Patrick P. Solan

The influence of room temperature aging on ball shear strength and microstructure of area array solder balls

Solder ball shear strength is determined for ball grid array (BGA) packages in the as-received condition and after solder reflow preconditioning (exposure to multiple solder reflow profiles). Immediately after reflow, the solder shear strength is considerably higher than the solder shear strength measured on packages in the as-received condition. When the shear strength is monitored as a function of the time after solder reflow preconditioning, it is found to decrease significantly with time. Microhardness tests made in parallel with the shear measurements, provide quantitative confirmation of a degradation in mechanical properties of the solder over time. The observed changes in mechanical properties are the result of room temperature annealing, or age-softening of the solder. Cross sectioning and scanning electron microscopy are used to identify and track microstructural changes in the solder balls with room temperature aging. The microstructural changes are correlated to the decrease in shear strength and microhardness and in some cases, to a change in failure mode. These results are discussed in terms of accurate interpretation of shear test data and suggestions are made for test procedures that can minimize data variations induced by solder aging effects.

The effect of modifications to the nickel/gold surface finish on assembly quality and attachment reliability of a plastic ball grid array (peer review version)

Electrolytic and electroless Ni/Au are common pad surface finishes on area array (BGA or CSP) packages and printed wiring boards (PWB). The electroless nickel/immersion gold (ENIG) process often is implemented when there is insufficient space to allow bussing for the more common electrolytic Ni/Au plating. The ENIG process continues to be used despite evidence that it may cause catastrophic, brittle, interfacial solder joint fractures. In this investigation a plastic ball grid array (PBGA) test vehicle is used to compare quality and reliability of standard and experimentally modified ENIG surface finishes. The standard electrolytic Ni/Au surface finish is used as the control cell for the experiment. Ball shear tests and optical and scanning electron microscopy are performed on as-received and thermally preconditioned packages to evaluate package quality prior to assembly. Accelerated temperature cycling (0/+100/spl deg/C and -40/+125/spl deg/C) is used to evaluate solder joint attachment reliability. Detailed failure mode analysis is used to compare the fracture modes in the ball shear and thermal cycled samples in the electroless and electrolytic packages. The results are discussed in terms of the failure modes and the characteristics of the different Ni/Au surface finishes.

Ball shear versus ball pull test methods for evaluating interfacial failures in area array packages

In this investigation, a ball pull (tensile) test is investigated as an alternative to the ball shear test for evaluating the solder joint integrity of area array packages. The relative effectiveness of the pull and shear methods is compared using BGA packages with documented susceptibility to brittle interfacial failure during accelerated temperature cycling tests or isothermal aging. Accelerated temperature cycling is used typically to measure long term solder joint attachment reliability in various use environments and isothermal aging is used to measure susceptibility to degradation following high temperature storage. The shear and pull tests are conducted on packages in the as received condition and after thermal preconditioning. Metallographic failure analysis and scanning electron microscopy with energy dispersive X-ray analyses are used to characterize the solder joints and fracture modes. The ball shear and ball pull results are compared and discussed in terms of the ability to predict susceptibility to interfacial failures in area array packages.

Laser weld process improvements for optical isolator assembly

Optical isolators are used in lightwave systems to reduce feedback that could be propagated in the signal fiber. Several methods can be used to assemble and join the various components which comprise the completed device. Since the single mode fiber core is 8 microns in diameter, it is desirable to utilize a robust assembly technique which will produce a minimum of distortion during the assembly process. Process induced distortion could randomly change the device coupling which may have an adverse effect on operation yield. With the development of laser welding systems, devices can be manufactured with localized welds providing small amounts of thermal distortion. This paper compares two isolator assembly techniques: resistance welding and laser welding. A comparison of device power losses after the weld sequence and thermal cycling data shows that laser welding is a more robust and stable technique for isolator manufacture.

