Mixed technology assembly combines surface-mount and through-hole components on the same PCB. A suitable production plan identifies how each component will be fitted and joined, which thermal and cleaning processes it can tolerate, and how the completed board will be inspected and tested.
For a buyer, the practical question is whether the factory can build the whole assembly consistently when one board contains fine-pitch chips, relays, terminal blocks and large connectors.
The component mix alone does not determine the answer. Review the exact part numbers, board layout and proposed sequence before comparing assembly prices.
What Does Mixed Technology Assembly Include?
A mixed assembly may combine reflow soldering for SMT parts with selective, wave or controlled manual soldering for through-hole parts. Some designs can use through-hole reflow for suitable components.
The project should identify which process applies to which positions.
| Component Group | Possible Assembly Route | Confirmation Needed |
|---|---|---|
| Surface-mount chips and passives | Paste printing, placement and reflow | Package requirements, board side and approved thermal process |
| Reflow-compatible through-hole connectors | Through-hole reflow, where suitable | Exact component approval, paste delivery and joint formation |
| Conventional through-hole parts | Selective, wave or manual soldering | Thermal limits, access, fixtures and acceptance requirements |
| Press-fit connectors | Controlled insertion process | Approved pin/hole combination, tooling and board support |
These are possible routes rather than interchangeable choices for every part.
If the BOM includes press-fit components, identify them separately. The press-fit assembly process needs its own preparation and acceptance controls.
For the individual technology basics, see SMT PCB assembly and through-hole PCB assembly. This guide focuses on how the processes interact on one board.
Can Through-Hole Components Go Through Reflow?
Some can. Confirm that the exact component supports the intended through-hole reflow process, often abbreviated THR or described as pin-in-paste.
For example, Phoenix Contact’s 1955400 PCB header listing identifies THR and wave-soldering mounting options and provides processing information. This illustrates the type of manufacturer evidence to request; it is not a component recommendation for your design.
A similar-looking connector may have different processing limits. Pitch and pin count do not establish reflow suitability.
Würth Elektronik’s WE-RJ45 through-hole reflow application note discusses the need for temperature-compatible materials, heat access to solder joints and suitable handling. Its discussion concerns the connector family and process it describes.
What Should Engineering Check?
Confirm the component’s permitted soldering conditions, any cycle limits and the proposed mounting method. Do not infer soldering compatibility from its normal operating-temperature range.
Then review how the process will deliver sufficient solder to the plated holes and form acceptable joints. A reflow-compatible housing is only one requirement.
Also assess the effect on the surrounding SMT process. A large connector can change access for printing, placement or heating. Ask the team to approve the combined assembly rather than reviewing each component in isolation.
If the existing connector cannot support the proposed process, compare a separately soldered route with an approved alternative component. Include validation and design-change costs in that comparison.
How Should A Mixed Technology Assembly Sequence Work?
Consider an illustrative industrial controller with SMT components on both sides, through-hole terminal blocks and a relay that the proposed process must keep out of reflow.
One possible route is:
| Stage | Work To Plan | Release Check |
|---|---|---|
| First SMT side | Print, place and reflow | Confirm setup and inspect the completed SMT work |
| Second SMT side | Repeat the approved SMT process | Verify thermal exposure, support and first-side retention requirements |
| Through-hole insertion | Fit the specified connectors and relay | Check part identity, orientation and seating |
| Through-hole joining | Use the approved selective or manual process | Inspect joints and check nearby components for process effects |
| Cleaning and final inspection | Apply the approved cleaning or residue-control plan | Confirm component compatibility and finished-board condition |
| Programming and testing | Verify the completed assembly | Exercise the required functions through the intended interfaces |
This route assumes engineering has accepted those steps for the example. The actual order can change with the parts, equipment, layout and product requirements.
Ask the factory to mark any exceptions on the assembly drawing or work instructions. Purchasing and production should be able to identify the same sequence from the released documents.
Why Does The Second SMT Pass Need Review?
After the first side has been assembled, a second pass creates another thermal exposure for components already on the board.
Confirm that the packages, solder materials and board support suit the proposed process. Where component mass or geometry creates a retention concern, ask how the team will verify it.
Avoid interpreting “double-sided assembly available” as approval of every component combination. The capability still needs to match the submitted design.
Selective, Wave Or Manual Soldering: How Do You Choose?
Choose the process around joint access, material limits, volume and repeatability.
Ersa’s miniwave soldering explanation describes selective soldering of leaded components using localized fluxing and soldering, together with preheating.
