2. Solder paste printing reflow soldering voids and their generation mechanism

2.1　Welding void

After reflow soldering, the product is inspected under X-ray, and the lighter color of the soldering zone is the void due to insufficient solder in the soldering layer.

 

(A) QFN solder paste soldering voids (b) Bare chip solder paste soldering voids

 

 

2.2　Welding void generation mechanism

    Take SAC305 solder paste as an example. The main composition and functions are shown in Table 1. The flux and tin beads are glued together in a paste form. The weight ratio of tin solder to flux is about 9:1, and the volume ratio is about 1:1. .

Element

weight ratio/%

Function

tin

85~92

Welding, connecting various electronic components and circuit boards

Flux

2~8

Increase viscosity and remove metal oxides

Adhesive

1~2

Prevent oxidation of solder surface

Active agent

0~1

Removal of metal oxides

Solvent

1~7

Tack and printability adjustment

 

    After the solder paste is printed and mounted with various electronic components, when it passes through the reflow oven, the solder paste will go through four stages: preheating, activation, reflow, and cooling. The temperature in different stages is different, and the state of the solder paste is also different. as shown in picture 2.

picture

Figure 2 Schematic diagram of solder paste temperature curve at different stages of reflow soldering

    In the preheating and activation stages, the volatile components in the solder paste in the solder paste will volatilize into gas when heated. At the same time, gas will also be generated when the surface oxide of the solder layer is removed. Part of these gas will volatilize and separate from the solder paste. The solder balls are caused by flux. Volatile and tightly gathered; in the reflow stage, the remaining flux in the solder paste will quickly volatilize, the tin beads will melt, a small amount of flux volatile gas and most of the air between the tin beads are not dispersed in time, and remain in the molten tin Under the tension of molten tin, it forms a hamburger sandwich structure and is clamped by circuit board pads and electronic components. The gas wrapped in liquid tin can hardly escape only by buoyancy, and the molten state time is very short. When the molten tin cools down and becomes solid tin, pores appear in the solder layer [7], and solder voids are generated, as shown in Figure 3.

 

Figure 3 Schematic diagram of the generation principle of solder paste reflow soldering voids

    The root cause of solder voids is that the air or volatile gas contained in the solder paste is not completely discharged after the solder paste is melted. The influencing factors include solder paste material, solder paste printing shape, solder paste printing volume, reflow temperature, reflow time, solder size, structure Wait.