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Mechanical Shock and Drop Reliability Evaluation of the BGA Solder Joint Stack-Ups



Mechanical Shock and Drop Reliability Evaluation of the BGA Solder Joint Stack-Ups
This paper investigates the Mechanical Shock and Drop Reliability of SnAgCu ball Flip Chip BGA solder joints.
Production Floor

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Authored By:


Olivia H. Chen and Al Molina
Intel Corporation
Folsom, CA, USA

Raiyo Aspandiar, Kevin Byrd, and Scott Mokler
Intel Corporation
Hillsboro, OR, USA

Kok Kwan Tang
Intel Corporation
Kulim, Malaysia

Summary


Printed circuit boards (PCB) used in the consumer electronic products such as smart phones, tablets and mobile client personal computers are commonly assembled by reflow soldering with lead-free SnAgCu (SAC) solder pastes with peak relow temperature in the 240C to 260C ranges. However, due to the decreasing size of electronic devices which demands lower z-height stackup, the ultra-thin flip chip ball grid array (FCBGA) often experienced SMT soldering challenges with typical lead-free solder reflow due to dynamic warpage on package substrate and PCB.

As a result, low temperature solder pastes of the Bi-Sn metallurgical system have been proposed as alternatives to SnAgCu solder pastes for assembly of boards in the mobile consumer electronics market segments. Besides improvements in solder joint yield due to lower package and board warpages at the peak reflow temperatures, the lower melting points of these solder pastes will also enable significant cost savings.

However, presence of significant amounts of Bi in Sn based solder has previously been shown to induce embrittlement in the solder joints which leads to lower mechanical shock resistance. Hence, solder pastes, containing resin, have been developed recently to provide a post reflow polymeric reinforcement encapsulation around the mixed BiSn+SnAgCu BGA solder joints to mitigate this risk by enhancing the shock reliability of these solder joints.

This paper will investigate the Mechanical Shock and Drop Reliability of SnAgCu ball Flip Chip BGA solder joints soldered with BiSnAg (BSA), and Resin Reinforced BiSn system solder pastes (termed Joint Reinforced Pastes, JRP). This data will be compared to that obtained for SnAgCu ball Flip Chip BGA solder joints soldered with standard SAC solder paste, with and without board level adhesive reinforcement.

Reflow processing conditions, the solder joint shape, microstructure, morphology and failure modes, for each of these cases, will also be described and discussed. Based on in-situ failures recorded during shock event, JRP and SAC305 showed comparable performance, while BSA showed lowest number of drops to failure (less shock resistant). BSA leg showed more failure propagation on additional package locations while SAC305 and JRP legs did not show additional failures at other package locations other than the first two rows of solder joints from the package corners.

In summary, BSA shock margin was significantly improved after the addition of L-shaped corner glue on all four package corners. No failures were seen on the BSA+CG leg. Hence board level adhesive and polymeric joint reinforce pastes, JRP both showed promising result to mitigate shock margin loss due to the brittle nature of BSA paste.

Conclusions


The brittleness of Bi-Sn alloys, even after addition of Ag to the system, limits its use in consumer products where the risk of multiple shock impacts is high. Resin reinforcement solder pastes have been developed recently to provide a polymeric reinforcement around the mixed BiSn+SnAgCu BGA solder joints to mitigate the risk of embrittlement in Bi-Sn solder joints in order to enhance shock reliability.

In summary:

Based on in-situ failures recorded during shock event, JRP and SAC305 showed comparable performance, while BSA showed lowest number of drops to failure (less shock resistant).

BSA leg showed more failure propagation on additional package locations while SAC305 and JRP legs did not show additional failures at other package locations other than the first two rows of solder joints from the package corners.

BSA shock margin was significantly improved after the addition of L-shaped corner glue on all four package corners. No failures were seen on the BSA+CG leg, outperforming both the SAC305 and JRP legs.

Initially Published in the SMTA Proceedings

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