That makes access to the intended joints an important review point. The nozzle, tooling and nearby parts must fit the actual assembly geometry.
| Route | Question To Resolve Before Approval |
|---|---|
| Selective soldering | Can the proposed equipment reach the required joints with suitable clearance and support? |
| Wave soldering | Which underside components or areas need compatibility review or protection? |
| Manual soldering | Can the documented method consistently meet the specified joint and thermal requirements? |
| Through-hole reflow | Do component approval, paste delivery and the thermal process support the required joints? |
A board with SMT devices close to through-hole pins may need a different route or additional tooling. Review the intended process with the layout, rather than assuming the factory can compensate for every access problem afterward.
The selective soldering guide covers that method in more detail.
For manual operations, ask which instructions, tools, operator qualifications and inspection controls the quotation includes. For automated operations, ask the corresponding questions about programming, fixtures and setup verification.
Automation level alone does not define the delivered acceptance evidence.
Which Layout Details Can Increase Mixed Technology Assembly Cost?
A layout can pass electrical checks while leaving difficult manufacturing access.
Review the following areas before releasing fabrication data:
- Through-hole pads close to underside SMT components.
- Tall components that obstruct insertion, soldering or inspection.
- Large copper connections that affect heating around selected joints.
- Hole and pin geometry that affects insertion and joint formation.
- Fixture contact areas, board support and panel clearances.
- Access needed for programming, testing and permitted rework.
Ask the assembler to identify the exact location and consequence of each issue. “DFM problem” is less useful than a marked drawing showing the restricted joint and proposed remedy.
A proposed remedy might involve a component move, a revised pad or hole, different tooling or a separate operation. Compare the consequences before approving the change.
Freeze the accepted layout, panel and assembly instructions together. A late connector change can affect more than the BOM.
How Do Relays And Connectors Affect Cleaning?
Identify cleaning restrictions by exact component and process.
A relay, switch or connector may impose limits on exposure to cleaning chemistry, moisture or temperature. Confirm the manufacturer’s instructions rather than assuming that a sealed appearance establishes compatibility.
The cleaning plan should identify:
- Which fluxes and residues the assembly processes introduce.
- Which areas require cleaning or other residue control.
- Which components restrict the available method.
- When cleaning occurs in the assembly sequence.
- What verification the project requires afterward.
If a restricted component must be installed after cleaning, account for the joining operation that follows. That later step can introduce additional residues, so the plan still needs a disposition for them.
Use the PCB assembly cleaning review to connect material compatibility with the finished-board requirement.
A label such as “no-clean” should not replace agreement on the actual process, application needs and acceptance conditions.
Why Does An SMT Inspection Pass Leave More Work?
An inspection performed after reflow covers the assembly at that stage. Through-hole insertion, later soldering, cleaning and handling occur afterward.
Plan the necessary checks after those operations. Depending on the product, they may address part identity, orientation, seating, joint quality, nearby-component condition and cleanliness.
Define any through-hole joint requirements using the agreed acceptance documents, revision, product class and customer-specific conditions. Avoid copying one hole-fill percentage into every order without reviewing its applicability.
Test Through The Completed Interfaces
A processor boot test performed before connector installation cannot demonstrate that the later connector joints carry the required signals or load.
For an industrial controller, the final test might need to exercise input channels, switched outputs, communications and power behavior using the assembled interfaces.
A relay clicking proves that something moved; the product may also need verification of the intended switched path under defined conditions.
Specify those conditions in the PCBA functional test requirements. Keep electrical testing and joint inspection as distinct parts of the acceptance plan.
What Should A Mixed Technology Assembly Quote Show?
Ask suppliers to price the same released design and delivery condition.
The useful breakdown includes SMT setup and processing, through-hole insertion and joining, fixtures or pallets where required, cleaning, inspection, programming and testing.
Also identify any development work and operations outside the quoted scope.
Two offers can use the same “SMT + THT” description while including different work. One may include through-hole joining but leave test development open. Another may include a fixture and a defined final test sequence.
Request an operation-level explanation where the difference affects cost or schedule. You do not need every internal machine setting to compare complete deliverables.
For lead time, confirm the assumptions about material acceptance, engineering approval, tooling readiness and production scheduling.
Prepare A Mixed Technology Assembly RFQ
Submit the BOM, placement data, fabrication package and assembly drawing. Mark through-hole and press-fit positions, populated board sides, orientation requirements and any components with known processing restrictions.
Include cleaning, programming, inspection and functional-test requirements. If a requirement remains open, identify it so the supplier can quote the review or development work separately.
You can adapt this request:
Please review the attached mixed SMT and through-hole design and propose the assembly sequence. Identify any component, clearance, thermal or cleaning restriction that affects the route. Separate SMT work, through-hole operations, tooling, inspection and testing in the quotation. Please state the acceptance records and approvals required before production.
For an enquiry to Haode PCBA, organize the package using the PCB assembly quote checklist and flag the parts that constrain the manufacturing route.
A useful mixed technology assembly proposal shows how the factory will complete the whole board: each operation has a defined purpose, compatible inputs and a check that supports the next production decision.
