Reflow Soldering in Inert Atmosphere

CO2 Soldering

CO2 Soldering

Reflow soldering in toaster oven or on a skillet is popular among hobbyists and small-scale manufacturers. The process is simple and the equipment necessary is inexpensive and readily available. The results are very good if several simple rules are followed. One of these rules is to use freshly made PCBs – the shelf life of hot air leveled tin PCB finish is 6 months. A PCB which was used close to the end or past its shelf life will exhibit poor solder wetting resulting in bad joints.

Poor wetting can be overcome by increasing soak time and/or rising reflow temperature to give solder flux more time to act on oxide layer formed on the PCB finish during storage and reflow. This approach worked well in good old times of leaded solder, however, modern lead free solder reflow temperature is already quite high and increasing it may not be possible. In this case, soldering in inert atmosphere could be better option. In this article I will explain the process that I have started using some time ago – with great success.

Conceptually, soldering in inert atmosphere is easy: substituting air (which contains oxygen) in the oven with inert gas eliminates the cause of oxidation. The best gas for this purpose is nitrogen; it is non-reactive at soldering temperatures and being the major component of air it is the cheapest of all industrial gases. Being air-like it is also easy to use – all that is needed is a constant low volume flow into the oven during soldering cycle.

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Neoden TM240A Pick and Place machine – first impressions

TM240A building USB Host Shields

TM240A building USB Host Shields

Some time ago I noticed that I’m spending more time building boards and less time developing and needed to increase my manufacturing capabilities. After thorough reading Dangerous Prototypes’ Chinese desktop pick and place machine forum thread I got in contact with a factory and bought TM240A – the big brother of TM220A. Earlier this week a DHL van carrying 70kg crate pulled in my driveway. After a day of hands-on learning I started building boards. This article was written after 2 days of using the machine and contains my first impressions as well as a couple of hints.

First, it is a real Chinese machine – well built, simple, and reasonably priced. At the same time, an owner must be prepared to fix mechanical issues and work around software bugs without relying on manufacturer’s support – the folks at Neoden are helpful but due to a time difference a reply to an e-mail would arrive the next day. Fortunately, the user base for these machines is expanding and the thread linked above as well as videos by Ian@DP and other people provide lots of useful info.

I was ready to face issues like air lines clogged by small pieces of styrofoam, non-functioning vacuum pumps and such; luckily, the only problem out-of-the box was racked gantry causing feeding fault. Thanks to this post in DP thread I was already aware about the symptoms as well as the fix – so I fixed it. While doing this I learned that to implement the fix no tools were necessary – a typical human finger jammed between the front support and the gantry works just as well as originally specified screwdriver.

I loaded some tapes and proceeded to stuffing boards. During test runs double sided removable scotch tape placed over the pads helped keeping parts in place. “Removable” type is preferable since it leaves no residue. Also, since the machine has no vision, accurate board registration is paramount. Here is how I do it.

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A reel of ADuM4160 ICs has arrived

A reel of ADuM4160s

A reel of ADuM4160s

I’m pleased to announce that the long wait is finally over – I’m now in possession of a reel (this is 1000 pcs!) of ADuM4160 – the core chip of USB Isolator. I will be shipping back orders today and tomorrow – if you have ordered any of USB Isolator products from me in February, March, or April, and will not have gotten shipping notification by Monday, please contact me.

USB Isolators are back in stock. By popular demand, I also started selling ADuM4160 in single quantities – look in components section of the store.

On a side note, I received an e-mail from a company in Denmark offering ADuM4160-based USB Isolators in “complete” form – sporting a nice looking case and bus-powered supply for the peripheral side. I bought one yesterday to take a look. I’m going to write a review and, if I like it, start reselling it.


On reflow soldering.

Tools of the trade

Tools of the trade

I noticed recently that I’m spending too much time building boards – it seems that my trusty air rework station just can’t work any harder. With ski season just a week away, I was looking at ways to minimize any work-related activity to save energy for skiing. As luck would have it, weather this weekend was extremely unpleasant – perfect time to stay home and upgrade my production to small scale plastic stencil/toaster oven process pioneered by Sparkfun and improved by many others. Below is my experience with it together with some pictures – enjoy!

In addition to typical household equipment – old decommissioned toaster oven, putty knife, paper towel, rubbing alcohol, and pair of good tweezers the process calls for two special items, which would have to be ordered in advance. One of it is a solder paste, another is a stencil to apply solder paste over PCB pads.

A Gerber for a stencil can be made from tCream layer in Eagle. This layer is generated automatically and works as-is with no modifications in most cases. However, if board contains packages with power pads, default footprint needs to be modified. Screaming Circuits has an article about proper QFN power pad solder paste pattern.

The stencil itself can be made from several different materials. Large scale production shops use stainless steel. Stainless steel stencil can be cut very precisely and work for many application cycles; it is, however, expensive. Another popular material is Mylar; it is not very durable but cheap. Kapton is more sturdy than Mylar (NASA used it as a micrometeoroid protection layer in space suits for Moon mission and Skylab project) and cost is roughly the same as Mylar – I prefer this material for my stencils.

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