This guide covers: wave soldering versus selective soldering — process mechanics, strengths, limitations, and the decision criteria for choosing between them (POINT 01); the four selective soldering system types and their respective applications (POINT 02); the major equipment manufacturers (POINT 03); the six process parameters that determine selective soldering quality and how to optimise each (POINT 04); IPC-A-610 quality standards for through-hole joints and inspection methods (POINT 05); lead-free (SAC305) considerations and copper dissolution management (POINT 06); and troubleshooting the three most common defect categories (POINT 07).
Both wave soldering and selective soldering address the same fundamental challenge: applying molten solder to the leads of through-hole components in a controlled, repeatable, production-ready process. The difference lies in scope. Wave soldering applies solder to the entire bottom surface of the board simultaneously. Selective soldering applies solder only to designated locations, leaving everything else untouched. Understanding when each is appropriate is the first decision in through-hole process planning.
WAVE SOLDERING
Conventional Wave Soldering
A continuous wave of molten solder is generated in the solder bath; the PCB passes over the wave, and all exposed through-hole leads and pads on the bottom surface are soldered simultaneously. The process has been in use since the 1950s and remains the highest-throughput method for predominantly through-hole boards. Process steps: flux application → preheat → wave contact → cooldown.
✓ Strengths
High throughput — entire board soldered in a single pass. Low per-unit cost at volume. Simple process with few variables. Mature, well-understood technology.
✗ Limitations
Entire board bottom surface exposed to solder and heat — SMT components on the bottom side require protection masking or are at risk of damage or displacement. Not suitable for boards with heat-sensitive bottom-side SMT components. Flux residue management is more demanding. Large, warped boards may have inconsistent wave contact.
SELECTIVE SOLDERING
Selective Soldering
A precisely positioned nozzle ejects or presents molten solder only at programmed locations — the through-hole joint to be soldered. The rest of the board, including adjacent SMT components, remains unaffected. Flux is applied locally rather than to the whole board. Process steps: localised flux application → preheat → selective solder contact → cooldown. Each joint or joint group is processed individually under CNC control.
✓ Strengths
Minimal thermal impact on adjacent SMT components. Compatible with high-density mixed-technology boards. Lower flux consumption and easier residue management. Programmable, repeatable, and fully documented process. Suitable for high-reliability product classes.
✗ Limitations
Higher equipment cost. Slower per-joint cycle time than wave soldering. Program creation and setup time required for each new product. Nozzle maintenance and solder bath management required.
Application Selection Guidelines
USE WAVE SOLDERING WHEN
Predominantly Through-Hole, High Volume, Cost-Priority
The board has many through-hole components and few or no heat-sensitive SMT components on the bottom side. Production volume is high enough to justify the tooling investment. Cost per unit is the primary production metric. Typical applications: consumer electronics power supplies, industrial control boards with DIP ICs, commodity transformers and relays.
USE SELECTIVE SOLDERING WHEN
Mixed Technology, High Density, or Quality-Critical
The board has both SMT and through-hole components in proximity, and the bottom-side SMT components cannot tolerate wave solder heat exposure. Or the product is Class 3 (medical, automotive, aerospace) requiring documented, repeatable soldering with full traceability. Or the production mix is high-variety, lower-volume where wave tooling per product is not economical.
Selective soldering systems differ in how the solder is applied to the joint. Each architecture has a distinct combination of throughput, positional precision, and flexibility — and each is optimised for a different range of assembly configurations.
CONTINUOUS WAVE
Mini-Wave (Jet Wave)
A small, localised solder wave is generated at a nozzle tip. The board (or nozzle) moves under CNC control, sweeping the mini-wave over a programmed path that covers multiple through-hole joints sequentially. The solder wave remains in continuous contact with the joint as the nozzle traverses the path — similar in principle to conventional wave soldering, but applied only to the defined path.
Best for: rows of connectors, multiple pins along a defined line, medium-to-high volume with consistent joint patterns.
POINT-PRECISE
Drip / Drop-Jet
The nozzle deposits precise, discrete quantities of solder onto individual joints — a controlled drip or micro-jet rather than a continuous wave. Highest positional precision of all selective methods; solder lands exactly where programmed, with minimal thermal influence on adjacent areas. Slowest throughput due to the point-by-point nature of the process.
Best for: isolated single pins at close proximity to sensitive SMT components, fine-pitch through-hole, precision applications requiring minimal thermal spread.
MULTI-LEAD SIMULTANEOUS
Dip (Localised Bath)
The board is lowered into a localised solder bath that is shaped and positioned to contact only the specific area of the board to be soldered. Multiple leads are soldered simultaneously in the dip contact. The solder bath shape — determined by a mask or nozzle geometry — defines which area is soldered. Faster than point-by-point methods for multi-lead connectors with compatible geometry.
