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.
The machine has a laser pointer shaped as crosshairs. With this shape it is easier to aim if a reference board feature AKA fiducial resembles a cross. When there is no fiducial, a reference point can be produced from existing land pattern, as follows.
A picture below shows the Eagle file for a panel of USB Host Minis. Each board contains MAX3421E chip packaged in TQFP-32.
Next picture shows closeup of MAX3421E land pattern on lower left board in the panel. I need coordinates of intersection of a grid lines passing through centers of pads 16 and 17. The coordinates can be derived from the library part (it is X of pad 17 and Y of pad 16 subtracted from the position of the center of the part itself) or simply read from the Eagle screen by placing a cursor on the intersection. This gives me one fiducial. Second one can be obtained from the MAX3421E land pattern on the upper right board in a similar manner.
After that, both pairs of coordinates can be used to produce fiducial parts in a placement file ( see DP TM220A wiki for more information about file format). Here’s the fragment of mine:
When file is loaded in the machine the parts can be used to check/adjust board alignment. Next picture shows laser pointer showing position of Fiducial1. The difference between tinned pad and surrounding solder mask is clearly visible, especially when USB microscope (not shown) is used.
After one diagonal point is aligned the machine can be moved to the second one to check. The issue with registration on this machine is that one side of a board will always be parallel to the gantry. Some of my boards are cut at a slight angle due to the boardhouse being sloppy – for those I will need to fabricate a jig which would allow board rotation. This is going to be my next project. Stay tuned!
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