Wettability test method for surface mount technology assessment

This study is part three in a series of papers on voiding in solder joints and methods for mitigation of voids. In this study several new variables were tested and compared to previous data on voiding. A new circuit board design was used which is different than the circuit board used in previous studies. The new circuit board design includes two sizes of QFNs (Quad Flat No Lead), BGA's (Ball Grid Array), and LGA (Land Grid Array) components which are susceptible to voiding. The following variables were evaluated with respect to voiding: Solder powder size was varied using a no-clean lead-free solder paste and IPC Type 3, Type 4 and Type 5 SAC305 solder powders. Solder alloy was studied using SAC305 alloy, SN100C® alloy, SN100CV® alloy and a mixture of SAC305/SN100C alloys. Surface finish on the circuit board is thought to have an impact on voiding. Organic solderability preservative (OSP) surface finish is more difficult to wet with solder than other surface finishes and in theory should produce higher voiding levels, and this was evaluated with multiple solder pastes. The performance of a new “low voiding” no clean solder paste was compared to other industry standard no clean solder pastes. Several new stencil designs on QFN thermal pads were tested. Data was gathered and statistical analysis performed to compare and contrast the voiding data. What was learned from this work is summarized below: Solder powder particle size affects voiding for some solder pastes. Voiding tends to decrease as solder powder size becomes smaller. Solder alloy affects voiding and seems to follow a trend with melting range. Wider melting ranges tend to generate lower voiding levels. Organic solderability preservative (OSP) surface finish gave higher voiding than electroless nickel immersion gold (ENIG) surface finish for a no clean lead free solder paste that was tested. Two different water soluble lead free solder pastes showed similar voiding levels for OSP and ENIG finishes. New “low voiding” solder pastes are becoming more common in our industry. In this work, low voiding solder paste F showed lower voiding than an industry standard solder paste C. Stencil design has an impact on voiding. In this study, area of printed solder paste coverage was varied. Voiding levels were much higher for 50 to 60% solder paste coverage than for 70 to 80% solder paste coverage. Based on the results of this work, the following recommendations were made to reduce voiding. Use of smaller solder powder sizes can reduce voiding with certain types of solder paste. Use of specific solder alloys may help mitigate voiding. Use a solder paste that works well with the surface finish to minimize voiding. Use “low voiding” solder pastes to reduce the overall potential for voiding. Optimize the stencil design for the components to allow for complete wetting and gas escape routes. Applying these recommendations may create issues on the printed circuit board assembly (PCBA). For example, use of smaller solder powder particle sizes may lead to increased random solder balling and/or graping. Increasing the volume of printed solder paste may lead to bridging for micro BGA and ultra-fine pitch components. Reduction of voiding may be important enough to risk these potential issues. This is left to the reader's judgement. It is the intent of this work to analyze general trends in voiding and give recommendations to help the reader to “Fill the Void.

The move to replace lead in electronic assemblies is rapidly gaining momentum. The driving factors are potential legislation, primarily in Europe, and global market pressures for more environmentally friendly products. This work examines the compatibility of lead free solders and printed wiring board (PWB) finishes. The solder alloys evaluated were Sn-3.5Ag-1.5In, Sn-3.4Ag-4.8Bi, Sn-3.8Ag-0.7Cu, Sn-3.5Ag and Sn-0.7Cu. The PWB surface finishes were Sn, Ag, Pd, Ni/Au and OSP. Eutectic Sn-37Pb solder was used for baseline comparisons. Surface finishes as received and after one reflow cycle in nitrogen with a peak temperature of 260/spl deg/C were studied. Double-sided reflow is common and the ability of the finish to survive at least one reflow cycle prior to assembly is critical. The surface finishes (as-received and after one reflow cycle) were characterized by Rutherford backscattering (RBS), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Wetting balance measurements were used to quantify solder wetting of each finish and alloy. Both a no-clean and a water soluble flux were used in the wetting balance tests.

