What are the factors that affect solder paste activity?

Factors affecting solder paste activity include the following:

I, the flux composition and active agent content

Active agent type and ratio: active agent in the flux (such as organic acids, amines halogen active agent, etc.) is the key component in determining the activity of the solder paste.The active agent content directly affects the ability of the paste to remove oxides and promote wetting.Too much active agent may lead to corrosion of the PCB, need to be adjusted according to specific process requirements.

Flux stability: the stability of the active ingredients in the flux in the storage and use of the process affects the paste activity of the retention time.If the flux stability is poor, the active ingredient is easy to volatile or failure, resulting in reduced paste activity.

II, solder paste storage and use conditions

Storage temperature and time: solder paste usually need to be stored in 0-10 ℃ refrigeration, beyond the shelf life or storage temperature is too high will lead to accelerated reaction between the flux and the tin powder, the activity decreases.

Temperature treatment: Before use, the solder paste should be placed at room temperature for 2-4 hours, insufficient temperature may lead to condensation of water vapour, affecting the activity; excessive temperature may accelerate the flux volatility.

Stirring time: Stirring time is too long will make the solder paste exposed in the air for too long, resulting in active ingredients volatilisation, reducing the quality of the solder paste.

III，Environmental temperature and humidity

Ambient temperature: too high a temperature (such as more than 28 ℃) will accelerate the flux in the solvent volatilisation and flux and tin powder reaction speed, resulting in dry solder paste, activity is reduced; too low a temperature affects the viscosity of the paste and printing performance.

Environmental humidity: high humidity will increase the amount of solder paste moisture absorption, resulting in soldering fried tin; humidity is too low will accelerate the solvent evaporation, so that the solder paste dry.

IV, solder paste powder and particle size

Powder (particle size) selection: the larger the number of tin paste powder, the smaller the alloy particles, although it does not directly affect the activity, but can enhance the clarity of printing and filling capacity of small components, the surface area of the tin powder is large, the need for more activity, indirectly affecting the welding effect.Powder selection needs to match the printing process and component type size.

V. Other process factors

Printing parameters: printing speed, pressure and other parameters affect the uniformity of the distribution of solder paste on the PCB, which in turn affects the effect of active ingredients.

Reflow soldering process: preheating temperature, time and other parameters if not set properly, may lead to premature volatility of the flux or active ingredient failure.

Summary and notes

Solder paste activity by the flux composition, storage and use conditions, environmental temperature and humidity, tin powder type and process parameters and other factors, in order to ensure the activity of the solder paste, need to:

Select the appropriate activity level of solder paste (such as RMA, RA level), to avoid excessive or insufficient activity.

Strictly follow the specification of solder paste storage, rewarming and stirring.

Control ambient temperature and humidity in the recommended range (temperature 20-25°C, humidity 40%-60%).

Optimise printing and reflow process parameters.

Regularly test solder paste activity and avoid using expired or deteriorated solder paste.