2.2 Fluxes, Alloys & Aids

Hand soldering is a controlled process, but its chemical core relies on the quality and purity of the materials used. This chapter details the critical selection of wire solder, flux core chemistry, and supplementary aids. Using the wrong alloy, excessive flux, or contaminants drastically compromises joint integrity and accelerates rework bench corrosion.

2.2.1 Solder Wire Mandates

Solder wire is a composite material consisting of the alloy, the diameter, and the flux core. Each parameter must be specified to ensure efficiency and joint quality.

A) Wire Diameter and Alloy

Diameter: The wire diameter must be sized to match the volume of the joint. 0.3 – 0.5 mm wire is standard for SMT pads and fine-pitch rework. 0.8 – 1.0 mm wire is preferred for high-mass THT pins and lugs. Using wire that is too thick leads to excessive solder deposition and bridging.
Alloy Purity: The alloy must be sourced with a documented Certificate of Analysis (CoA) to confirm metal percentages and minimize impurities that cause dull, granular joints or premature oxidation.

B) Flux Core Selection

The flux core cleans the pad during the initial wetting phase. The core chemistry must be compatible with the assembly's overall cleaning requirements.

Flux Core Type	Requirement	Rationale
No-Clean (NC)	Halide-free formulation required.	Leaves a benign residue that does not require cleaning, maximizing throughput. Halides introduce corrosion risk.
Water-Soluble (WS)	Mandates mandatory cleaning immediately after soldering.	Provides superior oxide removal on stubborn or aged components but leaves highly corrosive residue if left on the board.

2.2.2 Solder Alloy Choice

The selected alloy must match the Bill of Materials (BOM) and the manufacturing thermal requirements.

Sn63/Pb37 (Tin-Lead): Offers a single, low eutectic melting point (183°C), providing a wide, forgiving thermal process window. It achieves superior wetting at lower temperatures. Use is restricted by RoHS compliance mandates.
SAC305 (SnAg3.0Cu0.5): The standard lead-free alloy. Requires a higher operating temperature (liquifies at 217°C). It offers superior long-term reliability against thermal cycling fatigue compared to tin-lead.
Low-Temp Bismuth-Based: Used exclusively for highly heat-sensitive components (e.g., plastics, displays) or specialized step-soldering processes. These joints are mechanically weaker and require strict qualification for shock/vibration.

2.2.3 Supplementary Fluxes

Extra flux is mandatory for rework, as the core flux is typically exhausted by the time the technician begins work.

Purpose: Supplementary flux chemically reactivates the surface, removes residual oxides, and improves the solder's surface tension, aiding drag soldering and wick usage.
Form: Flux is typically applied via pen, liquid dropper, or gel. The pen is preferred for precise SMT application; gel is often used for large rework areas like BGA reballing.
Mandate: The chemical compatibility of the supplementary flux must match the flux core. Do not mix flux chemistries (e.g., water-soluble pen on a no-clean assembly).

2.2.4 Essential Rework Aids and Tools

Rework requires specialized tools to ensure safe component removal and site preparation.

Aid / Tool	Function	Rationale
Solder Wick (Braid)	Removes solder from pads and through-holes via capillary action.	The wick must be flux-coated and wider than the pad to achieve efficient thermal transfer and removal.
Desoldering Pump (Vacuum)	Removes bulk solder from THT barrels.	Requires a clean, grounded tip and nozzle to prevent thermal damage to the hole plating.
Bottom-Side Preheater	Raises the local board temperature to 80 – 120°C.	Mandatory for soldering heavy copper layers or large ground planes; dramatically reduces the iron's required dwell time and minimizes thermal shock.
Tinner/Cleaner	Used to revive oxidized, black tips.	Chemical maintenance for the tip, allowing rapid recovery without excessive sanding or scraping.

Final Checklist: Materials and Aids Selection

Parameter	Mandate	Rationale
Solder Alloy	Must match BOM specification and thermal profile requirements (SnPb vs. SAC).	Ensures correct joint integrity and melting point control.
Wire Diameter	0.3 – 0.5 mm for SMT; 0.8 – 1.0 mm for THT.	Prevents bridging and ensures controlled solder volume deposition.
Flux Compatibility	Core flux and supplementary liquid flux must be chemically compatible (e.g., both No-Clean and Halide-Free).	Prevents corrosive residues and residue interference with electrical testing.
Preheater	Mandatory use on all heavy copper or multi-layer ground plane rework.	Reduces required iron dwell time to less than 6 seconds, protecting the PCB laminate.
Tip Maintenance	Tinner/Cleaner used for immediate tip care; brass wool used for quick cleaning.	Maintains optimal heat transfer efficiency and extends tip life.