From the slice, the IMC remains on the BGA pad side, and at the same time, the crack solder ball side surface appears to be in a molten state.
First look at the heated environment in which the BGA is located during the furnace. The use of the tray basically eliminates the deformation of the PCB during the soldering process, but at the same time changes the heating environment of the top surface BGA together with the mirror facing BGA, so that the solder joint (bump) temperature of the BGA center part on the top surface is higher than the surrounding area. Solder joint temperature. This leads to two results:
(1) The solder joints (solder balls) at the center of the BGA on the top surface are solidified at the surrounding solder joints.
(2) The BGA on the top surface gradually becomes a crying face (inverted U-shape) from the smiling face (U-shaped) during the cooling process, and the intermediate solder joint will be stretched under the support of the surrounding solidified solder joints. If the solder joint at the center is in a semi-suspiced state, a shrinkage of tin can occur.
This is the same as the normal occurrence of tin-breaking, which occurs during the period of solidification. The reason why BGA's shrinkage tin break occurred in the BGA center in this case is because the mirror image and the use of the tray changed the solidification sequence of the BGA center and the edge solder joint. An example does not determine that the tin shrinkage position is fixed, but from the wiring point of view, the failed solder joint only occupies half of the conduction hole's thermal conductivity, which means that the cooling is faster, and there is a certain reason for the occurrence of shrinkage.
As you can see from this case, the BGA is very sensitive to the state of heat, which is one reason why BGA welding takes longer. The temperature profile should be set so that all the solder joints of the BGA are solidified at the same time, that is, the delay time of the reflow soldering is reduced.
Tin-fracture solder joints are a complex and unpredictable weld defect that is very random depending on a variety of factors. The general probability of occurrence is not high, with an average of about 0.3%, which is mostly related to air humidity. Tin-failed solder joints are sometimes not detected and pose a threat to reliability. Fortunately, the shrinkage tin crack only occurs on a specific package and PCB thick PCBA, and only appears at the four corners. There are more mature methods to prevent - the four-point solder joints increase the amount of solder paste, the BGA is dried before going online, and the dynamic deformation of the PCB using the tray is an important factor affecting the shrinkage of tin and stress fracture. A very effective process control measure.