 and press RETURN
  6. At the * prompt, press CTRL+E, then press RETURN
  7. You’ll see a line of text like M=00 A=08 X=00 Y=00 P=00 S=B7.
    If the line of text begins with M, then type
    :0 E0 60 1 and press RETURN
    Else if the line of text begins with A, then type
    :E0 60 1 and press RETURN
  8. Type C60BG and press RETURN

The //c will immediately begin booting ProDOS from Drive 2!

Andru developed this method by examining code from Apple //c ROM version 255, which includes this feature natively as PR#7. The above monitor hacking makes it possible to do the same thing on other ROM versions of the Apple //c.

I was successful using this method with ProDOS v1.9, as well as with a few other utilities and games. Unfortunately most games won’t work using this method. They’re hard-coded to expect booting from Drive 1, so if you try this method they’ll start to boot from Drive 2, but then you’ll hear Drive 1 suddenly begin to grind away, and the game will freeze or display an error. Despite this limitation, booting 5.25 inch disk images from Floppy Emu as Drive 2 is still a very handy trick!

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Reflow Soldering Still Doesn’t #*&@ Work

After four semi-failed attempts, I am not destined for success with reflow soldering using an electric hotplate. At least not with chips having a 0.5 mm pin spacing. At this point I’m only motivated to continue on by pure stubbornness, as any hope that this would “save me time” is long gone.

Building on the results of my most recent previous attempt, I tried again with a new aluminum plate and new solder paste. I was pinning my hopes on the new solder paste, after having discovered that my original solder paste syringe had already expired by the time I bought it. I suspected that the poor wetting behavior and large number of solder bridges were due to spoiled paste. Judging by its date code, the new solder paste was only five months old, and before purchase it was stored refrigerated by DigiKey to prevent spoilage. They shipped it in a cold pack, at least, so I assume it was refrigerated before that.

The results were sadly the same as my prior attempts: poor wetting and tons of solder bridges. Apparently it wasn’t my solder paste’s fault, but my technique. The video shows the story in all its gory detail.

After having more-or-less exhausted the other possibilities, I’ve come to the conclusion that using a stencil must be essential for success. Maybe no-stencil application of solder paste direct from the syringe can work for components with nice big pads, but not for chips with 0.5 mm pin spacing, unless you want to spend time afterwards manually fixing solder bridges with an iron. Lacking a stencil, I prepared a second PCB by carefully smoothing out the solder paste with a cotton swab, trying to approximate the result of a stencil. I couldn’t avoid getting solder paste between the pads, of course, but I tried to get a thin and even coating over the entire pad area. Here’s what this second attempt looked like, prior to placing the components:

Looking at this photo now, the solder paste coating is kind of terrible, but at the time it seemed good. It’s hard to appreciate just how small those pads are, and it’s very difficult to spread the solder paste around evenly with a cotton swab.

After reflow, this second board turned out better than the first. There were fewer solder bridges, and the overall distribution of solder was more uniform. It was still far from acceptable, though.

Now it’s time for a decision. Do I order a stencil, and try again? Out of pure stubbornness and a desire to see this finally work, I want to say yes. But the practical part of my brain says no, I’ve already proven that it’s faster to assemble these boards with my current technique of drag-soldering. That makes success or failure with reflow and a stencil irrelevant. Maybe I’ll try it anyway for “educational purposes” – a sure way to justify any questionable idea.

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BMOW on https

padlock

The entire Big Mess o’ Wires site should now be usable via the encrypted https protocol. I had some trouble with this a year ago, when I unintentionally enabled https for the blog and discovered that lots of things broke. At the time, I wasn’t ready to make the effort to fix it all, so I implemented a security-unfriendly solution of redirecting all https requests to plain unencrypted http instead. As of today that’s no longer necessary, which is good news.

