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.
