Guidelines

Intel
®
7500 Chipset Thermal Mechanical Design Guide 41
Design Recommendations for Solder Joint Reliability
8 Design Recommendations for
Solder Joint Reliability
Solder Joint Reliability (SJR) remains a major topic of concern in designing systems
especially for surface mounted components. Solder ball cracking and fracture is a
failure mode associated with overstressing the surface mounted component on the
motherboard. The over-stressing typically occurs when the motherboard is subjected to
bending deflection. The deflection of the motherboard applies loads to these surface
mounted components that attempt to peel the component from the board. These loads
stress the solder balls of the component and either initiate cracks, which grow through
the solder during thermal and power cycling, or cause fracture, which results in an
electrical open.
Loading conditions such as shock typically stress the motherboard and generate
stresses at the solder joints that leads to either crack initiation or complete fracture of
the balls. This section will discuss guidance specific to the Intel 7500 chipset. Please
refer to the System Mechanical Design Guidance for Dynamic Events -Application
Notes/Briefs for more information on system design guidance, and best practices.
Section 2.1 describes the function of the Non-Critical to Function (NCTF) Solder Balls.
These balls are located in the corners of the ball grid array, where they are most
susceptible to stressing from motherboard flexure, and under the die shadow. These
NCTF balls mitigate degradation to component performance once damage has occurred
at the solder balls. Monitoring of these NCTF balls during shock testing is described in
the Platform Design Guide. General design guidance is available in the System
Mechanical Design Guidance for Dynamic Events-Application Notes/Briefs. The NCTF
solder balls provide for load shedding during solder ball loading events.
8.1 Solder Pad Recommendation
Additional protection from pad cratering on the motherboard has been demonstrated
through the usage of thick traces at the corner NCTF ball locations. The NCTF trace
thicknesses from 60-80% of the pad diameter were tested in board level shock tests
with metal define pads and reduced the occurrence of pad cratering failures. Pad
cratering is the failure mode in which solder pads in the motherboard separate from the
PCB.
The thick traces shown in Figure 8-1 are an example of how thick traces may be used
at NCTF pads. Note the NCTF locations shown in Figure 8-1 are not the NCTF locations
of the name of product package and is shown to illustrate the application of thick
traces. Designers are encouraged to use thick traces in designs where pad cratering
has occurred along the corners of the package. The thick traces effectively increase the
strength of the pad to motherboard interface and may cause a crack to initiate in a
different failure mode in the NCTF solder ball while increasing the shock margin.