The BMOW store has always been https-only, but I never thought the blog section needed https. After all, there are no passwords or financial data or other secrets to protect here. The trouble with enabling https for the blog is the zillions of hard-coded references to http://www.bigmessowires.com in the text of old posts, image URLs, forms, and elsewhere. Http elements in an https page cause the browser to give security warnings, and some features like forms just plain don’t work. Fixing this was much less trouble than I’d feared – it only took one careful search-and-replace operation on the database to fix most of them. Yes I have backups, but I still quadruple-checked my search parameters before bulk-modifying 10+ years of posts.

Why is encrypted https useful for browsing public information, like the contents of the BMOW blog? Depending on your level of paranoia, it’s not. However, if you’re especially concerned about privacy, browsing the blog using https instead of http will provide some extra protection. It will prevent snoopers from seeing exactly what content you’re viewing on the BMOW site, or what you posted in the content forms. They’ll still be able to see that you interacted with bigmessowires.com, but no details about what you did there.

Https also provides more confidence that the content you’re viewing is the same content that the server sent you. With an unencrypted connection, a man-in-the-middle (your ISP, for example) could modify the pages you’re viewing on the fly, inserting extra advertisements or tracking elements or malware. I’m not certain this protection is completely guaranteed, however. While I’m no expert, I’ve read about SSL interception proxies that sit in the middle of an https connection, while making both ends think they’re communicating directly with the other end. Nevertheless, using https for all your web browsing should greatly reduce the risk of this type of tampering.

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Hot Plate + Aluminum Pie Pan = No Joy

I’m back with another episode of my DIY reflow soldering adventures! The first attempt using hot air didn’t go well, and the second attempt with a hot plate and half-inch thick aluminum block wasn’t great either. This time I used a lightweight disposable aluminum pie plate instead of the half-inch aluminum block. The idea was to reduce the thermal lag observed with the thicker block, and make it easier to remove the PCB from the hot plate after reflow was complete.

The lightweight pie plate proved to be a bad idea. Its bottom was embossed with small ridges, and was slightly warped and dented. The pie plate wouldn’t sit flat on the burner, and the PCB wouldn’t sit flat on the pie plate. To make matters worse, the pie plate also deformed as it heated. With such poor thermal contact between the burner, pie plate, and PCB, the PCB heated very slowly and unevenly. It did eventually reflow successfully after 10 minutes of heating, but I had to push down one side of the PCB with tweezers to improve the thermal contact.

The results of reflowing looked similar to my first reflow attempt using hot air: not good. There were solder bridges everywhere, but also clear evidence that many pads were dry, with only a portion of the pad area covered in solder. The photo above shows an example. The bare pads are ENIG plated and have a gold color. After reflow is finished it’s easy to see the areas that are still gold-colored, where solder never flowed. Ideally there should be no such areas.

Such poor flowing of solder makes me think the flux in my solder paste has gone bad. A few days ago I discovered that my newly-purchased paste was already beyond its shelf life, so I’ve got more solder paste on order. Hopefully that will result in better solder flow, with pads that are completely covered in solder, and few or no solder bridges.

 
It’s All About Time

My main goal with this whole reflow adventure is speed. I hope to assemble batches of a few dozen ROM-inator II PCBs faster than I can by drag-soldering with a standard iron. I can finish one PCB every 10 minutes with drag-soldering, and at least half of that time is consumed by fixing solder bridges. If I have to spend a similar amount of time fixing solder bridges on the reflowed PCBs as on the drag-soldered ones, then it’s unlikely reflow will provide the speed-up I’m looking for.

Even if new solder paste resolves my solder bridge problems, I’m also concerned by the setup time required to dispense the solder paste and position the chips. I timed these steps, and they took 6 minutes, all before the reflow process even started. With practice I could probably speed that up by a minute or two, but that still puts the total time including reflow close to 10 minutes, even assuming everything goes perfectly with no solder bridges. Only by reflowing multiple PCBs at once could I hope to save any appreciable time vs drag-soldering.

