Pillow-Head Effect in BGA and CSP Package

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With the BGA, CSP package devices to dense pitch, miniaturisation of the direction of development, the widespread application of lead-free processes to the electronic assembly process has brought new challenges. Pillow-head effect are BGA, CSP device reflow soldering is unique to a defective form. As shown in the figure, the solder ball and the solder between the solder is not completely fused, but like a pillow on a nest or a pile. This defect often occurs in the reflow soldering of BGA and CSP devices, and is more obvious in lead-free processes.

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Figure 1. Pillow head effect.

Pillow-head effect is not easily detected because there is often a partial connection between the solder ball and the solder. So the circuit may be able to pass functional testing, optical inspection and ICT testing. However, since the solder is not really fused and mixed with each other, the solder joints are not strong and may lead to failures in subsequent processes or in use. For example, after the soldering process, PCBAs with pillow-head effect may fail in subsequent assembly processes, during transport due to thermal expansion and contraction, or in the field when subjected to prolonged current loads. Therefore, pillow-head effect are extremely hazardous.

The mechanism of pillow-head effect

1. Poor solderability of solder paste or solder balls.

2. Impact of solder paste printing and placement accuracy. If the printing of solder paste is uneven or inaccurate, or the placement position is shifted, it will lead to poor contact between the solder ball and the solder, thus forming a pillow-head effect.

4. Poor coplanarity of BGA solder balls. If the height of the solder ball of the BGA package device is not consistent, it will lead to uneven force on some of the solder joints, resulting in warpage or fracture.

5. Chip warpage or uneven chip temperature, there is a temperature difference. If the chip is unevenly heated during reflow soldering, or warpage occurs before or after reflow soldering, it will lead to a mismatch in the coefficient of thermal expansion between the chip and the PCB substrate, resulting in stress and deformation. This can cause some of the solder joints to lose contact or separate, forming a pillow-head effect.

Measures to inhibit the pillow-head effect phenomenon

1. Optimisation of the reflow temperature profile ensures that the solder paste is completely liquefied and fully fused to the solder balls.

2. Selection of suitable solder paste: the use of anti-thermal collapse ability, go CuO (Cu2O), SnO (SnO2) good effect of the solder paste.

3. Improve the hot air reflow heat capacity, it is best to use "hot air + infrared" composite heating mode, can effectively improve the uniformity of the temperature on the package body.

4. Strengthen the reflow furnace exhaust system monitoring to ensure that the exhaust pipe is smooth and effective.

Fitech solder paste

Shenzhen Fitech company independent research and development and production of SiP system level encapsulation solder paste Fitech siperiorTM 1550/1565, particle size T8, T9, the smallest printing / dispensing point φ = 70/50μm, can be stably used for μBGA pre-set. It shows excellent printability, release transferability, shape and stability retention, and sufficient and uniform print volume in practical applications. No bead or bridge defects after prolonged printing. Inquiries are welcome.