What is the difference between solid crystal solder paste and conventional SMT solder paste?

Solid crystal solder paste and conventional SMT solder paste in electronic manufacturing were used in different processes, the main difference is reflected in the following aspects:

1. Different application scenarios

Solid crystal solder paste:

Mainly used in semiconductor packaging or LED packaging, used to fix the chip (Die) to the substrate (such as ceramic, glass, metal or PCB), belongs to the chip bonding material.

Typical applications: LED chip bonding, IC packaging chip mounting.

Conventional SMT solder paste:

Used in surface mount technology (SMT) to solder electronic components (e.g. resistors, capacitors, ICs, etc.) to the surface of a PCB.

Typical application: component soldering in PCB assembly.

2. Material Composition Differences

Solid crystal solder paste:

Alloy composition: generally SAC305 or special alloys (such as SnAg, AuSn, etc.), need to match the coefficient of thermal expansion (CTE) of the chip and substrate.

Flux: low content, curing needs to be less residual, to avoid contamination of the chip or affect the heat dissipation.

Additives: may contain silver, bismuth and other metals to improve thermal/electrical conductivity, or add fillers (e.g. aluminium oxide) to enhance mechanical strength.

Conventional SMT solder paste:

Alloy composition: SnAgCu (lead-free) or SnPb based, need to optimise solder wettability and reliability.

Flux: high content, need to remove the pad oxide layer and promote wetting, curing residue needs to comply with electrical safety standards.

Additives: may contain bismuth, antimony, etc. to adjust the melting point or improve performance.

3. Differences in process characteristics

Curing method:

Solid crystal solder paste: usually cured by heat (lower temperature, such as 150-200 ℃), to avoid high temperature damage to the chip; some products support rapid UV curing.

SMT solder paste: need to go through reflow soldering high temperature (peak temperature 217-260 ℃), through the melt to form a solder joint.

Dispensing precision:

Solid State Solder Paste: High precision dispensing is required (e.g., jet valve or needle dispensing), and the amount of glue is controlled to ensure the precision of chip position.

SMT solder paste: through stencil printing, focus on printing thickness and coverage.

Reliability requirements:

Solid crystal solder paste: need to withstand long-term thermal cycling (e.g. LED junction temperature changes) and mechanical stress (e.g. vibration), requiring low stress and high bond strength.

SMT solder paste: need to pass drop test, thermal shock, etc., focus on the fatigue resistance and corrosion resistance of the solder joints.

4. Differences in performance focus

5. Typical failure modes

Solid crystal solder paste:

Cracking of the bonding layer (CTE mismatch)

Loss of bond strength due to thermal ageing

Poor curing (insufficient dispensing volume or improper curing temperature)

SMT solder paste:

Solder voids (flux evaporation or pad contamination)

Solder joint embrittlement (improper alloy composition or temperature profile)

Cold/void soldering (printing or reflow process problems)

Summary

The core difference between solid crystal solder paste and SMT solder paste is:

Functional positioning: the former is a bonding material, the latter is a soldering material.

Process requirements: the former need low-temperature curing, high bonding strength; the latter need high-temperature welding, high reliability solder joints.

If confused use (such as solid crystal with SMT solder paste), may lead to heat damage to the chip or bonding strength is insufficient; Conversely, solid crystal solder paste welding components may be due to poor wettability resulting in poor welding.