Best for: multi-lead connectors where all leads in a defined area can be dipped simultaneously; high-volume applications with consistent connector placement.
HIGHEST FLEXIBILITY
Robotic (Soldering Iron or Nozzle on Robot Arm)
A soldering iron tip, solder wire feeder, or solder nozzle is mounted on a programmable multi-axis robot arm. The robot follows a three-dimensional programmed path, applying solder at each defined joint. Maximum geometric flexibility — can access joints at varying heights and orientations that fixed nozzle systems cannot reach. Higher programming time and maintenance complexity than fixed-nozzle systems.
Best for: complex 3D assembly geometries, low-to-medium volume, high-mix environments, repairs, and specialised applications requiring non-planar soldering paths.
The selective soldering equipment market is served by a small number of established manufacturers, primarily European. Equipment prices range from approximately $150,000–$600,000 USD for production-grade selective soldering systems; conventional wave soldering systems are generally lower cost. Evaluate equipment against the specific nozzle types, flux systems, and process parameter ranges required for your products.
Selective Soldering Equipment
ERSA
Germany
One of the largest global SMT equipment manufacturers. Full range of selective and wave soldering systems, including the VERSAFLOW selective and POWERFLOW wave lines. Strong process engineering support and global service network.
Pillarhouse International
UK
Specialist selective soldering equipment manufacturer with a strong reputation in high-reliability markets. The Jade and Orissa series cover a range of throughput and flexibility requirements. Particularly strong in Class 3 process documentation and traceability features.
Nordson SELECT
USA
Part of the Nordson EFD / SELECT group. Mini-wave selective soldering systems with integrated flux management. Strong presence in North American automotive and industrial electronics manufacturing.
JUKI
Japan
Primarily known for SMT placement equipment; also offers selective soldering systems. Strong position in Japanese electronics manufacturing and Asian EMS. Good integration with JUKI SMT line management software.
Vitronics Soltec
Netherlands
Long-established wave and selective soldering specialist, now part of Heller Industries group. The Delta series selective soldering systems are used in volume electronics manufacturing across Europe and Asia.
SEHO
Germany
German manufacturer of wave and selective soldering systems, including the SelectLine range. Strong in European automotive supply chain electronics manufacturing.
Conventional Wave Soldering Equipment
For conventional wave soldering, the main equipment suppliers include ERSA and SEHO (Germany), Rehm Thermal Systems (Germany), KIRSTEN (Switzerland), Senju Metal Industry and TAMURA (Japan), and Heller Industries. Chinese manufacturers including Folungwin offer lower-cost wave systems for price-sensitive applications. Wave soldering equipment is a more mature and competitive market than selective soldering, with a wider range of entry-level options.
Selective soldering joint quality is determined by six process parameters. Each parameter has an optimal range; deviation in either direction produces characteristic defects. The table below describes each parameter, its typical working range for lead-free SAC305 processes, and the consequence of operating outside the range.
| Parameter | Typical Range (SAC305) | Effect if Too Low | Effect if Too High |
|---|
| Flux type and volume |
No-clean or water-wash; validated volume per joint |
Insufficient flux activation → non-wetting; dry joints with high contact angle |
Flux splatter during solder contact → solder balls; flux residue bridging; entrapment under component body |
| Preheat temperature |
100–150 °C (board surface) |
Thermal shock on solder contact; flux activation before solder contact may be incomplete; risk of board delamination from steep ΔT |
Premature flux evaporation before solder contact; component damage if sensitive components are near preheat zone |
| Solder temperature |
260–290 °C |
Insufficient wetting; incomplete hole fill; cold joint appearance; icicles on lead exit |
PCB base material damage (delamination, measling); component damage; accelerated copper dissolution in bath |
| Contact time (dwell) |
2–5 s (joint-dependent) |
Insufficient heat transfer into barrel; non-wetting; low hole fill percentage |
Heat damage to component and PCB; bridging; solder draw-back on separation; excessive intermetallic layer growth |
| Nozzle size and geometry |
Matched to joint pitch and group layout |
Point nozzle too small for connector row → multiple passes required; increased cycle time |
Nozzle too large contacts adjacent pads or component bodies → bridging or component damage |
| Travel speed (board or nozzle) |
Validated per nozzle type and joint pattern |
Slow speed on mini-wave: excessive solder deposit; bridging; thermal damage accumulation |
High speed: insufficient solder contact time; icicle and solder tail formation on lead exit; reduced hole fill |
⚠ Validate parameters against cross-sections before production release: Process parameter optimisation should always be validated by cross-sectioning representative joints from the first production run. Hole fill percentage, fillet formation on both sides, and the absence of voids must be verified against IPC-A-610 acceptance criteria before releasing the process for production. Parameter windows that look correct in test coupon work may not be adequate for the specific thermal mass of the actual production assembly. Cross-section a minimum of three joints per connector type per new product introduction.
