It must not exhibit a tendency to separate and/or settle, demonstrate adequate tackiness to hold components after placement for at least 10-12 hours without slumping or sagging. Several factors influence this rheology, including the amount and size of the metallic particles suspended in the paste, and the differences in the density of the metallic particles and that of the organic mix.ДЄn ideal solder paste should deliver the shelf life and stability claimed by the manufacturer. The characteristics of flow and deformation of solder paste, also known as rheology, affects the quality of paste deposition on the solder pads.

Incoming inspection programs should include the test temperature for assessing viscosity and slump of the solder paste. Best practices include maintaining a first-in-first-out program, routine inspection of expiry dates for shelf life control, refrigeration at temperatures specified by the manufacturer for storage, and bringing the paste up to room temperature before opening the container. Therefore, it may be necessary to consult the supplier to obtain a product best suited to a particular requirement, and the selection may require some trial, inspection, and retrial.Д¬areful handling of solder paste will ensure it remains effective during both its shelf life and its operational life. Several types of solder pastes are available, developed for specific sets of processes and profiles. The solder paste is the most crucial ingredient in the entire reflow process.

Cooling the assembly to the cleaning temperature.

Heating the assembly to allow the solder paste to melt (reflow) and wet the PCB pad and the ends of the SMD parts, resulting in the properly soldered connection.

Depositing solder paste on specific pads on a PCB using a predesigned stencil.

The basic reflow process consists of four broad steps:

Minimizing the movement of parts during the soldering processДЄchieving the above goals requires a good understanding of the reflow soldering process, and the methods to modify it to ensure the product remains protected.

Minimizing the stress and damage to the PCB and SMD components.

The second goal has a wider scope and includes:

Achieving uniform, durable, and effective solder joints.

Achieving maximum flexibility in allowing soldering of a large number of components while minimizing the changeover time.

The first goal is more traditional, including: Reflow soldering aims to satisfy two basic goals. The advent of Surface Mount Technology (SMT) and lead-free soldering has further expanded the methods and techniques the industry uses for reflow soldering. Therefore, one cannot consider reflow soldering a new manufacturing process. For many years now, the hybrid electronic industry has been using the art of reflow soldering and refining it.