Fill the Void III

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.

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 this research, the microstructure evolution of Sn-Cu based solder paste on the electroless nickel immersion gold substrate was investigated. The aims are to study the microstructure formation of as-reflowed Sn-10 wt.% Cu paste on the substrate. Besides, the evolution of the interfacial intermetallic compound layer and bulk solder microstructure of the solder joint that subjected to multiple reflowed were also investigated in this paper. The Sn-0.7 wt. % Cu solder paste was mixed with copper powder to produce the Sn-10 wt. % Cu solder paste. The mixed solder paste then reflowed on the electroless nickel immersion gold surface finish substrate to form a solder joint in a reflow oven. Then, the as-reflowed solder joint was undergo multiple reflowed processes until the sixth time. It was noted that multiple reflowed process influenced the microstructure evolution of Sn-0.7 wt. % Cu and Sn-10 wt. % Cu solder samples on electroless nickel immersion gold substrate. The Cu6Sn5 IMC in the bulk solder microstructure for both solder samples become coarsen after 6th times reflowed cycles. Meanwhile, the primary Cu3Sn phase in Sn-10 wt. % Cu solder paste was found to dissolve. The morphology of the needle-shaped interfacial intermetallic compound layer was changed to layer-shape and the intermetallic compound thickness of both solder joints on electroless nickel immersion gold substrate was increased with the number of the reflow process cycles. The grain size and thickness of IMC formation for both solder joints are directly proportional to the multiple reflow cycles.

The article also investigates the min. solder volume request for solving Gold embrittlement issue in the limited condition. Gold (Au) embrittlement was known and had been studied for a long time. It will reduce the solder joint toughness not just only in reliability test but in manufacturing process. This kind of failure even occurs just after SMT process, it means we could not apply glue to protect the component from drop-off. The concentration of Gold is the major concern of this failure mode. How to dilute the concentration of Gold from substrate design, component manufacturing, soldering material selection and substrate manufacturing is final goal of forming a robust solder joint. The component drop-off was found in the latter process of SMT. The investigation started from failure mode definition. CSA (Cross Section Analysis) and EDS (Energy Dispersive Spectroscopy) were performed to observe the solder joint structure and alloy composition. Abnormal high concentration of Gold element reveals the Gold embrittlement is the major root cause of solder joint crack. Reducing Gold concentration started from the source of Gold surveying. Soldering related material investigation result shows no abnormality in substrate pad and solder paste but high risk in the soldering terminals of the component. The thicker Gold plating of the component is the major source of high Gold concentration in the solder joint. In general substrate manufacturing process such as desktop motherboard, it might not occur due to the sufficient solder volume provided by enough space and thicker stencil could cover the issue. In this case, the Gold plating thickness is unchangeable, so we only focus on substrate surface treatment and fine tune SMT process parameters to dilute the Gold concentration. The improvement DoEs were conducted for different substrate surface treatment, solder volume and reflow profile with different component batch. For substrate surface treatment, we changed it from ENIG (Electroless Nickel/Immersion Gold) to ENEPIG (Electroless Nickel/Electroless Palladium/Immersion Gold) for better solder joint strength. To increase solder volume, we enlarge the stencil aperture. For reflow profile fine tune, higher and lower peak with longer soaking time were verified for Gold diffusion. CSA and shear test were performed to confirm the solder joint and compare the joint strength. From the DoE results known, the positive factors solder joint strength enhancements are surface treatment and solder volume. ENEPIG solder joint interface is stronger than ENIG solder joint, but it can't dilute the Gold concentration directly. Solder volume increasing is most obvious factor to dilute the Gold concentration. Therefore, in SiP (System in Package) module production, tiny solder joint and high component density restrict the stencil design. The article also investigates the min. solder volume request for solving Gold embrittlement issue in the limited condition.