Tin-Bismuth (SnBi) solder alloys are being explored in the consumer electronics industry due to the benefit of low reflow temperatures when compared to Tin-Silver-Copper (SAC solders). These benefits include cost savings due to lower reflow temperatures in reflow ovens, reduction of temperature-driven component warpage, and elimination of the high-temperature material costs for components and printed circuit boards (PCB) to survive a SAC reflow process. An additional benefit of the SnBi’s low reflow temperature is the potential for eliminating the selective or wave soldering process and associated operating costs and factory space by soldering through-hole components with a Pin-in-Paste (PiP) process. The cost of a wave soldering tool can exceed $300,000 and switching to the PiP process can help eliminate this expense along with savings associated with tool maintenance, raw materials, and headcount. PiP uses a Surface Mount Technology (SMT) process to print solder paste at and around the Through Hole Mount (THM) location. Leads of the THM part can then be placed or inserted in the paste deposit either by an automated machine or manually. Paste forms a solder joint with the leads and via when the boards are sent through the reflow oven. Thus, this process can be combined with routine SMT parts and improve the throughput. Many papers have successfully demonstrated SAC PiP through hole soldering. However, the SAC PiP process has not gained traction due to the increased material costs for through-hole components that can survive SAC reflow temperatures which can be between 230ºC-250ºC. LTS solder paste reduces reflow temperatures to below 190ºC which makes it feasible for components with a lower temperature rating to be soldered during SMT reflow via Pin in Paste process. A drawback of the PiP process is that it can be difficult to achieve sufficient and uniform barrel fill as it involves the use of a stencil to print the necessary paste volume on the board. Stencil printing has an inherent limitation on the amount of paste volume it can deposit due to restrictions on the X, Y, and Z dimensions of the stencil aperture. This limitation is not observed at the wave and selective as they provide a continuous flow of solder paste to the PTH component to meet the higher paste volumes needed to fill the PTH barrel. This paper addresses various methods to overcome the limitations of the PiP process by defining the formula needed to calculate necessary solder paste volume. It recommends a ratio for the barrel to lead diameter, a comparison between the effect of board thickness on barrel fill percentage, and a proposal to use 100% lead coverage criteria for boards thicker than 62 mils. A lead pull test was performed on the 2x4 headers built with 100% lead coverage solder joint quality (SJQ) criteria using SnBi PiP process to study its solder joint strength and co-relate solder joint quality between the PiP and the Wave process. The test results were inconclusive in comparing SJQ between PiP and Wave as the pull test fixture failed to hold on to the header after 1kg of pulling force due to slippage.

The solder paste has become one of the most indispensable brazing and soldering materials in surface mount technology (SMT), and its performance affects the quality of welding directly. The independently-designed lead-free solder paste in the lab has the fine performance when synthesized initially. However, the performance of solder paste degraded during the storage. So the aim of the article is to analyze solder paste based on performance degradation. In the paper, the independently-designed Sn-0.3Ag-0.7Cu solder paste in the lab was compared with commercial Senju solder paste based on the observation of wettability and viscosity during being stored at room temperature. Subsequently, the solder paste was divided into solder power and flux after 1, 3 and 5 months' storage. The solder power was analyzed by scanning electron microscopy. The results show that during the solder paste was sealed at room temperature, the wettability of solder paste became worse. And the viscosity was tend to increase, which lead solder paste to become dry, sticky, and even fail in welding. In addition, solder powder would be corroded and oxidized during the storage. So the activation component of the solder paste reacts chemically with the solder powder, which results in the performance degradation of solder paste.

The primary objective of the research presented in this paper is to qualify the reliability of mixed assemblies by comparing them to the conventional Sn-Pb assembly and completely Pb-free assembly. The research investigates both forward and backward compatibility in electronic assemblies using a design of experiments (DOE) approach. The investigation utilized a test vehicle containing an area array component (BGA169) and chip components (0603 resistors). Hot air solder leveling (HASL) and organic solderability preservative (OSP) surface finishes were used on the test vehicles to represent Sn-Pb and Pb-free alternatives, respectively. The assembled test vehicles were cut into two panels - one containing a resistor section for isothermal aging and the other containing a BGA and another resistor section for thermal shock. The assemblies were subjected to isothermal aging and thermal shock tests as per Interconnecting and Packaging Electronic Circuits/Joint Electronic Device Engineering Council (IPC/JEDEC) standards. The resistors were sheared in the as-soldered condition, and at various isothermal aging intervals and thermal shock cycles. In order to simulate an intermetallic failure in isothermal aging, a reduced shear height (20 mum) was used for the shear test. The performance of the resistor solder joints were quantified in terms of the shear force. The performance of the ball grid array (BGA) solder joint during thermal shock testing was quantified in terms of the number of cycles to failure. Experimental results from shear analysis of resistor solder joints show that Pb-free alloy assemblies' performance is superior to those assembled using Sn-Pb alloy. Also, the ductile nature of the Sn-Ag-Cu (SAC) alloy provides the joints a better fatigue life. With the area array components, Pb-free assembly joints outlived the traditional Sn-Pb joints when subjected to thermal shock loading. Among the mixed assemblies, backward-compatible assemblies (Pb-free bumps and Sn-Pb solder alloy) showed superior performance. The microstructural analysis of the solder joints indicate a uniform distribution of lead in the solder joint matrix.

