The PCB industry is ever changing and adapting to new technologies. OEM specifications and requirements have also advanced due to these technologies. In some cases the OEMs are asking for a low resistance test to be performed on some or all electrical test nets of the PCB or on the holes of the PCB. This requirement is typically not well defined on the fabrication drawing and that leads to misleading conclusions by the fabrication house.
4-Wire Kelvin testing has been around for many years but using this type of measurement on bare PCB’s is a relatively new requirement. The requirement for PCB 4-Wire Kelvin testing was originally requested by digital commercial OEMs in the US with the aim to set out to improve the overall quality of the products. The first 4-Wire Kelvin test requirement for PCB’s were on a limited hole criteria. Since that time automotive companies in Japan have also adopted such requirements. Medical applications are also joining in with their own 4-Wire Kelvin requirement.
This paper will use the data gathered by the company’s operations to outline what a 4-wire Kelvin test is and how it can be used. Several examples will be illustrated of what the 4 wire Kelvin test can and cannot do. A clear definition of what limitations are present during the testing operation will be defined. The paper will assist designers in understanding how the low resistance test can assist them and also identify causes that can identify unwanted concerns/issues.

Many researchers have used different failure criteria in the published solder joint reliability studies. Since the reported timeto-
failure would be different if different failure criteria were used,it would be difficult to compare the reported reliability life of solder joints from one study to another. The purpose of this study is to evaluate the effect of failure criteria on the reported thermal fatigue life and find out which failure criterion can detect failure sooner. First,the application of the control-chart based method in a thermal cycling reliability study is described. The reported time-to-failure data were then compared based on four different failure criteria: a control-chart based method,a 20% resistance increase from IPC-9701A,a resistance
threshold of 500O,and an infinite resistance. Over 3.5 GB resistance data measured by data loggers from a low-silver solder joint reliability study were analyzed. The results show that estimated time-to-failure based on the control-chart method is very similar to that when the IPC-9701A failure criterion is used. Both methods detected failure much earlier than the failure criterion of a resistance threshold of 500O or an infinite resistance. A scientific explanation is made of why the 20% increase
in IPC-9701A is a reasonable failure criterion and why the IPC-9701A and the control-chart based method produced similar results. Three different stages in resistance change were identified: stable,crack,and open. It is recommended that the control-chart based method be used as failure criterion because it not only monitors the average of resistance,but also
monitors the dispersion of resistance in each thermal cycle over time.

Remote sensing products designed for ocean environments sustain the harshness of cold oceans. The reliability of these telemetry devices needs to be very high to measure,collect and transmit data over a long period of time.
One of the biggest challenges for ocean technology products is moisture. Moisture poses a significant threat to the reliability of microelectronic assemblies,especially for scientific research products that are designed for marine environments and can be attributed as being one of the principal causes of many early-life failures. The presence of moisture in plastic packaging alters thermal stress through alteration of thermo-mechanical properties like,change of elastic modulus,shear strength and glass transition temperatures. Moisture also induces hygroscopic stress through differential swelling,reduces interfacial adhesion strength,induces corrosion and acts as an unwanted resistance when present between the two nodes of component and result in lowering the resistance which results in faster depletion of budgeted power.
In this study,failure modes in preliminary tests were analyzed through Weibull analysis. Design fault tree analysis made it easy to isolate the root cause of the early life failures,moisture intrusion. An analytical model was developed and validated both by experiments and simulation to determine the ingress rate of the moisture through the bi-material interface. After calculating diffusion coefficients of the two polyurethane materials,moisture ingress rate was calculated using an analytical model and also simulated through finite element analysis. Once the diffusivity coefficient is known,the theoretical Fickian curve is plotted with the experimental data to see if the absorption is Fickian or not. The 99% saturation approach helps to define the limit of Fickian diffusion hence eliminate error caused by non-fickian absorption. Since the diffusion coefficient is constant for a particular material,for bi-material analysis,interfacial concentration discontinuity cannot be analyzed as an interfacial discontinuity result where two materials having different saturated concentrations are joined. The results of ingress rate through FEA simulation came close to the calculated values hence validating the model.
Based on results and understanding of ingress rates through different materials and considering deployment designed life of product,proper selection of materials is made possible thus increasing the reliability of the product which is evident in plotted comparison survival graphs.

Solder joint reliability of SMT components connected to printed circuit boards is well documented. However,much of the
testing and data is related to high-strain energy thermal cycling experiments relevant to product qualification testing (i.e.,
-55C to 125C). Relatively little information is available on low-strain,high-cycle fatigue behavior of solder joints,even
though this is increasingly common in a number of applications due to energy savings sleep mode,high variation in
bandwidth usage and computational requirements,and normal operational profiles in a number of power supply applications.
In this paper,2512 chip resistors were subjected to a high (>50,000) number of short duration (<10 min) power cycles.
Environmental conditions and relevant material properties were documented and the information was inputted into a number
of published solder joint fatigue models. The requirements of each model,its approach (crack growth or damage
accumulation) and its relevance to high cycle fatigue are discussed. Predicted cycles to failure are compared to test results as
well as warranty information from fielded product. Failure modes were confirmed through cross-sectioning. Results were
used to evaluate if failures during accelerated reliability testing indicate a high risk of failures to units in the field. Potential
design changes are evaluated to quantify the change in expected life of the solder joint.