Fatigue performance and microstructure of lead-free solder joints in BGA assembly at room temperature

Solder paste is widely used to connect surface mount devices to printed circuit boards [1]. As the components become smaller and thinner, solderability and voiding are more challenging [2]. It has been reported that a majority of assembly defects occur in the reason of quality decay of solder paste, resulting in solder paste dried-out and short shelf life. The chemical reactions between flux and solder powders, which are believed in detecting metal ions concentration in flux by inductively coupled plasma-mass spectrometer (ICP), are very critical and relevant to the quality of solder paste. Hence, a new coating technology on solder powders is introduced to eliminate this issue. In the first part of this research, a new coating solution with polymer compositions inside is developed [3]. Thermal characteristics were analyzed by thermogravimetric analyzer (TGA) to find out the optima temperature for solution activating, forming a coating layer on solder powders. The coating efficiency was found related to the ratio of coating solution and solder powders, and carried out to observe the powders surface by Focused Ion Beam (FIB). The study on effects of coating technology on solderability was performed in the second part of the research. It is realized that viscosity and rheological properties of solder paste in the condition of long-time storage are more stable for the solder paste with coating technology. On the other hand, dried-out issue was observed in the case of normal solder paste without coating. Besides, solderability and voiding were also improved by coating technology. Therefore, based on the innovation of this study, coating technology can be applied in a variety of metal surface to offer a layer to prevent from interference.

Solder pastes are complex products which are designed to meet the conflicting requirements of printability, slump resistance, good tack and a range of reflow conditions. This paper describes how the metal content and solder particle size distributions in solder pastes affect these properties for a typical RMA type product. The major effects are explained by reference to the purely physical effects of alloy density, metal content and solder powder size on the mean particle separation within the paste. Lower alloy density, high metal content and smaller solder powder size all reduce inter‐particle separations causing viscosity to increase, slump resistance to improve and peak tack force to increase. The paper also discusses the more subtle effects of the chemical interactions between solder powder and flux medium on the same properties. These are illustrated by reference to the changes which take place in performance of a typical solder paste during storage over 12 months. Ageing by solvent loss and rosin drying brought about by the effects of metal soaps formed between solder and activators probably contribute to these changes. Ageing can result in improved performance in tests like slump resistance and consequently solder balling, while in others, such as open time, there is a decrease in performance.

The increasing demand for portable electronics continues to be a dominant factor for technological advancements in the development of fine pitch devices. Assembling these highly compact and high Input/Output (I/O) density devices efficiently on Printed Circuit Board (PCB) assemblies can be a challenging task. Extensive literature is available focusing on stencil printing challenges and reliability studies of fine pitch devices. However, for 0.40 mm pitch Ball Grid Array (BGA) components, the effect of key factors like pad design of the PCB (Solder Mask Defined – SMD and Non-Solder Mask Defined - NSMD), solder paste type, and stencil technology on the stencil printing process requires further research. Also, the solder paste print definition for fine pitch printing greatly depends on the capabilities of the stencil printer and on printing process parameters. In this paper, the effect of PCB pad design (SMD/NSMD), and solder paste type (Type 3/Type 4) have been studied and discussed. Additionally, the performances of three different stencil printers were studied to determine their effect on the solder paste volume and print definition. Automated Solder Paste Inspection (SPI) was used to measure the print quality in terms of print volume, print height as well as printing defects such as solder bridging. Based on the results, the volume of solder paste deposited was higher in the case of NSMD pads as compared to SMD pads. Both type 3 and type 4 solder pastes were found to be compatible for the assembly of a 0.40 mm pitch BGA. It was also observed that the printing process and print quality were more dependent on the stencil printer used in the case of the 0.40 mm pitch components as compared to coarser pitch components, such as 0.50 or 0.80 mm pitch devices.

