After the product is switched to the lead-free process, when the PCB board is subjected to mechanical stress testing (such as impact and vibration), the cracking of the substrate under the pad is significantly increased. Directly lead to two types of failure: When the Surface Mount , the BGA pad and the wire are broken or two wires with potential difference in the board "establish" the metal migration channel, as shown in Figure 9-19 and Figure 9-20, respectively.
(1) The hardness of the lead-free solder becomes high, and the stress transmitted to the PCB pad interface is greater at the specified strain level.
(2) The temperature difference ΔT of the PCB from molten solder to solidified solder becomes larger, resulting in a more mismatched CTE in the X/Y plane between the PCB and the component, and a greater stress on the solder joint.
(3) High Tg material (Tg > 150 ° C) are more brittle.
These three conditions can to an increase in base material cracking.
PCB surface resin cracking occurs on a mobile phone board, as shown in Figure 9-21.
High Tg FR4 material is more susceptible to pad peel failure than standard Tg FR4 material when other factors are fixed. Therefore, it is better to use a intermediate Tg FR4 material in a lead-free process.
After the product is switched to lead-free, the main failure mode of the BGA solder joint mechanical impact test is the BGA of the lead solder joint to the lead-free solder joint because of the rigidity of the lead-free solder joint itself and the higher assembly temperature. The cracking of the surface resin of the PCB under the pad, the strain resistance of the lead solder joint and the lead-free solder joint is shown in Figure 9-22.
A Comparison of maximum tensile force of high Tg material and standard Tg material for strip stripping is shown in Figure 9-23.
After some companies changed to lead-free, several PCB subsurface resin cracking events occurred, indicating that lead-free changes are risky in using large-size BGAs.
In order to avoid delamination of PCB during lead-free soldering, it is required to increase Td . Generally, Dicy (dicyandiamide) curing agent which is very polar and easy to absorb water for ordinary FR-4 is used as PN which is not easy to absorb water ( The phenolic resin) curing agent increases the content from 5% of Dicy to 25% Wt, but at the same time increases the CTE in the Z direction. In order to lower the CTE in the Z direction, 20% of SiO2 was added. The result is an increase in T while increasing the rigidity of the PCB, reducing the toughness, or making the PCB brittle.
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