IPC-A-610 (Acceptability of Electronic Assemblies) is the industry standard for through-hole solder joint quality. The primary metric for through-hole joints is vertical hole fill — the percentage of the barrel height that is filled with solder. This metric determines both mechanical holding strength and electrical connection reliability.
Minimum Hole Fill Requirements by IPC-A-610 Class
IPC Class 2
75%
Minimum vertical hole fill
Applies to general industrial, commercial, and consumer electronics assemblies. Solder must be continuously present from the solder-side fillet to 75% of the barrel height as measured from the solder side. A solder-side fillet and component-side wetting evidence (not necessarily a full fillet) are also required.
IPC Class 3
100%
Minimum vertical hole fill
Applies to high-reliability assemblies: medical devices, aerospace, military, and automotive electronics. Complete fill of the barrel from solder side to component side is required, with a full solder fillet visible on the component side. Zero barrel voids are acceptable as a target; limited voids may be accept to Class 3 criteria only if they do not reduce the minimum fill below 100% in total.
Inspection Methods
VISUAL / AOI
First-Pass Inspection
Manual visual inspection under magnification evaluates fillet shape, surface texture, and wetting angle. Automated Optical Inspection (AOI) can detect missing solder, gross bridging, and solder balls. Neither method can directly measure hole fill — they evaluate the visible fillet geometry as a proxy for barrel fill quality. For Class 3 applications, visual inspection alone is insufficient for hole fill verification.
X-RAY
Hole Fill Verification
2D and 3D X-ray inspection directly images solder fill in the barrel, revealing voids, incomplete fill, and hidden bridges that visual inspection cannot detect. Essential for Class 3 hole fill verification and for process qualification cross-sections. 3D CT scanning provides the most complete barrel fill measurement but is slower and higher cost than 2D X-ray.
CROSS-SECTION
Process Qualification and Failure Analysis
Metallographic cross-sectioning — cutting, mounting, polishing, and microscopically examining the joint — provides direct measurement of hole fill percentage, intermetallic compound layer thickness, and void distribution. Destructive and time-consuming, but the definitive method for process qualification and failure analysis. Required for new product introductions on Class 3 products and for root cause investigation of hole fill defects.
FUNCTIONAL TEST
ICT and FCT
In-circuit test (ICT) and functional circuit test (FCT) verify electrical continuity and function of through-hole joints. ICT can detect high-resistance joints that may indicate marginal hole fill or cold solder. Functional test detects intermittent connections. Neither provides direct joint quality information — they are part of the overall inspection strategy, not a substitute for visual and X-ray inspection.
Common through-hole solder defect modes: The most frequently encountered through-hole soldering defects are: non-wetting (solder does not adhere — visible as retracted or absent fillet); insufficient hole fill (solder fills less than the required percentage of the barrel — identified by X-ray or cross-section); solder balls (small spheres of solder adjacent to the joint — cosmetic defect with potential to cause shorts); bridging (unintended solder connection between adjacent pads or pins — usually visible); icicles (pointed solder protrusions on the lead exit — caused by solder draw-back during nozzle separation); and solder cracks (fractures in the solidified joint — caused by mechanical stress during cooling or from vibration after assembly). Each defect mode has specific process parameter root causes, described in POINT 07.
The transition to RoHS-compliant lead-free soldering has been essentially complete for commercial electronics since 2006. SAC305 (Sn-96.5% / Ag-3.0% / Cu-0.5%) is the dominant lead-free alloy for selective and wave soldering. It differs from eutectic Sn-Pb in ways that require specific process adaptations — not just a temperature increase.
MELTING POINT
SAC305 Liquidus: 217°C vs Sn-Pb 183°C
SAC305's higher liquidus temperature requires process temperatures approximately 30–40°C higher than equivalent Sn-Pb processes. This has a downstream effect on every thermal parameter in the process — preheat, solder bath temperature, and contact time all need to be adjusted for the higher working temperature.
WETTABILITY
Reduced Wetting vs. Sn-Pb
SAC305 has lower wettability than Sn-Pb — it is more selective about surface cleanliness and flux activation. Flux selection is more critical: a flux that performs adequately with Sn-Pb may be insufficient to activate SAC305 at the through-hole joint within the contact time window. Validate flux activity with the specific flux, solder alloy, and surface finish combination to be used in production.
COPPER DISSOLUTION
Solder Bath Copper Content Management
SAC305 dissolves copper from PCB pads, via barrels, and component leads significantly faster than Sn-Pb. Copper content in the solder bath rises continuously during production use. When copper content exceeds approximately 0.3% by weight, the alloy's effective liquidus rises and joint surface quality deteriorates. Measure bath copper content regularly (weekly at minimum for production use) and replace solder when the limit is reached.