The video shows the entire process end to end, including dispensing the solder paste, placing the chips, reflow heating, and saying “woah” and “oh my gosh” about 20 times. I apologize for the poor quality of the video. My hand often blocks the view, and the camera isn’t positioned well to show the solder reflow in detail. Maybe you’ll find the “authentic amateur” style amusing!

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Hot Plate Reflow Soldering: Round 1

Last week’s attempt at reflow soldering with hot air didn’t go very well. Today I gave it another try with an electric hot plate, and it still didn’t go well, but maybe I learned something in the process. Will the third attempt bring success?

My goal this time wasn’t really to solder anything important, but simply to test how quickly the hot plate heats up, and where on the control dial’s warm-low-medium-high scale will produce the necessary temperatures. I used an infrared thermometer to measure the surface temperature of the burner.

 
Part 1 – Naked Hot Plate

With nothing on it, and the dial set to “medium”, the hot plate temperature increased by about 100 degrees Celsius per minute, reaching the 183 C melting point of solder after roughly 90 seconds. From there it kept on going to 300+ C before I aborted the test, so “medium” was clearly too hot. I later found a setting just above “warm” that produced a steady temperature about 150 C, which is about right for pre-soak. Unfortunately there was a large amount of temperature variation between different areas of the burner – about 45 deg C. This could cause problems form uneven heating if a PCB were placed directly on the burner.

 
Part 2 – With Aluminum Heat Spreader

For my second test, I placed a 6 x 6 x 0.5 inch aluminum block on top of the burner, to use as a heat spreader. I put an old PCB on the aluminum block, with solder paste and a few small capacitors. It was quickly obvious that the aluminum block was far too thick for this purpose, with much too high a thermal inertia (what’s the right term for this?). Its temperature barely rose at all initially, but then it continued to climb for several minutes even after the hot plate was turned off, as heat was absorbed from the still-warm hot plate.

When the aluminum and PCB reached about 100 C, I decided to turn the hot plate back on to “medium” and just let it rip. The temperature rose by about 15 deg/minute, which is pretty slow. Once it reached roughly 210 C and the solder paste was nice and melted, I turned off the hot plate, but the temperature kept going up! The hot plate burner was probably still at 300+ C, and the aluminum was still slowly absorbing heat. I had to slide the PCB off the aluminum while the solder was still molten in order to allow it to cool. Not good.

The temperature variation between different areas of the aluminum was only about 5 deg C. But the temperature variation between different areas of the PCB sitting on the aluminum was as much as 90 deg C – yikes! The test PCB was very slightly bowed, resulting in some areas not actually touching the aluminum, making them much colder than other areas just an inch away. I’m not sure how to fix that. Maybe I could press down on the PCB with a non-conductive tool while it’s cooking, but that would make it difficult to use the thermometer or adjust the control dial, and would make it almost impossible to reflow more than one board at a time.

I was surprised to see how long it took the hot plate to cool off after use. More than an hour after finishing these tests, the aluminum block was still at 55 C. That could be a problem if I need to run several batches of PCBs through hot plate reflow. Will I need to wait 10, 20, 30+ minutes between batches, to allow the hot plate to cool?

 
What Next?

Maybe a reflow toaster would be easier than a reflow hot plate, but most of the advice I’ve heard says the opposite. I’m also very reluctant to use a toaster, since even a small toaster would be hard to fit in my work area, and it’s tough to see inside a toaster to observe what’s happening during reflow. I really like the open loop simplicity of the hot plate method, and would prefer not to mess around with toasters and thermocouples and temperature controllers unless there’s no alternative. If it comes to that, I might just give up on reflow and stick with hand soldering.

For the next test, I’ll use a much thinner aluminum plate, or maybe just a couple of sheets of aluminum foil. I think the 100 deg/minute warming I observed without the aluminum block is actually about the right rate for reaching the pre-soak temperature, if I can find the necessary control setting to pause it there for a minute afterwards. But I probably still need something to help even out the hot spots on the hot plate burner, to ensure the PCB is warmed evenly.

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