Double-sided mixed-technology boards are common in industrial controllers, power modules, and complex IoT hardware. They combine surface-mount components on one or both sides with through-hole components that must be inserted and soldered afterward. The layout saves space and improves functional density, but it also creates a manufacturing sequence with multiple heat cycles, tight component spacing, and limited access for soldering and inspection.
For OEM hardware teams, the challenge is not whether a mixed-technology board can be built. It is whether it can be built consistently at volume, without hidden defects that only appear in the field. This is where PCB Assembly Manufacturing discipline matters—not as a single capability, but as the coordination of design review, process control, inspection, and production planning.
A double-sided mixed-technology assembly typically goes through reflow for the SMT side, then through-hole insertion, and then wave soldering or selective soldering for the DIP side. Some boards require a second reflow if SMT components are placed on both sides. Each heat cycle adds thermal stress. Each additional process step increases the chance of handling damage or component misalignment.
Two specific issues appear repeatedly:
Shadowing in wave soldering. When a tall through-hole connector or a large component sits close to smaller through-hole pins, it can block the solder wave from reaching adjacent joints. The result is insufficient fill or cold joints that may pass visual inspection but fail later.
Via and pin mismatch. If the PCB hole diameter is too large, the pin may not center properly and solder can wick away from the joint. If the hole is too small, insertion becomes difficult and may damage the plating. Both problems trace back to design decisions made before production starts.
These are not operator errors. They are process and design issues that need to be identified during NPI and controlled during volume production.
Double-sided mixed-technology PCBA 1
The most effective way to reduce DIP assembly defects is to address them before the board is released to production. Fumax’s engineering team performs a standard DFM review during NPI, covering:
When the review identifies a conflict—for example, a connector pin that is too close to a fine-pitch SMT component—we raise it with the customer before production. The goal is not to redesign the product, but to confirm that the design can be manufactured reliably at the required volume.
This review is part of our standard NPI support. It is not a separate consulting service, and it is not limited to a single board. For OEM teams transferring a design from prototype to production, this step often prevents expensive rework later.
Once the design is released, the through-hole soldering process must be controlled. For boards with a manageable number of through-hole joints, wave soldering is often the most efficient method. Key parameters include solder pot temperature, contact time, conveyor angle, and flux application. These are not universal settings—they depend on board thickness, copper weight, and component thermal mass.
For mixed-technology boards with high SMT density or thermally sensitive components near through-hole joints, selective soldering is sometimes the better choice. It applies solder only to the joints that need it, avoiding thermal exposure to adjacent SMT parts. Fumax supports both wave soldering and project-specific soldering processes, including manual soldering when the board layout or volume makes automation impractical.
The decision between wave, selective, and manual soldering is made during NPI, based on the board’s thermal profile, component mix, and production volume. It is documented in the process definition and verified on first articles before volume production begins. This approach aligns with recognized IPC standards for workmanship and process control.
selective soldering 1
Through-hole defects are not always visible from the top or bottom of the board. Barrel fill—how completely solder has risen through the plated hole—is a critical measure. A joint can look acceptable from the underside while the barrel remains partially unfilled. This is a latent reliability risk, not an immediate open circuit.
Fumax’s quality department uses a combination of SPI, AOI, and X-ray inspection to verify assembly quality. For through-hole joints, X-ray sampling is used to confirm barrel fill on first articles and periodically during production. Visual inspection remains in the flow for bridging and wetting, but fill decisions are based on X-ray data.
Inspection records are tied to production lots, and any recurring defect trend triggers a process review. This feedback loop is part of how we maintain consistency across batches—not by relying on final inspection alone, but by catching process drift before it becomes a shipment problem.
PCBA X-ray inspection 1
The process parameters that work for a pilot run are not automatically the right parameters for volume production. Board volume changes, panel utilization changes, and operator shift patterns change. When Fumax supports a project from NPI to mass production, we re-verify the through-hole process at production scale. We do not assume that a successful pilot guarantees a stable line.
Our engineering team and production team work from the same process definition. Any adjustment during volume production is documented and reviewed. This is how we help OEM customers transfer existing designs into stable production and scale output when demand increases.
Fumax’s electronics manufacturing services cover SMT assembly, through-hole assembly, manual assembly, inspection, functional testing, and box build. For mixed-technology boards, the through-hole stage is treated as its own process—not as a finishing step bolted onto an SMT line.
Double-sided mixed-technology boards require more than a capable SMT line. They require design review, process definition, inspection, and production planning working together. Fumax has supported OEM customers in industrial automation, test and measurement, and professional electronics for over 25 years. If your product includes through-hole components that cannot survive reflow, we can review your board files and discuss how our process supports your production timeline.
