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.
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:
The novel aspects of this project will be:
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
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.
The project has changed a lot since its inception. The pull test and shear tests are no longer a part of the project as they are unable to replicate the same failure mode and mechanisms in thermal cycling test, which is the most common reliability test for solder joint.
The three tests that have been selected are :
Animations showin cyclic displacement, bending and twisting test respectively.
These tests are able to induce plastic strains in the joints in the same way as thermal cycling. However, since the effect of CTE mismatch is not included in mechanical cycling, the failure mechanisms are not same. In thermal cycling failure the most common failure mechanism is inter-granular in nature, associated with low cyclic strains and accompanied with grain boundary sliding. In mechanical cycling, due to higher stresses, the failure mechanism in trans-granular in nature. Efforts are curently underway to combine thermal and mechanical cycling, in order to get the required failure mechanism. Initial results, both on QFP and BGA, are promising.
This project involves lot of simulation work as well. Simulation of thermal cyclingon QFP and BGA has been caried out in order to save time taken in experimentation. The simulation includes transient and non-linear behaviour of solder joints (In ANSYS Anand's model has been used). By calculating Plastic work energy, number of cycles to crack initiation and crack propagation rate during thermal cycling has been calculated. Fig. 6 shows the plastic work energy in Gull-wing lead during one cycle of thermal cycling. The figure clearly shows the crack growth path in the joint.
Fig. 6 Plastic Work energy in QFP solder Joint
This page was last updated on 8th Feb. 2001.
Copyright: Yasir A. Quadir, UCC, NMRC