Milos Dusek

The impact of thermal cycling regime on the shear strength of lead-free solder joints

Purpose – The purpose of this work is to undertake a comparison of accelerated test regimes for assessing the reliability of solder joints, in particular those made using lead‐free solders.Design/methodology/approach – Identical samples of 1206, 0805 and 0603 resistors were subjected to six different cycling regimes to investigate the effect of thermal excursions, ramp rates and temperature dwells.Findings – The most damage to joints was found to be caused by thermal cycling between −55 and 125°C, with a 10°C/min ramp rate and 5 min dwells. Large thermal excursions were shown to give faster results without compromising the failure mode.Research limitations/implications – Similar degrees of damage in the lead‐free solder joints were experienced from thermal shock regimes with ramp rates in excess of 50°C/min. However, these regimes, although faster to undertake, appeared to cause different crack propagation modes than observed with the thermal cycling regimes. However, these differences may be small and th...

Low cycle isothermal fatigue properties of lead‐free solders

Purpose – The purpose of this paper is to discuss a new method to measure the fatigue of single solder joints in shear, and hence calculate the joint strain energy density in each fatigue cycle.Design/methodology/approach – There has been a step change in the requirement to characterise solder joint reliability with the number of new alloys introduced as a result of the recent RoHS legislation. Experimental testing for every configuration is expensive and time consuming, and hence modelling has become more attractive. The accuracy of modelling predictions is limited by the accuracy of the materials data. The data for these new alloys must reflect the miniaturisation of electronics and that solder joints are loaded in shear, two aspects not well reflected in the existing SnPb data. The approach here has been to develop an instrument interconnect properties test machine, where the strain and stress can be measured directly for small solder volumes and in shear. A four‐point measurement system for resistance...

Assessing the performance of crack detection tests for solder joints

This paper presents both modelling and experimental test data to characterise the performance of four non-destructive tests. The focus is on determining the presence and rough magnitude of thermal fatigue cracks within the solder joints for a surface mount resistor on a strip of FR4 PCB. The tests all operate by applying mechanical loads to the PCB and monitoring the strain response at the top of the resistor. The modelling results show that of the four tests investigated, three are sensitive to the presence of a crack in the joint and its magnitude. Hence these tests show promise in being able to detect cracking caused by accelerated testing. The experimental data supports these results although more validation is required.

Correlation between jamming and skipping during solder paste printing

Purpose – To investigate how jamming of particles in a solder paste varies as a function of the gap through which the particles flow, and to correlate this with skipping defects during the printing process.Design/methodology/approach – Solder pastes with particle sizes of types 2, 3, 4 and 5 were sheared between the parallel plates of a rheometer. Jamming events that cause the solder particles to be forced against each other were detected by monitoring the electrical current flowing between the plates under a bias of 1.0 V or less. Solder paste printing trials were conducted with the same pastes, and solder paste skipping monitored.Findings – Jamming was detected when the ratio of plate gap to largest particle diameter is reduced to a value between 3.8 and 5.0. Decreasing the gap further results in increased jamming. A strong correlation between levels of skipping and jamming was found.Research limitations/implications – More extensive printing trials are required before rheometric jamming detection can b...

Fine pitch stencil printing using enclosed printing systems

Enclosed print heads have recently been developed as an improvement on the traditional squeegee methods for solder paste printing. They offer the opportunity of widening the printing process window and reducing process waste. Consequently, this work was undertaken to evaluate some aspects of enclosed print head printing, and it has been shown to be a robust process. A number of performance factors were established: with increased humidity the paste degradation was limited due to its sealed paste reservoir; the system also permitted successful intermittent printing over a 5 day period; printing is much more tolerant to distorted substrates than some squeegee blades, and hence improves printing on non‐planar surfaces; significant reduction in paste wastage occurs, since paste ageing is reduced.

Low Cycle Isothermal Fatigue Properties of Lead-free Solders

A major concern is the reliability of the solder joints, which has traditionally been predicted using modeling techniques with established SnPb solder materials data. Although the mechanical behavior of lead-free solders is known to be different from that of their traditional counterparts, there is a dearth of credible measurement methods, in order to evaluate the likely lead-free solder performance. However, currently available test instruments do not readily meet these constraints. This paper describes a new approach developed at NPL to overcome these constraints, resulting in the design of an instrument IPTM (Interconnect Properties Test Machine), with a sample geometry that permits small solder volumes to be studied in pure shear [Ref 1 pp.468]. Example data are presented and illustrate how the low cycle fatigue resistance of solders can be characterized. Advantages of the new approach include: (i) it can accommodate various solder alloys and surface coatings, (ii) the solder joint volumes mirror those in modern assemblies, (iii) the solder is under shear, (iv) the samples are relatively easy to manufacture, (v) the construction of the specimen allows direct microscopic examination during the test at room temperature. A 4-point measurement system for resistance monitoring has also been evaluated and found to correlate well with load decreases recorded during fatigue testing of solders. The results of this study reveal that lead-free solders at high temperatures have increased lifetimes when creep constitutes major part of mechanical cycle. A route to predict a fundamental reliability variable of solder has been identified.

