Pick and Place Machine 1

Pick and Place Machine 1

Postby admin » Thu Dec 13, 2012 3:24 pm

The delivery of the new Pick and Place machine was confirmed by the shipping company for 17 Dec 2012. Finally we will have an accurate, fast machine.

Pick and Place machines are car sized purchases and a lot of thought and research went into the choice of the machine. After sifting through the options for desktop machine only two machines were seriously considered. The Manncorp 7722FV and the DDM Novastar LE40V. Both machines are small enough to fit through the doors to our work area. This is a big deal, kind of a showstopper. Pick and Place machines can be large and heavy and are made to be rigid. This often means that the frame is welded or cast and can not be disassembled. Also, disassembling a machine means re-assembly and alignment. Alignment is also critical in the machine meeting its accuracy spec.

The runner up was the 7722FV. Several smaller manufacturers such as Adafruit and Eurobuilder went with the 7722FV and since they must have done their homework I had a very serious look at the machine. It is a benchtop machine and does not have a welded or cast frame. It is reasonably light, around 150 lbs. It has real feeders and optical alignment. It is reasonably fast at around 2200 components per hour and can handle all the components that I see in immediate future. Here are some photos http://www.adafruit.com/blog/2009/10/13 ... e-is-here/

In my evaluation of the 7722FV I was slightly uncomfortable with the stated accuracy of the machine and to a lesser extent the way in which the machine is constructed. The machine's accuracy is adequate for current parts but it has no room for the feature shrink in packages occuring now. A while ago, everything was dip packaged and pin pitch was 2.5mm. Then everything was leaded surface mount. Now lots of new parts are only available in leadless surface mount and ball grid (or chip scale) packages. Currently the ball pitch is already down to 0.4mm and t is likely to shrink in the future. 0.4mm is the limit of the 7722. Also, the 7722 is an open loop machine. Positioning takes place by counting pulses afer a reference was established, the gantry position is not measured. The reference is found by bringing the part over an up-looking camera and doing a pattern match to a known good position of a part on the nozzle. The nozzle (and gantry) is then moved to the desired position and the part is placed on the PCB and held in place by the solder paste. In any well designed machine the movement does not miss steps after the alignemnt and if the part does not move on the nozzle the only error is the postional error inside the step. This may appear to be neglible but in practice it depends on exactly where the motor is commanded to stop and the speed at which the motor slows down. Friction, bending and stretching and temperature effects all add to the complextities. That is why the accuracy is not equal to the resolution. The stated resolution of the machine is much better than the accuracy.

My second reservation about the machine is the construction using Aluminium profiles. When stiffness matters this is not the best way to build the frame. No doubt the machine meets its accuracy when it is new but I don't know how it will age. Secondary things that I took into consideration was the design of the feeders. The feeder design is 'classic'. It has an indexing mechanism and can advance the tape quite precisely and quickly. The feeder is a mechanical design. I can't remember if the machine had an option for 'smart' feeders but the default feeder for the LE40 is smart. A 'smart' feeder has an electrical interface to the machine and it can be identified by the machine to verify that it is in place. Smart feeders also maintain a count of the parts used making inventory tracking easier.

The LE40V was the winner. The machine base is welded steel and the motion is closed loop in the sense that the actual gantry position is measured using linear encoders. This gives me confidence that the long term accuracy will be the same as the 'factory' accuracy. The other major factor influencing my choice was that DDM Novastar gave me access to the manuals before purchase and the manuals are complete, legible and in good English. They even contain circit diagrams of the machine. From the manuals I recognized the motion controller used in the machine as a controller made by Galil. I have fisrt hand experience using their controllers in motion systems and they are used wherever high quality motion control is required. My current machine uses trapezoidal profiles for the motion and the jerk is not welcom. S-curve motion is definitely not a luxury.

The price of feeders are also a consideration. Very often you spend as much in feeders as on the machine. The LE40 has a very clever bank feeder for 8mm tapes. 8mm tapes are by far the most common size and you can never have enough of them. The bank feeder is a 'single' feeder for 12 8mm tapes and it costs less than a 5 individual 8mm feeders. Since I have existing products I knew from the beginning that I need more than 12 8mm feeders and this is an economical solution. The prices for the LE40 feeders are comparable to the feeders for the 7722 but they are smart feeders. Even though the initial price for the LE40 was higher than the 7722 by the time I factored in the price for the feeders, and external PC and a compressor, the LE40 ended up at roughly the same price.

As they say in the movies, watch this space. I will provide pictures and details of my experience with the machine.

Happy hacking.
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