Guidelines

Design Recommendations for Solder Joint Reliability
42 Intel
®
7500 Chipset Thermal Mechanical Design Guide
8.2 Shock Strain Guidance
A useful metric to compare the impact of design modifications to SJR and assess SJR
risk during shock events is strain measurement. This strain measurement, also referred
to as shock strain, utilizes strain gages to measure the surface strain of a motherboard.
Please note that Intel also publishes strain guidance specifically for manufacturing. This
manufacturing guidance is part of the Board Flexure Initiative (BFI) and those strain
limits are commonly referred to as BFI strain. More information is available in the BFI
Manufacturing Advantage Service (MAS). DO NOT use BFI strain values for shock
strain testing and DO NOT use shock strain guidance for BFI. These two strain
metrics are significantly different and are not interchangeable. Using the BFI strain
values for a design metric will likely result in a poor system design.
Given parameters unique to the board of interest, such as board thickness, the board
surface strain directly correlates to the amount of board curvature. The amount of
motherboard curvature in the critical locations directly beneath the solder balls is
indicative of the reliability of the component solder joints. This measurement is
typically made at the corners of the BGA components. The shock strain results are
sensitive to the application of the strain gages. Guidance for strain gage application is
available in the Shock Strain Monitoring Customer Reference Document (CRD) and the
local Intel Corporate Quality Engineer is also available for help with strain gage attach.
This Shock Strain Monitoring CRD outlines the proper selection, application, and usage
of the strain gages and strain instrumentation to attain repeatable and valid results.
The Shock Strain Monitoring CRD also discusses proper reduction of the data in order to
use the data to compare to the Intel strain guidance.
The strain guidelines will be developed from empirical testing under differing boundary
conditions and published in a subsequent release of this document. Three strain ranges
are determined to quantify associated SJR risk for the Critical to Function solder joints.
The Non-Critical to Function solder balls may have some cracking and fractures when
the strain measurements are within this guidance. Table 8-1 lists the three ranges for
the Intel 7500 chipset.
Figure 8-1. Example of Thick Traces used in a Desktop BGA
Package Edge
Thick Traces
attached to NCTF
solder pads
NCTF pads are
shown in blue