Avoiding solder paste failures through proper handling and preparation.

Our technical support team often sees the same preventable issues repeat themselves. Three times this week alone, I traced the root cause of a solder paste problem back to using paste straight out of the refrigerator without warming it up. In each case, once the user followed proper handling procedures, the problem resolved itself.

As more companies lose experienced staff and hire new engineers, these issues are occurring more frequently.

Solder paste is a carefully formulated mixture of metal powder and flux. Its properties – such as viscosity, tack and printability – can be affected significantly by temperature, humidity and agitation (Figure 1).

Figure 1. Solder paste is a metal powder and flux formulation and temperature, humidity and agitation affect its viscosity, tack and printability.

When paste is used straight from cold storage, condensation can form, altering its consistency and print behavior. Additionally, stirring the paste too aggressively or using mechanical mixing equipment can damage its rheological properties. Exposure to excessive heat or humidity during use also can break down the flux or shift the alloy-flux balance.

Best Practices for Proper Handling

Solder paste should be stored in a cool, stable environment. Refrigeration between 0° and 12°C (32°-55°F) is ideal, as it extends shelf life. But when it’s time to use it, the paste should be permitted to warm naturally to room temperature. This typically requires 4-6 hours and should be performed without applying heat.

Only after the paste has equilibrated should it be opened and stirred – gently and by hand. Stirring ensures uniformity after storage, but automated mixers should be avoided as they can alter the paste’s behavior in unpredictable ways.

During the application process, maintaining consistent environmental conditions is critical. Ideal conditions include a temperature range of 22°-26°C (72°-80°F) and relative humidity of around 45%, give or take 5%. In warmer or more humid climates, paste may dry out or slump more quickly, increasing the risk of printing defects. If manufacturing environmental conditions are outside this ideal window, it is advisable to use a smaller diameter bead of solder paste on the stencil and replenish it more frequently. By doing so, less material is exposed to the environmental factors and refreshed with new, uncompromised solder paste.

Advice for New Engineers

With the loss of experienced personnel, it’s not uncommon to see these errors repeated – often by no fault of the new engineers or operators involved. Proper paste handling may seem like a minor detail, but it has a significant impact on performance. If you’re new to this space, my advice is to ask questions early and check the documentation. Following a few simple procedures can prevent hours of troubleshooting and ensure high-quality results from the start.

Whether it’s summer heat, a high-humidity environment or just the fast pace of production, handle solder paste with care. Proper storage, preparation and environmental control are essential to maintaining paste integrity and ensuring reliable solder joints.

Kevin Pigeon is senior engineering manager at AIM Solder (aimsolder.com); kpigeon@aimsolder.com.

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