Introduction
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Fast or Accurate?
In the distant past electronics boards were assembled by hand. Wires were soldered from one component to another or a wire wrap wire was connected from one pin to another. Advances in PCB technology made the wire wrap tool obsolete. PCB traces and multilayer boards have pushed the forefront of technology. Along with the wire wrap tool, most through hole parts are also a thing of the past. Today high speed machines pick up IC’s and tiny resistors and capacitors and place them on a PCB coated with solder paste faster than the eye can follow.
This is a remarkable achievement and yet we take it for granted that the machine will know where to put the parts. I don’t mean it will forget where the part goes, I mean it will be able to place the part accurately on the board where it needs to go, board after board. Some boards have routed edges but others are scored and snapped. This means the edge of the board may not be a good registration point. To make matters more difficult the clamps that hold the board while the parts are loaded will not grab each board exactly the same way. What is needed is a way to register the copper on the top and bottom of the boards.
When is an eye not an eye?
Computer vision is used to make sure that the boards are in the correct orientation and to determine where parts are to be placed. Small marks on the surface (in copper and not silk screen) called fiducial marks are used to align or register the board.
A copper pad 1-2mm in diameter is a popular fiducial mark. The solder mask on the board does not cover this mark. Usually the solder mask will have a diameter of a few mm more than the mark itself. I mentioned that the computer vision system makes sure that the board is oriented correctly. To help with this an odd number of fiducials that are place in a non-symmetric pattern is useful. If the board is placed in the machine rotated 90 or 180 degrees it will be clear to the vision system there is an error.
There is another source of error you may not be acquainted with – shrinkage. I can just hear George Costanza urging me to tell you there will be shrinkage. Not only during manufacturing but during the soldering process the board can change dimensions enough to cause problems when you go to place the second side of parts. And if you are using ceramic as a substrate, the fabricators must compensate for a 20% shrinkage between green ceramic and the final fired board.
How many and where?
Just how many fiducials and where should they be placed depends on how tight the pitch and how dense the board. Often a PCB fabricator will make a panel of boards. Ten boards might be made in a 2 by 5 array. This array has a frame around it, called the rails, which is just board that is not used for parts. Often fiducials are placed in the rails. When the boars are stuffed, they are stuffed in the PCB array.
If the boards are densely packed or some of the parts are tight pitch, it can be helpful to have fiducials on the PCB itself close to the parts in question. It is best to discuss this with the board stuffing facility.
If you are placing bare die on the board you will need to have several fiducials placed on the board. You will need to work closely with the people doing the wire bonding to make sure the fiducials will meet their needs. Often, they will request specific locations for the fiducials.
You might ask why you will need fiducials on the PCB if they are on the rails. The reason is the error that exists in the measurement the farther you get from the fiducial. If you must travel 4 inches from the fiducial to the location where the part is located, you will have much more error in the measurement than if you only travel 0.5 inches. The other reason to place the fiducial near the part to be placed is the need for the camera to cover the entire scene. If the camera cannot see both the part location and the fiducial in the same field of view, it will need to return to the fiducial often to re-align itself. That takes extra time. If you are building 20 board that might not be an issue, but if you are building 20,000 boards it can add up to significant amounts of time.
Best advice
The best advice I can give anyone on this subject is to have a good working relationship with the people who will be stuffing the boards. They will be able to make suggestions that will make the board more manufacturable. This means faster turn arounds, fewer errors and higher quality product at a lower cost.
Final thoughts
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