1.3 Selective Solder Programming

Selective soldering is the high-precision solution for mixed-technology boards, solving the problem of soldering THT joints without exposing surrounding SMT components to the molten wave. This chapter outlines the digital translation of physical constraints—from defining the optimal nozzle motion and dwell time to managing the sequence—that ensures complete barrel fill and avoids costly thermal damage or solder splash.

1.3.1 The Selective Solder Mechanism

Selective soldering is the high-precision alternative to wave soldering, necessary for boards with mixed technology where sensitive SMT components are on the bottom side. Programming this process is a direct translation of the board's thermal and geometric constraints into machine motion. Success is measured by the quality of the smallest joint, achieved by optimizing nozzle size, path sequence, and solder dwell time.

Key Components

1. Flux Applicator: A micro-jet or spray nozzle applies flux only to the target pads.
2. Preheat Zone: Heaters (usually IR or convection) bring the joint area to the target top-side temperature (Chapter 1.2).
3. Mini-Wave Nozzle: A small, localized fountain of molten solder (often 2-15 mm in diameter) contacts the joint.

1.3.2 Programming Mandates: Motion and Thermal Management

The selective program must define the trajectory and the required contact time (dwell) for every joint.

A) Dwell Time and Penetration

Dwell time is the amount of time the joint remains in contact with the molten solder. This is the primary control for ensuring barrel fill.

• Standard Joint: 1.5 – 3.0 seconds is typical for standard leads and plated through-holes (PTHs).
• Heavy Thermal Load: Joints connected to large copper planes or thick boards require increased dwell time (greater than 3.0 seconds) to overcome the heat sink effect (Chapter 1.1).
• Verification: Dwell time is confirmed by inspecting the top-side fillet and measuring barrel fill percentage on a microsection.

B) Nozzle Choice and Pin Clearance

The nozzle size must be matched to the component pitch and the component-to-component spacing.

• Nozzle Size: Choose the smallest possible nozzle that covers the entire pad cluster. A typical range is 4 mm to 8 mm for standard connectors.
• Pin Clearance: The program must ensure the nozzle tip maintains a 3 – 4 mm keepout clearance from all surrounding SMT components, lead bodies, and nearby THT pins to prevent thermal damage or accidental solder contact.

1.3.3 Sequence and Path Optimization

The order in which joints are soldered is critical for managing heat distribution and preventing mechanical stress.

A) Soldering Sequence

The path of the mini-wave must be strategically planned:

1. Solder Least Thermally Demanding Joints First: Start with smaller pins or those on thin traces. This allows the system to build up heat in the local area.
2. Solder Most Thermally Demanding Joints Last: Address large planes, chassis lugs, and high-mass components at the end of the sequence. This ensures the maximum thermal energy is available without overheating nearby sensitive components.
3. Process Clusters Logically: Solder all pins of a single connector in one continuous motion (drag soldering) whenever possible to maximize throughput and minimize repositioning time.

B) Drag vs. Dip Methods

Selective soldering utilizes two primary motion patterns:

• Drag (Most Common): The nozzle maintains contact and moves along the pin row. This is fast and achieves high throughput, but requires perfect pin alignment.
• Dip (Spot Soldering): The nozzle contacts a single pin or small cluster, pauses for the required dwell time, and retracts. This is mandatory for single pins or where nearby SMT components restrict drag motion.

1.3.4 Tooling and Process Control Checkpoints

Selective soldering relies heavily on precise tooling and consistent process metrics.

Checkpoint	Process Control Requirement	Rationale
Nozzle Height	Controlled to 0.5 – 1.0 mm contact height relative to the pad/pallet.	Ensures the mini-wave top remains stable and minimizes solder spatter.
Flux Jet Calibration	Confirmed to apply flux only to the target pad cluster.	Prevents flux residue from contaminating adjacent SMT components or clean areas.
Nitrogen Purity	100% nitrogen blanket or inert atmosphere over the mini-wave.	Mandatory for lead-free soldering; minimizes dross formation and improves wetting performance by preventing oxidation.
Solder Pot Purity	Dross removed on a scheduled basis; alloy verified for copper contamination (Chapter 1.4).	Ensures consistent viscosity and prevents defects caused by impurities.

Final Checklist: Selective Program Optimization

Parameter	Optimization Mandate	Impact on Yield
Dwell Time	Set to 1.5 – 3.0 seconds baseline; increased for heavy planes/thick boards.	Guarantees barrel fill and top fillet formation.
Nozzle Path	Sequence moves from low-mass to high-mass joints.	Manages heat distribution; prevents thermal stress on sensitive parts.
Motion	Use drag for linear pin rows; dip for isolated pins.	Maximizes throughput and ensures precise contact geometry.
Keepout	3 – 4 mm clearance maintained from SMT components and plastic bodies.	Prevents thermal damage and solder splash.
Atmosphere	Nitrogen (N2) flow verified at the nozzle.	Critical for reducing dross and improving lead-free wetting.