At Fumax, the transition from working prototype to stable volume production is rarely blocked by the parts of the process that are automated. SMT lines are programmed, monitored, and documented. The variables are controlled by equipment, and the equipment tells us when something drifts. DIP post-soldering is different. It is the part of PCB Assembly Manufacturing where human hands still do the work, where process parameters are harder to verify, and where inconsistencies can stay invisible until they reach the field.
A customer came to us with an industrial motor controller that had already failed two pilot runs at another shop. The board was not complicated by component count — roughly four hundred parts — but it mixed two very different assembly worlds on the same substrate. The logic side was almost entirely surface-mount and behaved perfectly on any modern line. The power side carried electrolytic capacitors, a toroidal transformer, a relay, and two screw terminals that could not be reflowed at any temperature that would not cook the plastic housings. Every defect in both pilot runs sat on the through-hole side.
This case is a fairly typical example of where mixed-technology PCB Assembly Manufacturing goes wrong when the through-hole stage is treated as an afterthought. For any manufacturer preparing for volume production, this is where the risk concentrates. It is also where Fumax focuses significant engineering attention.
Surface-mount assembly is the default for good reasons: it is fast, dense, and highly repeatable under reflow. But a reflow oven assumes every part can survive the profile, and a meaningful share of power and interface components cannot. Large electrolytics dry out, transformer bobbins deform, and connector housings lose their retention force. Those parts need to be inserted into plated through-holes and soldered from the underside, which is a physically different operation from reflow and usually happens after it.
The consequence is that a mixed-technology board is really two builds sharing one substrate, and the second build can quietly damage the first. When we evaluate a manufacturing process at Fumax, the relevant question is not whether the operator knows how to solder. It is whether the process is built to keep those variables within limits across shifts, across operators, and across production batches.
PCB Assembly Manufacturing, Mixed-Techno 1
The two failed pilots shared the same failure signature. The reflow side was clean — first-pass yield above ninety-nine percent on the surface-mount joints. Every defect sat on the through-hole components, and it clustered in three places: insufficient solder fill in the barrel of the larger capacitor holes, bridges between the relay pins, and cold joints on the screw terminals where the copper area pulled heat away faster than the iron could supply it.
The root cause was not operator skill. The board had been run through a full wave with no selective masking, which meant the underside — already populated with surface-mount parts from the first pass — was exposed to a solder wave it did not need. The wave flooded pads it should never have touched, and the through-hole barrels still did not fill because the heavy copper planes underneath acted as a heat sink. The SMT placement work was fine; the board layout and the soldering strategy were fighting each other.
The fix started with the order of operations, which was already correct but was not being respected in the fixtures. Surface-mount reflow runs first and the board cools fully. Only then do the through-hole components go in. In our build we re-pinned that sequence with a physical hold point between the two stages, so a board cannot reach the insertion bench until the reflow record for that panel is closed.
The second change was pre-heating discipline. Through-hole barrels surrounded by large copper planes need the board at temperature before solder is applied, or the joint freezes before the barrel fills. We set a bottom-side pre-heat stage ahead of the soldering operation and verified it with a thermocouple on a sacrificial board rather than trusting the dial. That single measurement moved the capacitor barrel fill from sporadic to consistent.
SMT placement before the through-hole st 1
The largest decision was how to apply solder to the through-hole side without touching the surface-mount side. Two options were on the table: a selective soldering nozzle that addresses each joint individually, or a wave fixture with a mask that exposes only the through-hole pads. For this board’s mix — roughly sixty through-hole joints spread across a large area, including several heavy terminals — the selective approach won on quality and lost slightly on cycle time, which the customer accepted because the board is built in batches rather than at consumer volumes.
The relay, which had bridged in both pilots, was the clearest beneficiary. A selective nozzle applies flux and solder to the pin group under a controlled dwell, and the bridged pins stopped appearing entirely once the dwell and pull-away speed were dialled in on a test coupon. The screw terminals were handled separately with a hand soldering step under a documented profile, because their thermal mass and mechanical stress make them poor candidates for any automated nozzle. At Fumax, fixture design is part of the NPI engineering review, not an afterthought on the production floor.
Through-hole joints fail in ways surface-mount inspection is not designed to catch. The relevant measure is barrel fill — how completely solder has risen through the plated hole — and the target under IPC standards for class two work is generally seventy-five percent of the barrel height for a through-hole joint on a board of this type. The failed pilots were averaging well under that on the larger capacitors, which is a latent reliability problem rather than an immediate open circuit.
We verified fill on a sample basis with X-ray rather than visual inspection alone, because a joint can look acceptable on the underside while the barrel remains starved. Visual inspection stayed in the flow for bridging and wetting, but the fill decisions were made on X-ray images from the first articles of each batch and then held with periodic sampling. We treat inspection as a feedback loop, rather than a filter at the end of the line. This means inspection records are tied to production lots, and our review routine looks at trends rather than individual boards.
PCB Assembly Manufacturing, Through-Hole 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 the transition from NPI to mass production, we re-verify the process at production scale. We do not assume that a successful pilot guarantees a stable production line.
This is where the relationship between engineering and production matters. The engineering team defines the process. The production team executes it. If the two are not connected — if production runs on undocumented adjustments — the process drifts without anyone noticing until yield drops. At Fumax, engineering and production work from the same process definition, and any adjustment during volume production is documented and reviewed.
| Metric | Before (two pilot runs) | After the process change |
| Surface-mount first-pass yield | 99%+ (unchanged) | 99%+ (unchanged) |
| Through-hole barrel fill (large caps) | Below target, inconsistent | Consistently above 75% |
| Relay pin bridging | Present in both runs | Eliminated |
| Screw terminal cold joints | Recurring | Removed after documented hand profile |
| Rework hours per batch | High | Reduced to isolated touch-up |
Nothing about the surface-mount side improved, because nothing about it had been broken. The entire gain came from treating the through-hole stage as a process with its own parameters rather than a finishing step bolted onto a surface-mount line.
How Fumax Manages DIP Post-Soldering for Stable Production
Mixed-technology boards are not a special case at Fumax — they are part of our regular production. Over 25 years in PCBA manufacturing, we have built the process discipline that DIP post-soldering requires: documented through-hole parameters for each board, fixture design integrated into NPI engineering review, inspection data tied back to process adjustment, and a documented re-verification step when a project moves from pilot to volume production.
Our through-hole assembly capability is not a standalone service bolted onto an SMT line. It is part of a coordinated manufacturing flow that includes component sourcing, PCB fabrication support, SMT assembly, inspection, functional testing, and box build. When a board carries connectors, large capacitors, transformers, or any component that cannot survive reflow, the through-hole stage gets its own process definition — not a generic set of parameters applied to every project.
For OEM teams, the value of this approach is straightforward: fewer surprises during production scale-up, stable output across batches, and a partner who can support the product from pilot through volume. We do not ask our customers to become experts in DIP soldering. We ask them to share their product requirements, and we take responsibility for making the manufacturing process reliable.
Fumax’s electronics manufacturing services are built around long-term cooperation, not one-off builds. If your product includes mixed-technology content and you are planning for volume production, we are ready to review your board files and discuss how our process can support your timeline.
DIP post-soldering is not a secondary operation. It is a process that deserves its own parameters, its own fixtures, and its own inspection criteria. At Fumax, we treat it that way — not because it is complicated, but because stable production depends on it.
For OEM teams preparing for volume manufacturing, the question is not whether a supplier can solder through-hole components. It is whether they can do it consistently, at scale, with a process that does not drift. That is the standard we hold ourselves to, and it is how we build manufacturing partnerships that last.
