Mixed technology / SMT and THT on one floor
Most industrial products are not purely surface mount or purely through-hole. They carry a reflowed logic section, a connector row that needs mechanical strength and a mechanical stack on top. Keeping all three steps in one building removes the hand-off where boards get lost, re-inspected and re-quoted.
Scope
Twelve parameters that describe the process as it is actually run, including the ones that limit how many times a board can go through an oven.
| Parameter | What it covers | Limit or note |
|---|---|---|
| Technology mix | Reflowed SMT, through-hole, press-fit and electro-mechanical steps on one assembly | One build order, one serial |
| Reflow exposures | Two SMT passes plus one pin-in-paste pass before the thermal budget is reviewed | Maximum 3 passes |
| Second-side attachment | UV or thermal-cure adhesive dots under heavy bottom-side parts | $0.01-0.03 per component |
| Pin-in-paste | Through-hole connectors printed and reflowed with the SMT side | 1:1 aperture, 0.1-0.2 mm overprint |
| Selective soldering after SMT | Connector rows finished with a 3-6 mm nozzle under nitrogen | Oxygen below 1,000 ppm |
| Mixed alloy control | SAC305 and SnPb kept apart through pots, irons and profile libraries | Separate consumables and pots |
| Press-fit | Compliant-pin connectors inserted after reflow with no solder joint | 60-120 N per pin |
| Assembly thickness | Rigid boards from thin wearables up to thick power assemblies | 0.4-4.0 mm; above 4.0 mm selective only |
| Panel handling | Rail and tab design plus support pins under heavy through-hole parts | 5 mm rail, 2.0 mm tabs |
| Thermal mass control | Soak profile that brings a heavy connector and a 0402 part to liquidus together | Cold-spot delta under 12 C |
| Coating sequence | Conformal coating applied after every solder and press-fit step | Solderable surfaces masked |
| Mechanical steps | Torque-controlled fastening, staking, potting and labelling on the same traveler | Torque logged per fastener |
Build sequence
Every mixed-technology assembly is planned as this sequence. Moving a step changes the panel design, not just the schedule.
SEQ-01 / Print, mount, reflow
Paste is printed through a 100-150 um foil chosen for the finest pitch on the board, passives are shot at 40,000-80,000 CPH, and fine-pitch ICs are placed in vision mode. One 10-zone profile covers the side at a 217 C liquidus.
SEQ-02 / Second side
Adhesive dots hold anything above roughly 3 g before the board is flipped, then the second side is printed, placed and reflowed at a lower peak so the first pass is not disturbed. Cold-spot delta across the panel stays under 12 C.
SEQ-03 / THT finish
Connector rows that need mechanical strength are soldered by selective nozzle or wave, press-fit parts are inserted at 60-120 N per pin, and the assembly is cleaned, coated, labelled and functionally tested before it is packed.
Single source
Splitting SMT and through-hole work across two shops looks cheaper on a quotation and rarely is once the hand-offs are counted.
The panel ID is applied before the first print and follows the assembly through reflow, selective soldering, press-fit and system test. A single record shows laminate lot, component lot and date code, profile, and every inspection result.
Mixed-model changeover between a reflow-only board and a full mixed-technology build takes about 4 hours, including stencil swap, nozzle change and profile validation. That is what makes a 1-panel production release economic.
Reflow exposures are capped at 3 per assembly. Where a design needs a fourth pass, the DFM note proposes pin-in-paste, an adhesive-dot alternative or a selective route instead of quietly exceeding the budget.
If a connector fails functional test after the SMT side is complete, the same team desolders it under IPC-7711 and 7721 procedures rather than returning a partly assembled board to another supplier.
Comparison
The same board built two ways, scored on the eight factors that show up in a programme review after the first shipment.
| Programme factor | Split supply chain | Single source at GENESIS CIRCUITS |
|---|---|---|
| Hand-offs | Two packing, shipping and receiving cycles per lot | One traveler, no outbound leg |
| Lead time | Adds 3-7 working days per hand-off | Quoted as one schedule |
| Traceability | Two record sets that must be reconciled by part number | One serial covering every process step |
| Thermal history | Second supplier rarely knows the first profile | Reflow exposures capped at 3 and logged |
| Rework ownership | Contested when a joint fails after the second build | One team owns the rework record |
| Inventory | Two WIP pools and two minimum stock positions | One WIP pool, released from 1 panel |
| Change control | Revision must be frozen twice | One frozen revision across all steps |
| Failure cost | Scrapped value includes freight both ways | Scrap is detected before the mechanical stack is fitted |
Macro record
Questions
Yes, with pin-in-paste: the stencil aperture is opened 1:1 over the hole with a 0.1-0.2 mm paste overprint, and the connector is placed and reflowed with the SMT parts. It removes a whole selective-soldering step, which is usually worth more than the extra paste.
It does not reach the oven unprotected. Any bottom-side part above roughly 3 g gets a UV or thermal-cure adhesive dot at $0.01-0.03 per component before the board is flipped, and the second profile is run at a lower peak than the first.
Yes. Enclosure preparation, harness routing, torque-controlled fastening, staking, potting, labelling and system-level functional test run in the same building, so a mixed-technology board does not leave the site for its mechanical steps.
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Seven fields. A process engineer replies within one working day.