Authored By:
Yong (Hill) Liang, Hank Mao, YongGang Yan, Jindong (King) Lee.
AEG, Flextronics International, Inc.
Summary
Today's consumer electronic products are characterized by miniaturization, portability and light weight with high performance, especially for 3G mobile products. In the future more fine pitch CSPs (0.4mm) component will be required. However, the product reliability has been a big challenge with the fine pitch CSP. Firstly, the fine pitch CSPs are with smaller solder balls of 0.25mm diameter or even smaller.
The small solder ball and pad size do weaken the solder connection and the adhesion of the pad and substrate, thus the pad will peel off easily from the PCB substrate. In addition, miniature solder joints reduce the strength during mechanical vibration, thermal shock, fatigue failure, etc. Secondly, applying sufficient solder paste evenly on the small pad of the CSP is difficult because stencil opening is only 0.25mm or less. This issue can be solved using the high end type of stencil such as Electroforming, which will increase the cost.
Conclusions
Based on the work the following are recommended:
NSMD type is recommended for the CSP with 0.8mm pitch and above; SMD type is recommended for CSP with less than 0.8mm pitch with underfill.
A via hole away from the solderable pad will reduce the voids occurring in the solder joint which will help to improve reliability.
OSP with selective ENIG surface finish is recommended for fine pitch CSP on mobile product to help acquire higher solder joint reliability.
Underfill can be used for fine pitch CSP in mobile product to acquire high reliability.
Vacuum support blocks are helpful for solder paste printing; it can improve the reliability of the solder joint with an even and more uniform solder paste volume deposited on the board lands.
Square with round corner stencil aperture openings are recommended for fine pitch CSP stencil opening designs to acquire more solder paste deposited on the board land pads and to get higher standoff height.
Reflow profile is very critical for solder joint reliability, the parameters have to be controlled within specification based on the solder paste supplier and CSP component supplier recommendation.
Thermocouples should be attached on the component body and the solder joint of fine pitch CSPs to ensure there is sufficient heat at the solder joint and to avoid excessive heat on the component body.
Initially Published in the IPC Proceedings
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