Mixed Assembly Placement

Mixed assembly placement usually calls for three operations. Those are surface mount (SM), plated through hole (PTH), and in most cases implementing heat sinks, cables, or press fit connectors for PTH I/O communications. But now with high speed designs becoming more prominent, some of those connectors are converted from PTH to SM due to SM/s tighter tolerance.

Design-for-manufacture (DfM) thus becomes an increasingly important aspect of mixed assembly placement at the design layout stage. The OEM and EMS provider should therefore investigate certain considerations to assure mixed assembly placement is ultimately performed correctly.

• Careful study and analysis of SM and PTH placement to avoid pitfalls.
• Place SM components first since the 10 or 15 percent PTH can be placed by hand later in the assembly process.
• Place connectors close to the PCB’s edge to avoid interfering with neighboring cables or wires.
• Maintain tight tolerances on pick-and-place, solder, AOI, and other assembly equipment.
• Being alert to components that are heat sensitive to wave solder and making sure they are guarded with proper wave solder fixtures.
• Careful planning at the CAM level stage to pinpoint SM and PTH possible issues and problems.
• Mixed assembly placement of lead-free components requires especially careful considerations to avoid thermal profile issues.

Considerations like these and a host of others are embedded in a proper DfM plan the EMS provider must develop and put in practice. A solid DfM plan assures among other things that bifurcation is done correctly in the midst of countless restrictions. Those can be mechanical, height, or restrictions posed by cables, chassis or wires