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Products > Packages > Semiconductor Device Mount Manual > NOTES ON MOUNTING > BGA Bonding Process >

CHAPTER 4 NOTES ON MOUNTING


4.3 BGA Bonding Process
4.3.2 Case 2. Unmelted solder pt.1

Figure 4-9 shows an example of what is observed to happen to the solder joint when the solder has not melted. In this example, the solder balls and solder paste have completely separated and have not fused together. Even in this kind of case, at the early phase of mounting, the joint itself may be electrically conductible.


Example of Unmelted Solder
Figure 4-9. Example of Unmelted Solder

The possible causes of unmelted solder are given below. Unmelted solder may also occur due to a combination of these factors.

Factor 1.   Temperature profile:
The flux potency has degraded because either the preheating time was too long or the temperature was too high.
 
Factor 2.   Solder ball oxidation:
An oxide film has grown on the surface of the solder balls due to incorrect storage of the device. The oxidation on the surface of the solder balls has been further accelerated during preheating.
 
Factor 3.   Solder paste deterioration:
The flux potency has degraded, so the oxide film on the surface of the solder balls has not been sufficiently removed. The solder material in the paste has oxidized, making it impossible for the paste to fuse with the solder balls.
 
Factor 4.   Warping of mounted device/printed circuit board:
The device or printed circuit board has warped during heat application, causing the solder balls and solder paste to separate when the solder melted. The warped device or board then returned to normal while the solder was in a semi-molten state during cooling. time was too long or the temperature was too high.
 

Mechanism for Inferring Cause of Unmelted Solder
Figure 4-10. Mechanism for Inferring Cause of Unmelted Solder

Examples of the measures for the factors above are respectively shown on next page.
To 4.3.1 Case 1. Insufficient heat applicationTo 4.3.2 Case 2. Unmelted solder pt.2