PROCESS ADJUSTMENTS
Key Changes from Sn-Pb Process
Higher preheat temperature (within board and component capability limits) to manage the greater ΔT. Higher flux activation requirement — select flux rated for lead-free temperature range. Higher solder bath temperature: 265–285°C typical for selective soldering. Longer contact time may be needed for high-thermal-mass connectors to achieve equivalent hole fill to Sn-Pb. Monitor bath chemistry more actively than with Sn-Pb.
⚠ Mixed alloy contamination: In any facility that processes both leaded and lead-free products (even temporarily), preventing cross-contamination of solder baths is critical. A leaded solder bath contaminated with SAC305 solder has an elevated silver content that affects joint reliability; a lead-free SAC305 bath contaminated with Sn-Pb solder has depressed liquidus and altered wetting properties, and may not meet RoHS compliance requirements if lead content exceeds 0.1% by weight. Maintain separate, clearly labelled equipment for leaded and lead-free processes and implement process documentation that verifies bath alloy at each production run.
Selective soldering defects fall into three functional categories, each driven by a distinct set of process parameter root causes. Systematic defect analysis — comparing the observed defect mode against the process parameter signatures below — is more efficient than trial-and-error parameter adjustment.
🔬
Non-Wetting and Insufficient Hole Fill
Root causes: insufficient flux volume or wrong flux type (insufficient activation at operating temperature); flux degradation from improper storage or expired shelf life; preheat temperature too low (flux not fully activated before solder contact, or thermal shock cooling solder below liquidus at the joint); solder bath temperature too low; contact time too short for the thermal mass of the component; pad or lead surface contamination (oxidation, handling residue, incompatible surface finish); incorrect surface finish on the PCB (OSP oxidation, HASL roughness, inadequate ENIG gold thickness).
Corrective actions: verify flux type against alloy and surface finish; increase flux volume incrementally; increase preheat temperature in 5°C steps while monitoring component and board temperature; increase solder temperature and/or contact time; cross-section joints to measure actual hole fill and confirm the correction is effective.
⚡
Solder Balls, Bridging, and Solder Tails (Icicles)
Solder balls: caused by excessive flux volume (flux outgassing violently during solder contact ejects solder droplets); excessively high solder temperature accelerating flux outgassing; inadequate preheat allowing flux to contact solder before it is properly activated. Bridging: nozzle positional offset placing solder outside the intended landing area; nozzle too large contacting adjacent pins; excessive solder volume; pin-to-pin spacing too small for the selected nozzle. Icicles/solder tails: excessive solder volume deposited; board or nozzle travel speed too high on separation, drawing solder into a tail; solder temperature too low causing solder to solidify partially before full separation.
Corrective actions: reduce flux volume to minimum effective level; verify nozzle position with a test deposit on a sacrificial board; select a smaller nozzle if bridging adjacent pins; reduce solder volume; reduce travel speed on separation; if icicle formation persists, increase solder temperature slightly to improve solder fluidity at separation.
🌡️
Component and PCB Thermal Damage
Caused by any combination of: solder temperature too high; contact time too long (heat accumulation in component body and adjacent area); preheat too low creating steep thermal gradients (thermal shock from cold board rapidly heated by solder contact); mechanical contact between nozzle and component body; repeated re-soldering without adequate cool-down interval; component or PCB material not rated for the process temperature. Symptoms include discolouration, delamination, lifted pads, cracked component bodies, and failed components.
Corrective actions: verify solder temperature is at the minimum required for adequate wetting — do not overshoot; optimise contact time to the minimum that achieves required hole fill; increase preheat to reduce thermal gradient; verify nozzle height and position does not contact component bodies; check component and PCB material temperature ratings against actual process temperature profile using a thermocouple profiler. For Class 3 applications, document component temperature limits in the process specification.
Summary
Through-hole soldering remains essential in modern PCB assembly wherever connectors, high-current terminals, transformers, and mechanically robust components are required. Wave soldering is the efficient choice for boards dominated by through-hole components with few heat-sensitive SMT components; selective soldering is the correct choice for mixed-technology boards where through-hole and SMT coexist. The four selective soldering types — mini-wave, drip, dip, and robotic — each suit a distinct combination of throughput requirement and joint geometry complexity. Process quality is governed by six parameters (flux, preheat, solder temperature, contact time, nozzle geometry, and travel speed), all of which must be validated against IPC-A-610 hole fill requirements by cross-section for new product introductions. For lead-free SAC305 selective soldering: monitor bath copper content regularly, validate flux selection for the higher process temperature, and ensure preheat adequately compensates for SAC305's higher liquidus. The three defect categories — non-wetting, solder balls/bridging/icicles, and thermal damage — each have distinct process parameter root causes that systematic cross-section and process data analysis can identify and resolve without extended trial-and-error adjustment.