Experience sharing of BGA assembly process
(1) The viscosity of the flux is very important. Too high will affect the coating and transfer; if it is too low, it will affect the amount of coating. Generally, the viscosity should be selected in the range of (25000±5000)cp.
(2) The thickness of the flux is 60% of the solder ball. If it is too thick, it is easy to be smeared on the package body, causing vibration during soldering, and even optical centering recognition. The phenomena observed in different thickness flux tests are shown in Table 4-3
(3) Detection method. Generally, a saw tooth ruler can be used for detection, but this saw tooth ruler is due to the sampling position and operation method.
Factors such as (immersion speed, time), sawtooth size, etc., will be different from the actual BGA chip solder ball height.
Glass paste should be used for observation. A good solder height should obtain a uniform solder pattern under the glass plate, and the size should be at least larger than the solder ball. The device that rotates and smoothes the flux often obtains a different amount of flux due to the difference in viscosity.
As shown in Figure 4-54. Using X-rays to observe the size of the solder joints, it is found that the thicker the flux, the larger the diameter of the solder joints.
Shows that the amount of flux affects the degree of collapse of the solder joint.
Tests show that the thickness of the dip coating should reach 60% of the diameter of the solder ball.
(4) BGA packages should use large-size balls to eliminate the bridging caused by the sealing effect after excessive flux.
(5) The formation process of solder joints is shown in Figure 4-55, and the source and reflow soldering process are recorded in video. When the ML-PoP is heated above the melting point of the solder joint, the BGA solder ball and the PoP solder ball will melt and fuse successively.
The early fusion will be drawn into a columnar or waist shape, and then as most of the solder joints are fused, BGA will fall.
From this process, the melting and fusion process of BGA solder balls and PoP solder balls is gradually completed. As long as there is flux on the BGA solder balls, the BGA solder balls and PoP solder balls will fuse and will not form a ball socket; if BGA solder If there is no flux on the ball, a ball socket will be formed.
If there is flux on the bottom surface of the BGA, it will cause the BGA to vibrate up and down on the PoP. This vibration is beneficial to eliminate the discharge of gas inside and outside the fusion solder joint and the disconnection of the bridge solder joint, and has the effect of eliminating the bridge. PoP has never been bridging due to excessive flux, but excessive flux will affect the mounting of a BGA with flux, and the nozzle of the placement machine cannot absorb the components, and it also affects the pattern recognition.