A study was performed to develop a different experimental methodology to assess wettabilities of solders on various printed wiring board (PWB) finishes, based on a modified spreading test in which solder pastes were heated following temperature reflow profiles representative of those used in surface mount technology (SMT) instead of using a fixed rate temperature ramp. Three solder alloys (Sn63‐Pb37, Sn96.5‐Ag3.5, and CASTINTM: Sn96.2‐Ag2.5‐Cu0.8‐Sb0.5), two fluxes (rosin, mildly activated, RMA, and no‐clean, NC), and seven PWB finishes (Pd, Au/Ni, Ni, Ag, Sn, and organic solderability preservatives: OSP), and bare copper were involved in the study. Better wettabilities were observed in the current study than the results reported in the literature for conventional tests on the same combination of solder alloy, flux, and substrate. The different results in measurement of wettabilities obtained in the current study were attributed to the more adequate heating process allowing flux activation, which reduced reoxidation of solder powders and substrates during the reflow process and thus improved wettabilities of solders. Compared to the results obtained from the popular wetting balance test, the current study demonstrated a more realistic simulation of, and approach to, assessing the wettability of solder for SMT.

For SiP or SMT assembly process, solder paste has been the choice of primary soldering material. In the mean time, miniaturization has been the trend of electronic industry. While the size of components, pads, stencil thickness, aperture, pitch, and solder powder has been shrinking continuously so far, it is about time to ask whether the solder paste technology is able to support this trend forever. In this study, solder powder oxygen content was observed to increase linearly with increasing powder surface area up to Type 5. Beyond Type 5, it is getting difficult to maintain oxide thickness. Beyond Type 8, the oxide thickness always higher than Type 5 or coarser. The flux burn-off rate increased linearly with decreasing flux size plotted in log scale. However, for solder paste, the paste residue showed a sharp drop initially with decreasing flux quantity, then levelled off at 3 mg flux (~30 mg paste), and maintained at around 30% residue through the remaining range down to 0.3 mg flux (~3 mg paste). The constant residue level at small sample size was attributed to the surface adsorption phenomenon, or “Flux Shell” phenomenon, where a layer of flux was strongly adsorbed on the solder powder surface. For solder paste, the flux workload increased rapidly with decreasing powder size. The flux capacity needed in removing oxide was derived as volume fraction of COOH functional group in flux residue, with 10% maximum being set as a guideline for no-clean applications. Type 8 paste may be the limit of SiP or SMT printable no-clean solder paste due to corrosivity consideration. Oxygen barrier and alloy dopants may expand the miniaturization potential. When reviewing the brittleness of solder joint formed, Type 6 may be the limit of fine pitch applications. Beyond that, the joints may be too brittle to be reliable, unless underfill is applied.

As the trend toward further miniaturisation of pocket and handheld consumer electronic products continues apace, the requirements for even smaller solder joints will continue. With further reductions in the size of solder joints, the reliability of solder joints will become more and more critical to the long-term performance of electronic products. Solder joints play an important role in electronics packaging, serving both as electrical interconnections between the components and the board, and as mechanical support for components. With world-wide legislation for the removal/reduction of lead and other hazardous materials from electrical and electronic products, the electronics manufacturing industry has been faced with an urgent search for new lead-free solder alloy systems and other solder alternatives. In order to achieve high volume, low cost production, the stencil printing process and subsequent wafer bumping of solder paste has become indispensable. There is wide agreement in industry that the paste printing process accounts for the majority of assembly defects, and most defects originate from poor understanding of the effect of printing process parameters on printing performance. The printing of ICAs and lead-free solder pastes through the very small stencil apertures required for flip chip applications was expected to result in increased stencil clogging and incomplete transfer of paste to the printed circuit pads. Paste release from the stencil apertures is dependent on the interaction between the solder paste, surface pad and aperture wall; including its shape. At these very narrow aperture sizes the paste rheology becomes crucial for consistent paste withdrawal because for smaller paste volumes surface tension effects become dominant over viscous flow. Successful aperture filling and release will greatly depend on the rheology of the paste material. Wall-slip plays an important role in characterising the flow behaviour of solder paste materials. The wall- slip arises due to the various attractive and repulsive forces acting between the solder particles and the walls of the measuring geometry. These interactions could lead to the presence of a thin solvent layer adjacent to the wall, which gives rise to slippage. The wall slip effect can play an important role in ensuring successful paste release after the printing process. The aim of this study was to investigate the influence of the paste microstructure on slip formation for the paste materials (lead-free solder paste and isotropic conductive adhesives). The effect of surface roughness on the paste viscosity was investigated. It was also found that altering the surface roughness of the parallel plate measuring geometry did not significantly eliminate wall slip as was expected. But results indicate that the use of a relatively rough surface helps to increase paste adhesion to the plates, inducing structural breakdown of the paste. Most importantly, the study also demonstrated on how the wall slip formation in the paste material could be utilised for understanding of the paste microstructure and its flow behaviour