Organic soderability preservative (OSP) is the cheapest metal surface finish to apply for corrosion inhibition of bare copper. In conventional flip chip process, the various OSPs could be dissolved by rosin pre-fluxes before the chip placement to enable solder bonding. The thermal compression flip chip bonding with non-conductive paste (TCNCP) process is one type of no-flow underfill process. In a TCNCP process, the OSP layer must dissolve in self-fluxing agent in NCP, but the process does not have adequate time for allowing complete dissolution to occur. In order to assist in OSP removal, this work attempted to Ar/O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> microwave plasma to remove the OSP layer on Cu trace of a substrate. The SEM-FIB investigation revealed that almost all of OSP layer could be removed with plasma, while only few OSPs remained in the relatively deep concavity on the surface of Cu trace. The analysis results by SEM-EDX showed that the carbon content of the treated plasma residue decreased from 44.69% to 11.5%. The Raman spectrum also showed that the bands of vibration mode in benzimidaozle almost disappeared, indicating the successful removal of the OSP layer by Ar/O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> microwave plasma. The cross-sectional SEM image of the bump on trace produced with the TCNCP process indicated that thin intermetallic compound (IMC) layer formed at the solder/Cu trace interface when incorporated with Ar/O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> microwave plasma pre-treatment. The SEM-DEX composition analysis delineated that the IMC layer consists of Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn and Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> . These results indicate the successful removal of OSP.

In this paper, the performance of a complete halogen-free (HF) board assembled with five ball grid array components using daisy chains on an 8-layer high density interconnection printed circuit board (PCB) was examined using bending tests. The solder joint was composed of the SAC405 solder ball melted with the SAC105 solder paste on the Cu pad surface of either organic solderable preservative (OSP) or electroless nickel immersion gold (ENIG), and was tested to examine the extent of intermetallic compound (IMC) formed. A board-level cyclic bending test was executed to the as-reflowed assemblies according to the JESD22-B113 standard to evaluate the reliability of various surface finished HF PCB assemblies under a recommended load condition. By Weibull analyzes the characteristic lives of the assemblies were 99098 and 130290 cycles on OSP soldering system and ENIG system respectively, indicating that the ENIG had a higher bending fatigue endurance as compared to the OSP. The dominant failure mode was the fracture of Cu trace within the micro-via caused by the crack of the resin coated copper layer. At the interface of the solder joint, the IMC formed on both OSP and ENIG was with various Ni contents of 3.9 and 20.1 at%, and different thicknesses of 5.6 and 1.1 , respectively. Numerical simulations showed that both composition and thickness of IMC influenced the stress concentrations and characteristic life of the HF test vehicle as they could slightly increase localized solder joint rigidity and result in more serious pull and drag behaviors at the corner joint. Although the on the ENIG was stiffer than that on the OSP, the thicker IMC layer instead increased the stress on the OSP system due to size effect, and resulted in a lower characteristic life.

In the surface mount technology, Ball Grid Array (BGA) has been used in a production of PC boards, because of their excellent characters such as high density of the lead pin pitch, better lead rigidity and self-alignment during re-flow processing. This paper deals with the development of image analysis for the detection of defects at BGA solder joints in PC boards by using X-ray images. In the conventional IC boards, it is possible to detect defects of solder joints by visual inspection, because the lead of IC package is set on its outside. However, we can't detect visually defects at BGA solder joints, because they are hidden under the IC package. In a production line, the inspection of BGA in PC boards depends on the function test of electric circuits in the final process. To improve a cost performance and the reliability of PC boards, an inspection of BGA is required in the surface mount process. Types of defects at BGA solder joints are solder bridge, missing connection, solder voids, open connection and miss-registration of parts. As we can find mostly solder bridge in these defects, we pick up this to detect solder bridge in a production line. The problems of image analysis for the detection of defects at BGA solder joints are the detection accuracy and image processing time according to a line speed of production. To get design data for the development of the inspection system, which can be used easily in the surface mount process, it is important to develop image analysis techniques based on X-ray image data. At the first step of our study, we attempt to detect the characteristic of the solder bridges based on an image analysis.