Solder joint attachment reliability and assembly quality of a molded ball grid array socket

This paper summarizes efforts to improve the assembly quality and solder joint reliability of a molded, plastic ball grid array socket. Metallographic analysis shows that a ball grid array type solder interconnect has superior assembly quality compared to a butt type solder interconnect. Ball shear tests, coupled with reflow preconditioning show that the socket ball attachment exceeds industry requirements established for area array packages. Isothermal aging, ball shear tests, and X-ray fluorescence confirm that the electrolytic Ni/Au surface finish on the socket bond pads presents no risk for interfacial, intermetallic embrittlement. The coefficient of thermal expansion (CTE) of the socket is optimized through the proper selection of material and molding process. The physical property improvements are confirmed using bulk CTE measurements on sockets manufactured with and without material and process optimization. Sockets are assembled on PWB test vehicles using typical surface mount manufacturing processes and temperature cycling is used to assess the long-term, solder joint attachment reliability. Failure analysis of the thermally cycled assemblies shows that the reduction in the anisotropy and mismatch of the CTE alters the failure mode of the sockets, which results in a substantial improvement in the overall long-term attachment reliability. The improvements in solder joint reliability are correlated with the improvements in the thermal expansion properties and the enhanced reliability is discussed in terms of lowering the risk of using this socket technology in more demanding use environments.

The effect of variations in nickel/gold surface finish on the assembly quality and attachment reliability of a plastic ball grid array

Electrolytic and electroless Ni/Au are common pad surface finishes on area array (BGA or CSP) packages and printed wiring boards (PWB). The electroless nickel/immersion gold (ENIG) process often is implemented when there is insufficient space to allow bussing for the more common electrolytic Ni/Au plating. The ENIG process continues to be used despite evidence that it may cause or contribute to catastrophic, brittle, interfacial solder joint fractures. In this investigation a plastic ball grid array (PBGA) test vehicle is used to compare quality and reliability of four variations of the ENIG surface finish. The standard electrolytic Ni/Au surface finish is used as the control cell for the experiment. Ball shear tests and optical and scanning electron microscopy are performed on as-received and thermally preconditioned packages to evaluate package quality prior to assembly. Accelerated temperature cycling (0/+100/spl deg/C and -40/+125/spl deg/C) is used to evaluate solder joint attachment reliability. Detailed failure mode analysis is used to compare the fracture modes in the ball shear and thermal cycled samples in the electroless and electrolytic packages. The results are discussed in terms of the failure modes and the characteristics of the different Ni/Au surface finishes.

Safe laser system design for production

The majority of industrial lasers used in manufacturing are Class IV lasers. Engineering and administrative control measures appropriate to that class of lasers must be enforced to ensure their safe use in production environments, i.e. they should be designed into Class 1 laser systems. In addition, several other design characteristics unique to industrial applications are normally incorporated into these Class I systems, such as interlocked enclosures, process monitoring by means of closed circuit television (CCTV) or microscope optics, and optical fibers as well as conventional, fixed optics for laser beam delivery. The accessible emission for an integrated production laser system must be below the Accessible Emission Limit for a Class I laser. This paper addresses solutions to safety design issues through examples of production industrial laser systems. The appropriate administrative control programs that complement the engineering control measures are also discussed.The majority of industrial lasers used in manufacturing are Class IV lasers. Engineering and administrative control measures appropriate to that class of lasers must be enforced to ensure their safe use in production environments, i.e. they should be designed into Class 1 laser systems. In addition, several other design characteristics unique to industrial applications are normally incorporated into these Class I systems, such as interlocked enclosures, process monitoring by means of closed circuit television (CCTV) or microscope optics, and optical fibers as well as conventional, fixed optics for laser beam delivery. The accessible emission for an integrated production laser system must be below the Accessible Emission Limit for a Class I laser. This paper addresses solutions to safety design issues through examples of production industrial laser systems. The appropriate administrative control programs that complement the engineering control measures are also discussed.