Tamás Garami

Investigating the mechanical strength of Vapor Phase soldered chip components joints

In our experiment the mechanical strength of solder joints formed by Vapor Phase soldering was investigated. A testboard was designed, which allowed fifty pieces of 0603 size chip resistors to be placed. The soldering was carried out by infrared conventional reflow and by Vapor Phase reflow as well. After soldering the shear strength of joints formed by both of the soldering methods was measured, and the cross-section of solder joints was analyzed by Scanning Electron Microscopy. Based on the results, it can be said that the shear strength of joints formed by Vapor Phase soldering is slightly lower than the strength of joints formed by IR reflow. The thermal profile of Vapor Phase soldering needs further optimization. The detailed results are presented in the paper.

Comparing the intermetallic layer formation of Infrared and Vapour Phase soldering

In our experiment the intermetallic layer (IML) formation during Infrared (IR) and Vapour Phase (VP) soldering was investigated. A testboard was designed, onto which fifty pieces of 0603 (1.5 × 0.75 mm) size chip resistors were mounted. For the soldering, profiles with different Qη factors (750, 1000, 1250 s°C) were set for both the IR and VP soldering. After soldering, the shear strength and the intermetallic layer (IML) thickness were measured of joints formed by both of the soldering methods. The shear strength was measured according to industrial standards; the shearing speed was 100 µm/s. Afterwards, solder joints were cross-sectioned for the IML thickness measurements. The cross-section of the solder joints was inspected by Scanning Electron Microscopy and the IML thickness was measured by analyzing the SEM images. The image analysis method is based on image binarization and the code is developed in Matlab. The threshold level for the image binarization is determined by the mean value of a Gauss curve fitted onto the histogram of the SEM image. The image analysis method and the detailed results are presented in the paper.

Soldering tests with biodegradable printed circuit boards

The paper presents soldering tests on biodegradable printed circuit boards (PCB) focusing on different alternative base materials. The aim of the work was to investigate and optimize different low-temperature soldering profiles on cellulose-acetate (CA) and polylactide (PLA) biopolymer-based PCBs with the application of special lead-free solder alloy suited for the temperatures. The investigations were focused on the minimization of the heating effect on the heat-sensitive biopolymers, where on the other hand, sufficient thermal energy is required to melt the solder alloys. The soldering profiles were created with a custom vapour phase soldering (VPS) approach where the heat transfer is controlled with an adaptive height-setting, temperature tracking sample holder. The profiles were evaluated according to their heating factor values. The soldered joints were evaluated with X-Ray microscopy and Shear Tests. The deformation of the PCB tracks was also investigated. The preliminary results reveal the priority of CA for further experiments.

Method for validating CT length measurement of cracks inside solder joints

Purpose – This paper aims to develop a method to measure the length of cracks inside solder joints, which enables the validation of computed tomography (CT) crack length measurements. Design/methodology/approach – Cracks were formed inside solder joints intentionally by aging solder joints of 0603 size resistors with thermal shock (TS) test (−40 to +140°C, 2,000 cycles), and CT images were captured about them with different rotational increment (1/4, 1/2 and 1°) of sample projection. The length of cracks was also measured with our method, which is based on capturing high-resolution radiography X-ray images about the cracks in two perpendicular projection planes. The radiography results were compared to the CT measurements. The percentage error for the different CT rotational increment settings was calculated, and the optimal CT settings have been determined. Findings – The results have proven that reducing the rotational increment increases the sharpness of the captured images and the accuracy of crack le...

Characterizing the conductivity of ICA joints by the mean intercept length of Ag flakes

Purpose – The purpose of this paper is to present a novel and alternative method for the characterization of isotropic conductive adhesive (ICA) joints’ conductivity by the calculation of the mean intercept length of conductive flakes in the cured joint. ICAs are widely used in the field of hybrid electronics or special printed circuit board applications, such as temperature sensitive or flexible circuits. The main quality parameters of the ICA joints are the conductivity and the mechanical strength. Design/methodology/approach – For the experiments, one-component Ag-filled thermoset ICA paste was used on FR4 printed circuit test board to join zero-ohm resistors. Six different curing temperatures were applied: 120, 150, 175, 210, 230 and 250°C. The conductivity of the joints was measured in situ during the curing process. Micrographs were taken from the cross-sectioned joints, and the mean intercept length was calculated on them after image processing steps. Findings – Results of the measured conductivity...