Reliability of Microvia has been a concern since microvias were introduced to our industry. This study was designed to understand the reliability of Type 1,Type 2,and Type 3 Microvias. Reliability Test Coupon design was developed in co-operation with PWB Interconnect to include up to four stacks of microvias placed on and off a buried via. Standard FR4 material,meeting the requirement of IPC-4101/24,was selected and IST thermal cycling was chosen as a reliability test method. Staggered microvias were not considered because previous testing has shown that staggered microvias are as reliable as single stage microvias. It was also decided to have all the microvias plated shut during the copper plating process. Samples were produced as one lot,utilizing FTG’s standard manufacturing processes. Efforts were made to include all possible test conditions required to understand microvia reliability.
Introduction:
The Printed Circuit Board industry has seen a steady reduction in pitch from 1.0mm to 0.4mm; a segment of the industry is even using or considering a 0.25mm pitch. This has increased the use of stacked microvias in these designs. The process of stacking microvias has been practiced for several years in handheld devices; however,the devices generally do not operate in harsh conditions. Type 1 and Type 2 microvias have been tested over the years and have been found to be very reliable. We do not have enough test data for 3 and 4 stack microvias when placed on and off buried via. The main objective of this study was to understand the reliability of 3 and 4 stack microvias placed on and off a buried via.

The advancement of area-array packages,such as flip chips and chip scale packages,has driven the adoption of high density interconnects (HDIs) that allow for an increased number of I/Os with a smaller footprint area. HDI substrates and printed circuit boards use microvias as interconnects between conductor layers. HDIs have evolved from single-level microvias to stacked microvias that traverse sequential layers. A stacked microvia is filled with electroplated copper to make electrical interconnections and support the outer level(s) of the microvia or components mounted to the upper capture pad. A common problem in copper-filled microvia fabrication is that the copper plating process can result in incomplete filling,dimples,or voids. However,the effects of these copper filling defects on the reliability of microvias are unknown. This study is the first known investigation and analysis of the influence of voiding and incomplete copper filling defects on the thermomechanical stresses in microvias.
Single-level and stacked microvias were modeled using the finite element method to simulate fully filled and partially filled microvias,as well as filled microvias with voids of different sizes. The stress states of these microvia models under thermal shock loads were investigated to determine the effects of the filling defects on the reliability of microvias.
The finite element modeling and simulation results demonstrated that stacked microvias experienced greater stresses than single-level microvias. With the same microvia geometry and material properties,copper filling reduced the stress level on the microvia structure,where fully copper-filled microvias had a lower stress level than partially filled microvias. The presence of voids generally increased the stress level in the microvia structure,but with a very small void size,the maximum stress in the microvia can be less than in a non-voided microvia. The stress level and the location of the maximum stress varied with changes in the void size.

This paper documents test data on the effects of materials and processes on microvia structures. Thirteen sets of experiments were carried out to evaluate the effects of dielectric material,aspect ratio,via morphology,surface preparation,temperature and copper plating type and thickness on microvia reliability. Reliability was assessed by subjecting boards and coupons to thermomechanical stress using four test methods: hot oil immersion,thermal shock,oven reflow simulation and Interconnect Stress Test (IST).

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The choice and optimisation of underfills to maximise process productivity and end product performance has been widely studied,and generated numerous performance criteria,flow time cure schedule,CTE,and Tg to name but a few. The same can not be said of the rework process. By separating the rework process into a series of independent steps and setting performance criteria on each step it is possible to generate a rating system to compare both the performance of the underfill and the impact of changing the process setup. The weighting of the various steps in the final calculation can be adjusted to suit an individual application’s requirements to help optimise both the product and process for rework.

Smart phones are complex,costly devices and therefore need to be reworked correctly the first time.
In order to meet the ever-growing demand for performance,the complexity of mobile devices has increased immensely,with more than a 70% greater number of packages now found inside of them than just a few years ago. For instance,1080P HD camera and video capabilities are now available on most high end smart phones or tablet computers,making their production more elaborate and expensive.
The printed circuit boards for these devices are no longer considered disposable goods,and their bill of materials start from $150.00,with higher end smart phones going up to $238.00,and tablets well over $300.00.
The implementation of the surface mount devices have become key components for mobile products by offering increased component density and improved performance. For example,the newer style DDR memory integrated components use less power and work at twice the speed of former versions. It is not surprising that most component manufacturers now produce these surface mount devices as small as 1mm square.
Mobile products generally use an epoxy underfill to adhere components to the printed circuit board in order to meet the mechanical strength requirements of a drop test. Reworking glued components is the most difficult application in the electronics industry,and needs to be addressed as a process.