There are many variations on the PCB assembly sequences described here. In particular, the use of robots is increasing, and for high-volume production the robots can be arranged in assembly line fashion as illustrated in Figure below, with such an arrangement, the components are usually presented in standardized pallets roughly oriented. The robot must then provide the final orientation prior to ion and, for this purpose, vision systems are sometimes employed. Alterantively, some components or mechanical parts are delivered to a station using standard feeding and orienting techniques. The robot then acquires the preoriented component or part at the station.
Estimation of PCB assembly costs
The materials necessary for an estimation of PCB assembly costs are presented at the end of this section. Databases giving the times for manual ion and the costs for automuatic and robot ion are included and w worksheet is provided to assist in tabulating the results. Components and operations are entered on the worksheet in assembly order, one line for each basic type of component or operation.
The time for manual ion, obtained from the database, is entered on the worksheet and then multiplied by the operator rate to give the ion cost. After per component allowances for rework costs are added, the total operation cost is entered.
For automatic or robot ion the cost is obtained directly from the database, and then adjusted for programming, setup, and rework.
For machinial parts, the manual assembly times and costs can be obtained using the product design for assembly handbook. When all the operations have been entered the total cost is obtained by summing the figures in the cost column.
Figure below shows, by way of example, a completed worksheet for a PCB assembly taken from a microcomputer. Such an assembly is commonly referred to as a logic board, and this particular board contains 69 DIPs, 1 DIP socket, 16 axial components, and 32 radial components. In addition, 2 parts are attached machanically requiring 11 screws, nuts and washers. It is assumed that the DIPs, radials, and axials are autoed and the remaining components or parts manually ed or assembled, except ofr one DIP, which is assembled the corresponding DIP socket after wave solder. The completed worksheet for this example board gives a total estimated assembly cost of $3.48. Consideration of the avoidable costs indicates that elimination of the 11 fasteners would save 48.0 cents--a surprisingly high figure for a board with only one nonelectrical component, namely, the end plate.
The operation costs for the automatic ion processes include the cost of rework, which amounts to a total of 46.1 cents--clearly a significant item.
In estimating the cost of wave solder it was assumed that two boards would be processed together. However, the resulting cost of 75 cents is a significant item and in practice should be examined closely for accuracy.
Finally, it was assumed in this analysis that the manufacturer would have an automatic machine available for the ion of the 32 radial components. It is interesting that if these components were to be ed manually, an additional expense of $2.19 would be incurred--increasing the total cost of assembly by 63%.