Controlling the cooling rate of soldering processes with PIC microcontroller

This paper focuses on the cooling rate of soldering process. Our target was to create a well-controlled equipment that is used to set the adjustable slope of the cooling curve with three special sections. The three sections are: near the melting point, above and below the melting point. The unit uses cooling fans to create a forced convection air flow. K-type thermocouple is used for the temperature measurement in the temperature feedback loop. The cooling rate is adjustable in a wide range, between 0.25 K/s and 10 K/s. To understand the effect of different cooling rates, the undercooling mechanism near the solidus point of the solder has to be investigated. Experiments showed that there is a flat section in the temperature profile when the solder reaches the solidus point during cooling. With the control unit this phenomenon and the effect of various cooling rates can also be investigated.

The effect of cooling rate on the thermo-mechanical properties of micro-alloyed solders

In this paper, the effect of reflow cooling rate on the thermomechanical properties of a traditional SAC (Sn96.5/Ag3/Cu0.5) and of a micro-alloyed SAC (mSAC) solder (Sn98.9/Ag0.3/Cu0.7/Bi0.1/Sb0.01) was investigated. An FR4 based testboard was designed for the investigation; and preformed solder balls (0 600 μm) were soldered onto it with different cooling rates (1, 1.8, 2.8 K/s). After the soldering, the samples were subjected to Thermal Shock (TS) test (-40 to 140 °C, 30 min. cycle time) up to 2500 cycles. The thermomechanical properties of the samples were investigated by measuring the shear strength of the solder bumps, and by analysing the intermetallic layer growth on aged samples as well as on as-reflowed samples. Based on the shear strength measurement, the characteristic life of the samples was calculated. It was found that the characteristic life of the micro-alloyed solder is only slightly longer at faster cooling rates. On the contrary, the characteristic life of the micro-alloyed solder is considerably longer than that of the traditional alloy at the slowest cooling rate. Furthermore, the growth of Cu6Sn5 intermetallic layer was faster in the case of the traditional alloy because of the larger amount of grain boundaries; however, the growth of Cu3Sn was larger at the micro-alloyed solder because of the higher number of valleys between the grains of the Cu6Sn5 layer.

Effect of solder joint volume on its shear fracture mode

The crack behavior of the chip-scale 0603 (1.5 × 0.75 mm) size resistor was investigated during shear test, while the amount of the solder alloy was decreased. The resistors were soldered with the same thermal profile using vapor phase reflow soldering technique. During the sample preparation the aperture size of the stencils were decreased while the thickness of the stencil foils was the same in all cases. The minimum applied surface ratio was 0.66. After shear tests, the surface of the cracked area was investigated with optical and scanning electron microscopy (SEM). With energy dispersive spectroscopy (EDS), the composition of the investigated layers was detected and it helped to recognize the layer-structure.

Investigation of the pre-heating process during thermosonic wire bonding by FEM simulation

In this paper, the pre-heating process of the thermosonic wire bonding process was investigated. In the case of this method, the energy for the bonding process comes from three different sources: ultrasonic vibration, pressure (force) and elevated temperature. The pre-heating system contains a heating table and the sample is positioned on the top of it. In order to determine the exact temperature distribution, a numerical model was prepared, which takes into account the specific geometry of the arrangement and the heat transfer methods between the various surfaces. The heat transfer between the ceramic substrate and the heating plate depends on the surface roughness of the materials, and on the pressure which is exerted by the substrate to the heating table. In our investigation, we implemented a physical test setup with K-type thermocouples which can validate stationary numerical models. The temperature difference between the set temperature of the heating table and the measured temperature was 4-5 °C, while the difference between the set and modeled bond pads temperatures was only 2-3 °C, which information will help to set the temperature of the heating tables more accurate.

Quantitative analysis of Ag 3 Sn particle size in Sn3.5Ag solder alloy

Evaluation of the Ag3Sn intermetallic compounds in the solder joints is a difficult issue; so a new image processing technique is developed to give quantitative, numerical results for the grain structure. During the cooling phase of the soldering method, different grain structures could be generated with different cooling rates. It is worth quantifying the difference. The developed image processing algorithm requires simple cross section images for input. During the preparation of the samples, they should be etched with OPS (Oxide Polishing Suspensions). After increasing the contrast, binarizing and filtering the image, the mathematical calculations regarding the grain structure could be performed. By using our algorithm, we found that the magnitude of Ag3Sn grain size follows exponential distribution, so it can be characterized with the amplitude and the exponent of the fitted exponential curve. At last, these parameters were compared to commonly used quantitative parameters, like the mean intercept length or average area of Ag3Sn particles.