Basic assembly requirements
The purpose of the assembly is to install and weld various electronic components on the substrate according to the design requirements, and to fully specify the electrical interconnection to meet the performance requirements of the assembly parts.To be specific:
Electrical interconnection between components and pad is completed by current.
In the whole process of the device assembly process, the device is not allowed to cause damage to its performance.
The process can meet the requirements of performance, including reliability.For example, the welding spot can withstand thermal fatigue and mechanical fatigue test without failure, and there is no characteristic change or electric migration in the hot flash and life test.
At present, the "lead-free" electronic products have become an inevitable trend, and a considerable amount of products have been produced, especially the "lead-free" assembly.The industry has been deepening its understanding of the relevant design, materials, components, technology, equipment and other aspects of continuous improvement.
The main differences in technological performance between lead free and lead are as follows:
High temperature of welding: traditional tin-lead solder eutectic solder melting point is 183 ℃, and lead-free solder is above 217 ℃;
· self-alignment and poor penetration performance;
· narrow process window: due to the improvement of lead-free solder melting point and the limitation of components' temperature tolerance;
· poor appearance of welding spot.
According to the above differences and basic requirements of assembly, the influences of solder, welding interface and process on product quality will be discussed respectively below.
Solder characteristics and reliability
Solders include alloy solder and solder paste, applicable to wave solder and reflow solder respectively.
Alloy solder is the base material.Two or more substances are mixed together to form a uniform and stable dispersion system called a solution.Solder alloy is a solid solution or solid solution.Phase diagrams, also known as equilibrium diagrams, reflect the relationship between temperature, composition, and phase at equilibrium.Solder alloys usually present three basic forms under different components and different temperatures: solid phase, solid-liquid coexistence phase (with several different phases) and liquid phase.
The electronics packaging industry is concerned about the impact of solder joints formed after thermal fusion welding on reliability.In addition to electrical properties, mechanical and thermal properties (ultimate tensile strength, elongation, young's modulus, thermal expansion coefficient), etc., alloy solders pay special attention to the following characteristics of solders:
(1) melting characteristics
There is a definite melting point for eutectic solder alloys, and solid, liquid and melting points can be used to characterize the melting characteristics of solder alloys without eutectic compositions.
(2) infiltration characteristics
Wetting characteristic is the most important characteristic of solder.
Temperature has a great influence on wettability. Generally, the higher the temperature, the better the wettability, which is related to the reduction of surface tension and interfacial reaction.Lead-free solder has worse infiltration characteristics than SnPb solder.
(3) interface layer morphology
It has great influence on the reliability of the connection, especially when a very thick reaction layer is formed. As a result, Cu in Sn is dispersed in the solder joint in the form of compound. It can be considered that defects of the same size are formed and should be avoided as far as possible.As the reaction layer is an intermetallic compound (IMC), it is brittle and does not match the thermal expansion coefficient of the weld surface.