A mixed-technology PCB does not need one soldering method for every joint. A plausible route is to reflow the surface-mount components, then insert and solder the through-hole parts using selective or hand soldering. Wave soldering is another possibility when the underside layout, components, fixtures and geometry permit it.
The right choice depends on the whole board. Before an assembler commits to a route, it needs to know which components can tolerate each thermal exposure, whether soldering equipment can reach the through-hole joints, and how the finished joints will be inspected against the agreed criteria.
Compare the main soldering methods
| Method | First use to consider | What can make it unsuitable |
|---|---|---|
| Reflow | Paste-deposited surface-mount joints; compatible through-hole parts may also be candidates for pin-in-paste. | A part cannot tolerate the planned thermal exposures, or the joint cannot be supplied with adequate solder by the chosen process. |
| Wave | A group of exposed joints that can pass through a solder wave. | Underside parts, orientation, masking or fixture geometry prevent compatible exposure or access. |
| Selective soldering | Localized through-hole joints on a mixed assembly. | A nozzle or fixture cannot reach the joints, or nearby parts and the joint's thermal demand cannot be accommodated by the proposed process. |
| Hand soldering | A small number of joints, prototypes or exceptions that need individual access. | Joint access, heat control, operator process or inspection requirements cannot be met consistently for the job. |
| Laser soldering | A localized joint where a suitable laser process is available. | The beam or solder supply cannot reach the joint, or the assembly and process cannot be controlled and verified for the selected materials. |
These are screening questions, not automatic process rules. Availability and tooling need to be confirmed with the assembler. Laser soldering uses a focused heat source for a local joint; it still requires attention to alignment, solder delivery and the temperatures reached by the board and parts. An IPC technical paper, Options for Assembly using High Temperature Interconnection Technologies, describes one laser-soldering trial; its process settings are specific to that trial.
Start with the joint groups, not the equipment
Reflow is common for SMT assembly, but it is not the only way to solder surface-mount parts. Compatible underside SMT components can also be wave soldered, so wave soldering is not necessarily limited to through-hole joints. Conversely, an underside populated with SMT does not automatically rule out a wave process; it makes compatibility and access important questions.
For a board with SMT on both sides, the proposed sequence must account for each side’s exposure to reflow. Check component temperature limits, moisture sensitivity, the number of reflow passes and the actual board thermal profile. Component orientation and the ability to retain parts during any subsequent pass also need review.
For process-specific background, see reflow soldering, wave soldering, and the comparison of through-hole methods in through-hole soldering basics.
Hypothetical mixed-board route
Consider a board with top-side ICs, bottom-side SMT components and two through-hole connectors. One connector is close to a dense SMT region. The other has more apparent space around its joints. This is a planning example, not a completed assembly or a test result.
One candidate sequence is:
- Apply solder paste, place SMT components and reflow the SMT sides in a board-specific sequence.
- Insert the two through-hole connectors.
- Use selective soldering or controlled hand soldering for their joints, depending on access and thermal review.
- Inspect and test the assembly using methods and acceptance criteria agreed for the product.
The dense region is the critical handoff. Selective soldering heats a local area, but that does not eliminate the need to evaluate nearby components. Hand soldering may offer another way to reach the joints; it can be controlled and is not inherently defective. Neither method can be selected confidently from a component count alone.
| Joint group | Candidate method | Access or thermal concern | Evidence or decision still needed |
|---|---|---|---|
| Top-side ICs | Paste, placement and reflow | Component temperature limits and exposure during assembly of the opposite side | Component datasheets, moisture-sensitivity requirements, proposed pass count and board profile |
| Bottom-side SMT | Paste, placement and reflow; wave soldering for compatible parts may also be considered | Retention during the selected sequence; compatibility with any later soldering exposure | Component and material guidance, underside layout and proposed process review |
| Connector away from dense SMT | Selective or hand solder after insertion | Joint access, connector heat limits and support during soldering | Connector data, board geometry and assembler review of the proposed operation |
| Connector near dense SMT | Selective or hand solder after insertion | Tool access and possible heating of adjacent parts | Local layout and height data, access review, component limits and a plan for evaluating thermal exposure |
| Both connectors, as an alternative group | Wave soldering, if the board permits it | Underside SMT compatibility, connector geometry and whether fixtures or masking are needed | Full underside review, fixture approach and supplier assessment of the proposed route |
This matrix identifies questions, not a winning route. The connector that appears easier to access on a drawing still needs review with the actual component dimensions and board geometry.
For example, if the supplier finds that a selective-solder nozzle cannot reach the connector near the dense SMT area, ask whether a layout change, a different tool, or a controlled manual operation is feasible. If a wave fixture would cover a joint or leave a sensitive component exposed, ask for a revised fixture approach or another route. Record the chosen option and the joint-inspection method before releasing the assembly package; do not assume that a fixture or a different method automatically solves the problem.
Where wave and pin-in-paste fit
Wave as a conditional alternative. A wave route may be worth evaluating for the through-hole connectors and any compatible underside SMT joints. The decision depends on what the underside presents to the process: component compatibility, orientation, spacing, shadows, connector geometry and any proposed fixtures or masking. Ask the assembler to review the complete underside rather than assuming that a wave can reach every intended joint.
Pin-in-paste as a possible route reduction. A through-hole connector may be considered for pin-in-paste if its component reflow limits, available paste volume and the proposed process permit it. That review belongs before the route is fixed: a connector suitable for insertion soldering is not automatically suitable for reflow, and paste deposited for a through-hole joint must be evaluated as part of the process.
Supplier-specific component and material guidance matters here. Texas Instruments component datasheets and reflow guidance, for example, inform limits for the relevant parts; Indium Corporation solder paste product data sheets inform use of the relevant material. Neither is a universal profile for every board.
What to hand off before an assembler commits
Provide the information needed to evaluate the complete assembly:
- Board and assembly data: fabrication files, assembly drawings and component locations for both sides. Identify through-hole joints and nearby dense SMT regions that may restrict access.
- Component information: manufacturer part numbers and datasheets, including connector heat limits, SMT moisture sensitivity and permitted reflow exposure.
- Proposed sequence: which SMT side is assembled when, how many reflow passes are proposed, and which through-hole joints are candidates for selective, hand, wave or pin-in-paste soldering.
- Access information: component heights, connector geometry and areas where a tool, nozzle, wave or fixture may be obstructed. A selective-solder design review can help frame those questions.
- Acceptance and verification requirements: the applicable document revision or contract-specified criteria, inspection points and any required electrical or functional tests.
A usable BOM helps the assembler connect each placement and joint to the correct part data. See assembly BOM best practices.
Agree on acceptance and inspection
IPC distinguishes J-STD-001 soldering requirements from IPC-A-610 post-assembly acceptability. For a specific order, agree on the applicable documents, revisions, class and any additional acceptance criteria; consult the licensed standards for their requirements. NASA-STD-8739.3, Soldered Electrical Connections, is another workmanship document; use it when applicable to the work rather than treating it as a default for every commercial board.
Choose inspection around the actual geometry and agreed criteria. Visual inspection or automated optical inspection can help assess accessible features, but neither establishes every condition in a hidden joint. If important joints cannot be adequately evaluated visually, discuss what additional inspection or test is appropriate before production.
Solder paste inspection may support the SMT process. Electrical and functional testing answer different questions from workmanship inspection, so plan those separately where needed: PCBA functional test planning.
For a route review, include the board files, BOM, connector datasheets and proposed acceptance requirements with a quote request. If the process sequence or available evidence is still being defined, contact APTPCB to discuss what needs review before a commitment.