When assembling a custom PCB that combines both surface-mount and through-hole technologies, success depends on reflow soldering for SMT packages, wave soldering for DIP components, and a thorough review of the board's physical data before finalizing the process plan.
A board containing a QFN microcontroller, fine-pitch memory, and small passive components frequently requires connectors, relays, transformers, or terminal blocks that are more reliably soldered through drilled holes. Surface-mount packages work well for dense circuitry, whereas through-hole packages are better for parts needing mechanical retention or a larger conductive path through the board. Consequently, a single assembly service must be capable of reflow soldering for the SMT side and wave soldering for the DIP side without compromising either joint type. For a hardware team placing a custom PCB assembly order, the critical factor is not whether the supplier offers both processes, but rather how the process sequence and documentation review safeguard both joint structures.
Why a Mixed-Technology Board Creates Both SMT and DIP Solder Joints
The physical differences between SMT and DIP solder joints are fundamental. SMT joints are formed on copper land pads on the board surface: solder paste is printed, the component is placed onto the paste, and reflow soldering melts the paste to connect the component termination to the pad. In contrast, DIP joints are created inside the board: the component lead is inserted through a drilled hole, and wave soldering wets the lead, the annular pad, and the plated copper within the barrel. Because the through-hole connection extends through the board thickness, it offers greater mechanical strength and a larger conductive path. These physical distinctions determine what must be inspected prior to production. SMT reliability depends on the land pattern, solder mask opening, and stencil aperture controlling solder paste volume—layout-stage decisions addressed in references like Analog Devices AN-772 and STMicroelectronics AN2639. Through-hole reliability is governed by hole diameter, board thickness, plated barrel copper, and component thermal mass. A mixed board exists because the two package families address different engineering challenges. QFN, QFP, BGA, DFN, SOT/SOP, and small passive components allow dense circuitry to fit into a compact area. Connectors, relays, fuse holders, terminal blocks, and magnetic or power components require a stronger anchor than a surface pad can offer. While an SMT connector may survive reflow soldering, it can crack at the pad after repeated insertion cycles. A through-hole connector distributes that stress through the plated barrel and around the lead, which is why many industrial, power, and measurement designs retain those parts in DIP form. A PCBA service must be capable of assembling both groups on a single line, from fine-pitch IC packages to inserted DIP components.
Process Decisions That Keep SMT and Through-Hole Zones Reliable on the Same Board
Because both reflow and wave soldering introduce heat to the same laminated PCB, the assembly sequence determines whether one group of joints remains intact while the other is being created. The engineering review must decide which side is soldered first, how the finished side is shielded, and the maximum heat the through-hole process can apply near already-soldered SMT joints.
1. The Assembly Sequence Should Protect Reflow-Soldered SMT Pads from Later Through-Hole Handling
The standard sequence for a mixed board begins with solder paste printing, followed by SMT placement, reflow soldering, and inspection, then through-hole insertion and wave soldering. This order enables the dense SMT side to be completed and inspected before heavy connectors or relays are inserted. If DIP leads were inserted prior to the SMT reflow step, the tall components would obstruct solder paste printing and interfere with the fine-pitch placement needed for QFN, QFP, and similar packages. Reflow soldering also has component temperature limits, and the profile must remain within the limit of the most sensitive package on the board. Texas Instruments AN-1028 states that the maximum reflow temperature is a design constraint, not a parameter that can be increased to simplify soldering a difficult board. On a mixed board, the SMT side must also withstand board support, handling, and the additional heat encountered during through-hole soldering. The process plan should consider the reflowed SMT zone as a completed structure and position DIP insertion such that it does not bend or overstress those joints.
2. Wave Soldering Needs Enough Thermal Input for the Plated Barrel Without Damaging Nearby SMT Joints
Wave soldering must meet two conflicting requirements. The solder wave must provide sufficient heat and flux to wet the lead and fill the plated barrel, particularly on thicker boards and around high-thermal-mass components like transformers, relays, and heavy connectors. Simultaneously, the underside of the board may already have reflowed SMT joints. If the preheat stage or wave contact applies excessive heat to those areas, the SMT joints may suffer thermal damage. Conversely, if heat is insufficient, the through-hole joint may exhibit poor fill or incomplete wetting. Achieving the correct balance is not a one-size-fits-all formula. It depends on board thickness, copper plating inside the hole, component thermal mass, flux conditions, and the actual distance between DIP components and nearby SMT parts. Some through-hole components are better soldered using selective soldering or a dedicated pallet instead of full wave contact. These decisions require an engineering review of the physical board before the assembly line determines which DIP components can be wave soldered together and how the SMT side will be protected.
