SMT007 Magazine

SMT-Mar2015

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74 SMT Magazine • March 2015 racy and repeatability, and pick-and-place cen- tering methods. accuracy and repeatability For production machines, we typically rec- ommend 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 envi- ronment. 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 Method 2: Mechanical centering jaws or fingers. In this method, the component is picked up and moved into its center position in the X and Y axes on the pick-up head. Typi- cally, this method is easy to set up and repeat- able within +/-.001" accuracy. This centering hOW TO SELEcT an aUTOmaTIc PIck-anD-PLacE machInE continues method is generally found in low- to mid-range machines. • Pros: Easy to learn and set up; repeatable; one of the fastest method currently available; a true "on-the-fly" system; low cost • Con s: Physically touches the component, which may not be appropriate for certain types of parts, especially those with delicate leads • Size range: 0201 packages up to 35 mm square Method 3: Laser centering. In this method, the component is picked up inline with a la- ser beam which detects the component's center position on the tool head and recalculates the zero point of the part according to its position in the X, Y axes and rotational position relative to the head for an accurate placement on the board. • Pros: Touchless; on-the-fly (similar to mechanical method) • Cons: It is less reliable. There are limita- tions on the types of parts it can handle, such as very thin components (if they are .050" thick, they may need to be reset because of part variations, even from the same vendor); requires longer setup time, since the Z-axis (part thickness) must be defined; more costly than mechanical centering, but about the same as vision • Size range: cannot center parts below 0402 packages or larger than 35 mm square Method 4: Vision centering (look-down and look-up). Look-down vision will view the top of the component prior to picking it up for its pick-up location. It then calculates its center, compares it to its image file from the stored da- tabase, then picks up the component and trans- ports it to its position on the board. • Pros: True touchless centering; can handle odd-shaped and delicate compo- nents; accurate to +/-.004" capability • Cons: Typically longer setup times due to sMt quICk-tIPs

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