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Project Summary

    1.Title of Project: Rapid Ageing and Reliability Tests for Microelectronics Interconnections.



    A.  Background: Lead (Pb) containing solders are being replaced by lead-free solders because of environmental concerns. In addition to environmental problems, the technical limits of tin-lead solders, in particular its
       relatively low-strength, are currently being reached as component operating temperatures are increasing and finer pitch components with smaller solder joints are becoming the industry standard. The manufacturers
       have to change fast over to the lead-free alternatives because of legislation and laws being introduced to check the use of lead based materials in electronics industry. But, there is not much detail available on the
       thermo-mechanical behaviour of various lead-free solder alloys with respect to different components and finishes on PCB. The manufacturers also want to have a rapid test method to compare the joint reliability.
       This project proposes to develop a rapid reliability test method for comparing solder joints with different alloys and components.

    B.  Proposed Research: To date, a large proportion of the work undertaken on lead free solders has concentrated on chemical composition of alternative solder alloys and their compatibility with different sets of PCB
       finishes. Manufacturing related issues are also being reported. But there is little, if any, work reported on the mechanical reliability and strength of lead free joints. Currently in NMRC work has been going on to
       develop a means of measuring the mechanical behaviour of a range of lead-free solder materials through the design and test of specimens, which reflect real joint behaviour. Usually the reliability tests take too long
       and sometimes can go as long as six months. Some rapid testing has been reported for lead-based solder joints but currently there is no available rapid test method that allows judging the reliability of solder joints.

       Therefore the objectives of this project are:

                           To test and compare the reliability of conventional lead based solder joints and lead-free solder joints.

                           To develop a mean for rapid reliability testing of solder joints.

                           To test the solder joints in combined thermal and mechanical conditions (e.g., combined heat and vibration test).

                           To extrapolate the short term results for long term reliability.

                           To develop numerical models from the available data and results.

              The novel aspects of this project will be:

                           Development of new rapid test techniques for comparing joints quality with different alloys and components.

                           Linking of the test data to numerical models to provide a reliability prediction capability.

              The work plan for this project is to start with the review of literature of lead-free solder development and new rapid testing methods. This will go alongside in getting acquaintance and experience with
              various processes and equipments to be used for this project. This include the assembly process of the PCB, using mechanical testers (INSTRON and DAGE) and various reliability test processes like
              thermal vibration, thermal cycling tests, etc. After this stage, the reliability testing of conventional lead-based solder joints will start. This will go alongside with finding new techniques for rapid testing of
              these joints. This includes the combined heat and vibration testing and the behaviour of yield strength of solder joints with respect to temperature. Next, the numerical modelling will commence as well as the
              systematic study and comparison of various lead-free alloys and components. The above description is presented in the form of bar chart in the page attached.

              The end result of this project will be to develop a rapid reliability test method for the solder joints. It will also provide a database of results on the reliability of lead-free solders. The numerical model
              developed in this project will also provide a reliability prediction methodology strongly based on measured data.


                The above images show the solder pads after the leads have been pulled of. Voids on the pads can be seen very clearly.

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