Abstract

The solder joint reliability for BGA (Ball Grid Array) assembly is becoming a more concerned issue as these packages are featuring higher density interconnections, multiple functionality and higher speed combined with smaller size. The traditional test methods for second level PCBA (Printed Circuit Board Assembly) mechanical reliability monitor the electric resistance changes of Daisy chains in the test samples under 4-point bending. The method has been documented by Interconnecting and Packaging Electronic Circuits and Joint Electronic Devices Engineering Council in PC/JEDEC 9702 standard. The effectiveness of the test has been questioned when applied to the new lead-free soldered packages. Due to the failure mode shift from solder joinicopper pad interface cracking in Tin-lead PCBAs to pad-cratering cracking in leadfree packages, the electrical continuity monitoring becomes ineffective in detecting the interconneCt failure. On the other hand, the strain gauges recorded PCB strains during bend tests show little increase that would be indicative of an onset failure. This project applies Fiber Bragg Grating (FBG) strain sensors to detect the pad-cratering failure. FBO have been employed widely in different areas of engineering due to its advantages of small size, light weight and high sensitivity. In this project the FBG sensors are laid to the vicinity of the BGA substrate comers. By detecting and recording the solder joint fracture induced strain release, the onset of pad-cratering is explicitly revealed. The study has demonstrated that the FBG sensors are much more sensitive than electric resistance strain gauges in detecting the substrate strain release in BGA assembly 4-point bend testing due primarily to the sensor's much smaller geometric size. By placing the sensors very close to the comer solder joints, the new test obtains accurate strain information related to the first solder joint cracking. Furthermore, the recorded strain release enables the detecting, understanding and analysis of the critical load of the solder joint fracture, the brittle and ductile fractures and related strain relaxation phenomenon during the PBGA flexural loading, etc.

Highlights

  • The electronics industry is one of the most fascinating, dynamic and rapidly developing industries

  • An electronic package typically consists of basic elements including a silicon integrated circuit (IC) chip, a substrate and solder joints interconnecting the component to a printed circuit board (PCB)

  • The study focuses on the second level solder joints reliability under flexural mechanical loading in order to characterize the mechanisms of mechanically induced joint fracture and material damage development under flexural loading

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Summary

Introduction

Fiber Bragg Grating (FBG) is a type of distributed Bragg reflector that is constructed in a short segment of silica fiber. FBG sense numerous parameters such as strain, temperature, and pressure. Optical fiber technology has been the subject of considerable research and development to the point to transmit vast amounts of data and information from one point to another. The reasons of optical fiber to become an attractive sensor element are their low loss, high bandwidth, immunity to electromagnetic interference, small size, light weight, safety, relatively low cost, low maintenance, etc. The wide applications are due to their multiplex ability and durability for use at relatively high temperature and environmentally unfavourable conditions. Fiber Bragg grating sensors have attracted considerable attention for measurement application such as in mechanics, aerospace, chemistry and electronics. FBG technology is in particular used to monitor composite material curing and fatigue life evaluation, and to provide good measurement in large scale civil structures such as highway, buildings, bridges, as well as remote sensing for oil wells, power systems and pipelines

Reliability issue for second level assembly
Stress and strain induce during reflow assembly
Cycling thermal stresses and strains during working environment
Mechanical bending induced damages
Research objectives of the current project
Onset critical load detection by FBG
Chapter 2 Basics of electronic packaging and mechanics
Basic structure and function
Structure of a PCB
Materials of PCB lamina
Transition to lead-free solders
Peak reflow temperature increase
Mechanical and thermal properties for SAC solders
Young's modulus
The coefficient of thermal expansion
Chemical compositions of SAC solders
The ternary eutectic composition of SAC solders
Structure of BGA assembly (PBGA)
Effect of reflow cooling rate
Microstructure of SAC solder joints
Microstructure of solders and its impact on solder joint behaviors
Deformation of solder joints under a thermal load
Stress and strain
Interfacial stress
Stress distribution
Stress concentration
Creep - time dependent mechanical behaviours
Hall's Stress! Strain Hysteresis Loop
Metallization diffusion and solder flux burning
Effect of alloy element contents
Silver contents The beneficial effects of a reduced
Copper contents
Add other alloys It is report that a small
Flexural test methods review
Three- or four-point bending tests
Spherical bending tests
Literature review for BGA solder joint flexural reliability
Four-point bending tests
Critical load detection
Strain gauge placed on the substrate
Strain relaxation
Literature review of FBG sensor application for strain measurement
Theoretical background and operating principles
Strain measurement by FBG
Test method specification
Strain rate
Maximum crosshead travel distance and speed
Minimum crosshead speed
Failure criteria
Fabrication of FBG Sensors
Strain gauge
Gauge factor
Comparison of strain gauge and FBG
Quality tests for FBG grating
Flip chip ball grid array package
Strain gauge layout
Daisy-chain
Layout FBG on the substrate
Signal synchronization
Parameter set up
Balance adjust screws
Preload requirement
Test procedure
Chapter 6 Failure inspection and analysis
Fracture modes under mechanical bend loading
The second level interconnect failure mechanisms
Dye and pry inspection
Dye and pry tests
Failure maps
Chapter 7 Analysis and discussion
Brittle fracture
Ductile fracture
PCB thickness
Findings
Conclusions
Full Text
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