MCM RF assembly is not about placing parts correctly. It is about building a package where each interconnect decision compounds onto the next — and where a single placement error or bond geometry shift can degrade RF performance in ways that are difficult to trace after the build is sealed.
In multi-chip module work, precision is not a property of the final result. It is a discipline enforced at each step of the build. Die placement accuracy sets the stage for wire bond geometry. Wire bond geometry determines impedance and loop inductance. Loop inductance and bond pad access determine what the assembled module can actually do at RF frequencies. That chain has no weak links in a high-performance prototype.
Why RF packages demand more from interconnect
RF packages carry an additional constraint that general-purpose MCM work does not: the electrical behavior of the package itself is part of the circuit design. A wire bond in an RF package is not just a mechanical connection — it is an element with inductance, resistance, and geometry that the circuit relies on. Placement accuracy and bond consistency are not secondary concerns; they are the build.
The operations that set the ceiling on RF performance in a prototype MCM are the same ones that accumulate tolerance error if they are not controlled deliberately:
- Die placement accuracy. Micron-level alignment to substrate bond pads determines bond wire angle, length, and clearance to adjacent features. Small drift at this step multiplies through every subsequent interconnect in the package.
- Bond wire geometry consistency. Loop height, span, and profile must hold across all bonds in the package. A bond on an RF line with the wrong loop geometry changes the circuit’s impedance characteristics, not just the mechanical connection.
- Fine wire capability and process window. RF packages frequently require fine wire — 0.7 mil and below — with tighter loop control tolerances than standard production wire bonding. The characterization requirement is higher and the margin for drift is narrower.
- Repeatability across units. A prototype that worked on unit one is not a process. A build sequence that produces consistent bond geometry from unit one to unit ten is a process that can be characterized and eventually transferred to a volume shop.
Process control as the technical differentiator
In standard packaging work, operations can often be reviewed in isolation. A bond pull or shear test verifies the attach step; a visual inspection confirms the wire bond. In RF packaging, the interconnect only makes sense in the context of the full assembly. A bond that passes a pull test can still affect circuit performance if the loop geometry drifts from what the circuit model assumed.
That integration discipline — understanding how each step affects the next one — is the core technical requirement for prototype MCM and RF assembly. Process control is what makes it achievable at the prototype stage, before a program has the volume to average out variation and the schedule to rediscover what went wrong.
What good process control delivers
When placement accuracy is held and bond geometry is consistent, a prototype MCM can be characterized electrically and the results can be traced back to physical build parameters. That traceability is not just useful for the current build — it is the foundation for understanding how to improve the next article, and how to hand the process to a volume shop when the program is ready to scale.
At Heisler, MCM and RF prototype work is approached as a process-definition problem before it is an assembly problem. The build sequence, cleaning steps, and test points are designed around the specific package geometry and material stack, not borrowed from a generic flow. That is what makes the result repeatable and the data actionable.
If your team is building an MCM or RF prototype and needs a process-controlled approach to die bonding and fine wire interconnect, that is the work we take on.