SMT chip processing BFA pad under the PCB surface resin cracking reasons

- Sep 27, 2019-

SMT chip processing BFA pad under the PCB surface resin cracking reasons

After the product is switched to the lead-free process, when the board is subjected to mechanical stress testing (such as shock and vibration), the cracking of the substrate under the pad is significantly increased, which directly leads to two types of failure: smt patch BGA pads and wires. The fracture and the two wires with potential difference in the veneer "establish" the metal migration channel.

(1) The hardness of lead-free solder becomes high, and the stress transmitted to the PCB pad interface is greater at a given strain level.

(2) The temperature difference ΔT of the PCB from the molten solder to the room temperature becomes larger, resulting in a more mismatched CTE of the XY plane between the PCB and the component, and the stress on the solder joint is greater.

(3) High Tg sheets (Tg > 150 ° C) are more brittle.

These three "bigger" lead to an increase in substrate cracking.

PCB surface resin cracking occurred in a mobile phone board.

High TgFR4 resin is more susceptible to pad peel failure than standard TgFR4 resin materials when other factors are fixed. Therefore, it is better to use a medium TgFR4 sheet 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. Cracking of the surface resin of the PCB under the pad, with resistance to lead solder joints and lead-free solder joints. The high Tg is compared to the standard Tg plate pad peel peak pull.

After a company's lead-free switching, several PCB subsurface resin cracking events occurred, indicating that lead-free switching is risky in using large-size BGAs.

In order to avoid delamination of PCB during lead-free soldering, it is required to increase Td and 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.

The above is the industry consultation provided by smt patch processing factory Shenzhen Pengzexiang Technology Co., Ltd., I hope to help you! Please contact us if necessary:

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