What solder paste do I need to solder a mobile phone motherboard?

Soldering mobile phone motherboards requires the selection of solder paste suitable for high-density, fine solder joints, and the following are key factors and recommendations:

1. Type of solder paste

Lead-free solder paste: Comply with environmental regulations (such as RoHS), commonly used alloy SAC305 (tin 96.5% / silver 3% / copper 0.5%), melting point of about 217-219 ℃.Suitable for modern mobile phone motherboards, but note that higher reflow temperatures may affect heat sensitive components.

Leaded solder paste: e.g. Sn63/Pb37 (Sn63%/Pb37%), lower melting point (183°C), easier to control the soldering process, but subject to environmental constraints, usually used for non-export products or specific scenarios.

2. Alloy composition

SAC series (e.g. SAC305, SAC307): high mechanical strength, good fatigue resistance, suitable for precision packages such as BGA.

Bismuth-containing alloys (e.g. Sn42/Bi58): lower melting point (138°C), but more brittle, may be used for low-temperature soldering needs.

3. Particle size

Type 4 (20-38μm): Commonly used standard, balancing printability and fine pitch requirements.

Type 5 (10-25μm) or Type 6 (5-15μm): Suitable for ultra-fine pitch components (e.g. 01005 packages) to minimise the risk of bridging.

4. Flux types

No-clean type: less residue, no need for subsequent cleaning, suitable for high-density design of mobile phone motherboards.

Activity level: choose medium activity (e.g. ROL0 or ROL1) to ensure good wettability and avoid corrosive residues.

5. Viscosity control

Recommended viscosity range: 100-200 Pa-s (adjusted according to the printing speed), to prevent printing collapse and ensure the release effect.

6. Brand and Reliability

Recommended brands: Senju, Indium, Alpha, etc., providing proven and highly reliable products.

7. Process Adaptation

Reflow profile matching: Lead-free solder paste requires a peak temperature of 240-250°C, and the preheating phase requires a slow temperature rise to avoid thermal shock.

Storage and use: Refrigerate (0-10℃), rewarm for 2-4 hours before use, stir until uniform paste.

8. Special requirements

Low Voiding Rate: Select low voiding formula to enhance the reliability of BGA soldering.

Drop resistance: Silver alloy (e.g. SAC305) can enhance the mechanical strength of solder joints and adapt to the environment of mobile phone use.

Summary Recommendations

Lead-free Type 4 SAC305 solder paste with no-clean flux is recommended for soldering mobile phone motherboards to ensure environmental compliance and soldering quality.Strictly follow the reflow process parameters and choose a well-known brand to ensure consistency and reliability.For miniature components (e.g. CSP packages), consider Type 5 particles to optimise printing accuracy.