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This is the third in a series of articles designed to help buyers analyze and select SMT equipment for PCB assembly, and examines automatic pick-and-place machines, the most complex component in a circuit assembly operation. Because of this complexity, the next few columns will address different functions in this process.
Automatic pick-and-place machines are much more common than manual or semiautomatic machines, but they also have the greatest range of capabilities and cost. This column will focus on features and capabilities and include some cautions regarding reliability of low-cost machines, including:
• Production volume ranges (CPH)
• Accuracy and repeatability specs
• Pick-up and centering methods
In future columns, I will address size, machine accuracy, encoders, feeders and mechanical positioning methods.
When starting your evaluation process, there are two defining factors to keep in mind, which determine what category fits your machine needs. The No. 1 principal factor is components per hour (CPH), and the secondary factor is machine capability.
As in the previous column, it’s constructive to start by addressing production ranges for various types of machines, since this is the No. 1 factor in your evaluation process. For purposes of comparison, since all circuit boards vary in size and complexity, we talk about volumes in terms of components per hour, or CPH. The following table offers a general guideline of machine categories defined by their CPH.
This is the second defining factor in helping choose the correct auto pick-and-place machine for your needs. Here, we will only be talking about two aspects of machine capability: accuracy and repeatability, and pick-and-place centering methods.
Accuracy and Repeatability
For production machines, we typically recommend looking for a machine with accuracy of +/- .0001” and down to fine pitch capability of 12 mil on a repeated basis. Less expensive machines often don’t meet this spec. Most low-cost machines will also not come standard with a computer or software which could help with the repeatability aspects if not the accuracy. While some may offer enhanced technology, most do not.
Pick-and-place Centering Methods
There are four methods for pick-up and placement:
1. No centering mechanism
2. Mechanical (jaws)
3. Laser centering
4. Vision centering
Method 1: No centering mechanism other than relying on the component’s pick-up point for placement. In other words, the part is not physically centered after being picked up by the tool head, and if it’s picked off-center on the tool, it will be off-center when placed on the board. Obviously, this is not a very accurate placement method because there is no definable tolerance. You can expect to find this method used by hobbyists or instructors, but certainly not in any type of precision production environment. There are not many options available either, and long-term reliability is questionable.
• Pros: Low cost
• Cons: Low accuracy, repeatability and long-term reliability, no options, or spare parts
• Size range: No definable tolerances
Editor's Note: This article originally appeared in the March issue of SMT Magazine.