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In this month’s installment in this series of columns aimed at helping buyers analyze and select SMT equipment for printed circuit board assembly, I will address features that affect decisions on the selection and purchase of an appropriate automatic pick-and-place machine.
As with any complex machine, there will be tradeoffs between cost and capabilities, some of which specifically relate to production accuracy and yield. We will address:
1. Mechanical positioning methods
2. Machine construction
3. Solder paste fluid dispensing
4. Component feeders
To review, when starting the evaluation process, there are two defining factors to keep in mind which determine what category fits your machine needs. The first principal factor is CPH (components per hour), and the secondary factor is machine capability. While it’s constructive to start by understanding how production rates affect the type and performance of a pick-and-place machine, please refer to the prior two installments, in the February and March issues, for those ranges.
Machine capability is the second defining factor in helping choose the correct auto pick-and- place machine for your needs. In this installment, we will address three aspects of machine capability that have a direct impact on final board quality and production yield.
Accuracy and Repeatability
For production machines, we typically recommend looking for a machine with accuracy of +/- .001” and fine pitch capability, down to 12 mils, on a repeated basis. Be aware that less expensive machines rarely meet this spec.
There are several types of positioning systems employed, differences in construction methods, and a variety of component feeders, all of which have an impact on quality and yield.
Component Positioning Systems
After each component is picked up and centered in the tool by one of the methods described in Part 2, it must then be positioned accurately on the board, in an X-Y position. Following are three methods commonly used for positioning, along with pros and cons for each.
1. Positioning with no feedback system (open loop system)
2. Positioning with rotary encoders (closed loop system)
3. Positioning with linear encoders (closed loop system)
Editor's Note: This article originally appeared in the May issue of SMT Magazine.