Sources of placement inaccuracy

For both 0603 capacitor and the QFP-208, the sources of placement and accuracy arise from the machine, the board, and the component outline. The requirements for a large QFP are very much tighter than for a small capacitor, particularly in relation to the acceptable rotational accuracy, and this is usually reflected in the use of a slower precision placement machine for the large fine-pitch package.

The designer should work with both fabricator and assembly house to agree:

• An appropriate board specification, taking care to choose stable materials and design the board internally to avoid warping and any local material stretch
• To agree a standard for fiducials and location of holes, in terms of position and accuracy. It may well be necessary to have local fiducials for the QFP.

In addition, the designer should be aware of the influence of supplier on component outline, as apparently the same part may be substantially different physically, and this will have an effect on the placement process.

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