Design and Documentation Needed Before a Custom SMT/DIP Assembly Run Starts
A mixed-technology board cannot be reliably quoted from a netlist or brief component description. The service must know which components are SMT, which are through-hole, and where process conflicts are likely to arise. Gerber files and drill data define the copper structure: SMT land patterns, solder mask openings, drilled hole sizes, and the clearance between surface-mount pads and through-hole pads. The bill of materials must include exact manufacturer part numbers, package types, reference designators, and approved alternates. For a board with a QFN MCU and a DIP connector, an alternate part might appear similar in the BOM but require a different footprint or a different through-hole soldering method. Approved alternates prevent procurement and assembly teams from making assumptions that could disrupt the process order. The assembly drawing provides information separate from the BOM: pin-one orientation for ICs, polarity for polarized components, mechanical mounting details, parts inserted from the secondary side, and clearance limits around the DIP zone. It also indicates how the board should be supported during through-hole insertion and wave soldering. Thus, a mixed board requires a documentation package built for a dual-process assembly line: Gerber data for the physical pads and holes, a BOM for exact component identity and sourcing, and an assembly drawing for build intent. Maxipcb’s PCBA service follows this approach. Its assembly line supports SMT, DIP wave soldering, and reflow soldering, covering packages from DIP through SOT/SOP/QFP/BGA/QFN/DFN. Mixed-technology boards can be quoted after a DFM review of the Gerber data, BOM, and assembly drawing, which is the stage where process conflicts should be identified before the first assembled unit.
Conclusion
A mixed-technology board is a single electrical assembly containing two distinct solder joint types. SMT is the optimal choice for dense ICs and small passives. DIP is the optimal choice for connectors and power components requiring mechanical retention and a larger conductive path. The PCBA service chosen for that board must be capable of reflowing the SMT side first, protecting it during through-hole handling, and providing sufficient thermal input to fill the plated barrel during wave soldering. When preparing a mixed board, submit the Gerber files, a complete BOM with manufacturer part numbers and approved alternates, and the assembly drawing for review. Confirm the proposed assembly sequence, the through-hole soldering method for each DIP component, and how the SMT side is protected. A service that reviews those files before quoting is more likely to deliver a functional board with both joint types intact.
FAQ
Q:Why would a custom PCB assembly run need both SMT and DIP on the same board?
The two package families exist for different physical reasons. Dense ICs, memory, and small passives are surface-mount parts that can be placed and reflowed efficiently, while connectors, relays, transformers, terminal blocks, and other parts that carry higher current or face repeated mechanical stress are more reliable as through-hole components because their leads are soldered inside a plated barrel. Many industrial, power, and measurement boards therefore combine reflowed SMT zones with wave-soldered through-hole zones on the same PCB.
Q:What is the usual assembly order for a mixed-technology board with SMT and through-hole parts?
The typical sequence is solder paste printing, SMT placement and reflow, inspection, through-hole insertion, and then wave soldering. This order enables the dense SMT side to become a completed, inspected structure before heavy DIP components are inserted. It also prevents through-hole leads from interfering with paste printing and fine-pitch placement, and it provides the final wave soldering step access to the plated barrels around the inserted leads.
Q:What files does a PCBA service need to assess an SMT and DIP mixed assembly project?
The essential package consists of Gerber data, an accurate BOM, and an assembly drawing. Gerber and drill files define SMT land patterns, solder mask openings, hole sizes, and clearance around mixed-technology areas. The BOM must include exact manufacturer part numbers, package types, reference designators, and approved alternates. The assembly drawing indicates polarity, pin-one orientation, mechanical insertion details, and any special through-hole requirements. These files enable the service to perform a DFM review before quoting the mixed assembly.
Sources / References
Texas Instruments AN-1028 Maximum Solder Reflow Temperature for Surface Mount
Analog Devices AN-772: A Design and Manufacturing Guide for the Lead Frame Chip Scale Package
STMicroelectronics AN2639 Solder Pad Recommendations for Surface Mount Devices