If the pads in the centre of the board have been accurately printed, registration there must be correct. You need to look at the direction of the claimed misregistration towards the corners. The most likely findings will be that the misregistration gets worse with increasing distance from the centre, and that inaccuracies at opposite corners are in almost exactly opposite directions. For example, if deposits in the upper right quadrant are printed towards the bottom and left of the pads, those in the lower left quadrant will be printed towards the top and right. The effect can be visualised as being due to the image on the stencil being slightly enlarged (or reduced) compared to that on the board, and this is indeed a good description of the problem. Its cause is inaccuracy in either board or stencil, and is most likely to be the former, produced by material shrinkage during manufacture and dimensional changes with temperature.
In this case, the vision system is operating correctly, averaging the error between the fiducials: if in doubt, refer your colleague to the explanation in this unit. To confirm correct operation, examine whether the problem is consistent within a PCB batch – consistency indicates that any registration differences between boards are being compensated for by the vision system.
If critical areas are off-centre, most vision systems offer the opportunity of changing the weighting of the fiducials so as to move the position of the optimised area. Alternatively, where correct registration is required everywhere, and the batch is both consistent and of sufficient size, you could laser-machine a stencil to the actual board pattern, rather than to the design co-ordinates. If the problem lies in variable shrinkage of the board, you might also be able to purchase more stable base laminate.
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