Novel Testing Instrument for Lead-Free Solder Characterization

The European Union Restriction of Hazardous Substances Directive of 2006 has revolutionized the use of materials for electronic packaging: tin-lead solder has had to be replaced with lead-free solder. While years of experience and data acquired in the field are available for tin-lead solder, the same cannot be said for lead-free solder. The lack of data on these new alloys hinders the use of modeling for predicting the reliability of lead-free solder. This paper describes a new testing machine that can assess the properties of solder joints in a copper-solder-copper structure. Finite-element analysis models using the newly acquired data will improve lifetime predictions. During a typical test, a model solder joint is expanded and contracted in shear. This actuation is achieved from the thermal expansion of a steel tube, which provides a smooth movement at a rate that is equivalent to that encountered on printed circuit boards when thermally stressed. The applied force is measured using a load cell; the sample displacement is determined from laser displacement sensors. Hysteresis load-displacement loops are obtained and can be used for evaluating the material plastic response and loading history. A camera/microscope system captures time-lapse photography images, which digital image correlation software can analyze to obtain strain profiles and study crack propagation. The instrument is capable of investigating creep, stress relaxation, and isothermal and thermal-mechanical fatigue behavior of solder interconnects. Results from this new instrument are presented here for isothermal and thermal cycling fatigue tests.

Lead-free Solders and PCB Finish Effects on Solder Joint Reliability

As we move forward into the lead-free era there are numerous possible combinations of solder alloys and PCB finishes. This is further complicated by wide variations in processing resulting in different microstructural properties. To judge the relative impact this might have in accelerated life testing during thermal cycling. Test vehicles were assembled with combinations of three alloys (SnAgCu, SnAg and SnPbAg), two board finishes (ENIG and immersion Ag), various component sizes and two substrate materials (FR4 and CTE matched), plus a conformally coated PCB was included. The degree of damage was monitored using continuity testing, push-off shear testing and microsectioning. The salient points from these studies were that there were significant (measurable) difference between the reliability of joints soldered with SnAgCu and SnPbAg alloys. There was no evidence to suggest that thermal pre-treatment was detrimental to the long-term reliability of the joint. Additional reflow passes or extended thermal ageing did produce some differences in reliability, but these were component specific. The reliability of joints made using SnAgCu solder was significantly better than that of joints made using the simple binary SnAg alloy. There was no evidence to suggest that thermal pre-treatment (reflow, wave or isothermal ageing) was detrimental to the long-term reliability of the joint. The results suggest that ageing and microstructure are not linked in a straightforward way. Lead-free processing can affect the reliability of plated-through holes (vias). The latter generated cracks when subjected to more than 500 thermal cycles. Conformal coating improved the joint reliability performance, possibly by a retaining action of the more pliable coating, preventing complete detachment. Comments are offered on the techniques used to assess solder joint reliability

Crack Degradation Model Derived From Experimental Strain-Stress Data

A new concept of lifetime prediction of solder joints is presented, based on a fatigue cycling experiment under isothermal conditions. A common approach of lifetime prediction of solder joints is based on damage model based on the Coffin-Manson equation, which uses the plastic strain range from a single hysteresis loop to calculate a number of cycles to failure. This was later modified by Engelmaier. Then Morrow developed a damage model based on strain energy density, which takes the area of a single hysteresis loop. These stress-strain hysteresis loops are calculated in FEA modelling, but with the assumption that there is no crack. Hence only engineering values of strain and stress are estimated, and the load-bearing area of a solder joint is assumed to be constant and not cracked. This is clearly an over simplification as solder will crack under fatigue conditions during thermal cycling. A further simplification is that the stress is calculated from an inverse solution of secondary creep rate equation. The correct approach should reflect the true stress and strain (measurement of force and actual load bearing area) developed inside a solder joint as a function of time. A clear correlation of solder joint area with electrical resistance can be shown to exist, hence the load bearing area can be obtained by monitoring of solder joint electrical resistance, and therefore the true stress can be calculated. Measurements following this approach show that the strain-stress hysteresis loop area does not collapse (using a constant displacement controlled profile of the solder joint), but that the hysteresis loop expands on the stress axis. Since, an expansion of a hysteresis loop on the stress axis would cause a divergence of stress energy density, the true shear strain is shown to be decreasing under the same displacement in each cycle. A crack degradation model will be presented using the measured true stress strain hysteresis loop area through out the fatigue life of a solder joint.

Finite element modelling of crack detection tests

Four non-destructive tests for determining the length of fatigue cracks within the solder joints of a 2512 surface mount resistor are investigated. The sensitivity of the tests is obtained using finite element analysis with some experimental validation. Three of the tests are mechanically based and one is thermally based. The mechanical tests all operate by applying different loads to the PCB and monitoring the strain response at the top of the resistor. The thermal test operates by applying a heat source underneath the PCB, and monitoring the temperature response at the top of the resistor. From the modelling work done, two of these tests have shown to be sensitive to cracks. Some experimental results are presented but further work is required to fully validate the simulation results.