The mechanical and fatigue properties of microelectronic interconnection materials are important issues in critical applications in the life of electronic assemblies. Due to the growth in the applications of electronic components in new technological products used in tough conditions, evaluating the reliability of solder alloys has become crucial to the prediction of product life. SAC (Sn-Ag-Cu) solder alloys are common lead-free alloys used to form solder joints. In the current study, the reliability of individual SAC305 solder joints in actual setting conditions with an organic solderability preservative (OSP) surface finish was examined using an accelerated shear fatigue test at different load levels (16 MPa, 20 MPa, and 24 MPa). Four operating temperature levels (−10, 25, 60, and 100 °C) were also used. Seven samples were utilized at each experimental condition. An orthogonal array (L12) was employed for this experiment. The fatigue behavior of the SAC305 solder joints in actual setting conditions was described by a two-parameter Weibull distribution. The characteristic life and the shape parameter were extrapolated from the Weibull distribution for each experimental condition. Characteristic life was employed to represent the fatigue resistance of the solder joints. For each sample, the inelastic work per cycle and plastic deformation were determined. The results indicated a notable increase in the inelastic work per cycle and the plastic strain as a result of increasing either the stress load or the working temperature. Combinations of stress amplitude, working temperature, inelastic work, and plastic deformation were applied as inputs to the fuzzy system for computing a comprehensive output measure (COM-Value) using the Mamdani method. A negative relationship was observed between the solder life and the four fuzzy inputs. To fuzzify the inputs of the fuzzy system, two membership functions, “MFs”, were formed for each input, and five MFs were set for the output. The center-of-gravity (COG) theorem was utilized as a defuzzification method in the fuzzy inference system. The characteristic life of the solder joints was predicted by the COM-Value, which was used as an independent variable. Finally, The COM-Value was used to generate a general predicted reliability model for SAC305 solder joints with an acceptable model adequacy index.

A quantitative dynamic solder wettability mesurement technique was utilized to evaluate the effects of reflow processing on the wettability parameters associated with solder ball alloys. This technique enables the examination of the final degree of solder wetting and the continuous monitoring of wetting as a function of time during the reflow process under nitrogen atmosphere. An experimental design approach employing a 24 full factorial experiment was formulated to illustrate the use of this measurement technique investigating the final result of wetting. Solder wettability was determined with respect to the contact angle, base diameter and height of the reflowed solder ball alloy. The most significant effect estimates with respect to contact angle were solder flux and pad metallization. Solder ball alloy was found to significantly impact the base diameter and height of the reflowed solder. The effect of solder flux activators and pad metallizations on the subsequent continuous solder wettability wetting rates and amount of molten solder spread of solder ball alloys during reflow were measured. Reflow with a relatively more activated solder flux material was found to enhance the rate of solder wetting of the the pad metallization.

A quantitative dynamic solder wettability measurement technique was utilized to evaluate the effects of reflow processing on the wettability parameters associated with solder ball alloys. This technique enables the examination of the final degree of solder wetting and the continuous monitoring of wetting as a function of time during the reflow process under nitrogen atmosphere. An experimental design approach employing a 24 full factorial experiment was formulated to illustrate the use of this measurement technique investigating the final result of wetting. Solder wettability was determined with respect to the contact angle, base diameter and height of the reflowed solder ball alloy. The most significant effect estimates with respect to contact angle were solder flux and pad metallization. Solder ball alloy was found to significantly impact the base diameter and height of the reflowed solder. The effect of solder flux activators and pad metallizations on the subsequent continuous solder wettability wetting rates and amount of molten solder spread of solder ball alloys during reflow were measured. Reflow with a relatively more activated solder flux material was found to enhance the rate of solder wetting of the pad metallization.

Effect of temperature on the low cycle fatigue properties of BGA solder joints

Based on application, it was found that the rheological properties of the solder paste play an important role in the print performance. Temperature-dependent rheological behaviour of the lead-free solder paste and flux medium used for flip-chip assembly applications were studied. Two commercially available lead-free solder pastes P1 (LF318) and P2 (LF328) prepared from fluxes F1 and F2 were used. The temperatures under investigation were 15˚C, 25˚C, and 35˚C. All the results recorded were put into the Gauge calculator. The snap-off height variable and the condition of the printing equipment were used to determine the print height. Slump test was used to determine the slumping behaviour at room temperature and during reflow. Results show that the properties of solder paste are immensely affected by paste heights, high temperature variations, time and humidity. Keywords: Surface Mount Technology (SMT), Printed Circuit Board (PCB) Assembly, Slump Test, Reflow Soldering, Solder Paste. DOI : 10.7176/ISDE/11-1-05 Publication date: February 29 th 2020

Organic solderable preservative (OSP) has been adopted as the Cu substrate surface finish in flip-chip solder joints for many years. In this study, the electromigration behavior of lead-free Sn-Cu solder alloys with thin-film under bump metallization and OSP surface finish was investigated. The results showed that severe damage occurred on the substrate side (cathode side), whereas the damage on the chip side (cathode side) was not severe. The damage on the substrate side included void formation, copper dissolution, and formation of intermetallic compounds (IMCs). The OSP Cu interface on the substrate side became the weakest point in the solder joint even when thin-film metallization was used on the chip side. Three-dimensional simulations were employed to investigate the current density distribution in the area between the OSP Cu surface finish and the solder. The results indicated that the current density was higher along the periphery of the bonding area between the solder and the Cu pad, consistent with the area of IMC and void formation in our experimental results.

WL-CSP reliability with various solder alloys and die thicknesses