For SiP or SMT assembly process, solder paste has been the choice of primary soldering material. In the meantime, miniaturization has been the trend of electronic industry. While the size of components, pads, stencil thickness, aperture, pitch, and solder powder has been shrinking continuously so far, it is about time to ask whether the solder paste technology is able to support this trend forever. In this study, solder powder oxygen content was observed to increase linearly with increasing powder surface area up to Type 5. Beyond Type 5, it is getting difficult to maintain oxide thickness. Beyond Type 8, the oxide thickness always higher than Type 5 or coarser. The flux burn-off rate increased linearly with decreasing flux size plotted in log scale. However, for solder paste, the paste residue showed a sharp drop initially with decreasing flux quantity, then levelled off at 3 mg flux (~30 mg paste), and maintained at around 30% residue through the remaining range down to 0.3 mg flux (~3 mg paste). The constant residue level at small sample size was attributed to the surface adsorption phenomenon, or “Flux Shell” phenomenon, where a layer of flux was strongly adsorbed on the solder powder surface. For solder paste, the flux workload increased rapidly with decreasing powder size. The flux capacity needed in removing oxide was derived as volume fraction of COOH functional group in flux residue, with 10% maximum being set as a guideline for no-clean applications. Type 8 paste may be the limit of SiP or SMT printable no-clean solder paste due to corrosivity consideration. Oxygen barrier and alloy dopants may expand the miniaturization potential. When reviewing the brittleness of solder joint formed, Type 6 may be the limit of fine pitch applications. Beyond that, the joints may be too brittle to be reliable, unless underfill is applied.

PurposeSolder paste printing is the most common method for attaching surface mount devices to printed circuit boards (PCB), and it has been reported that a majority of all assembly defects occur during the stencil printing process. It is also recognized that the solder paste printing process is wholly responsible for the solder joint formation of leadless package technologies such as land grid array (LGA) and quad-flat no-lead (QFN) components and therefore is a determining factor in the long-term reliability of said devices. The aim of this experiment is to determine the acceptable lower limit for solder paste volume deposit tolerances during stencil printing process to ensure both good assembly yield and reliability expectations.Design/methodology/approachStencils with modified aperture dimensions at particular locations for LGA and QFN package footprints were designed to vary the solder paste volume deposited during the stencil printing process. Solder paste volumes were measured using solder paste inspection system. Low volume solder paste deposits were generated using the modified stencil designs to evaluate assemble yield. Accelerated thermal cycling (ATC) was used to determine the reliability of the solder joints. Failure analysis was used to determine if the failure was attributed to the low paste volume locations.FindingsSolder joints formed with nominal paste volume survived longer in ATC compared to intentionally low volume joints. Transfer efficiency numbers for both good assembly yield and good reliability are reported for LGA and QFN devices. A lower volume limit is reported for leadless devices that should not significantly affect yield and reliability in thermal cycling.Originality/valueVery little literature is available on solder paste volume tolerance limits in terms of assembly yield and reliability. Manufacturers often use ±50 or ±30 per cent of stencil aperture volume with no evidence of its effectiveness in determining yield and reliability of the solder joints.

The increasing industry awareness of lead-free activities has prompted OEMs and suppliers to investigate lead-free solder systems in detail. Reliability of these new systems is not as well understood as those manufactured using conventional eutectic tin-lead solder. In particular, limited data are available regarding thermal fatigue resistance of lead-free solder joints and the impact of the package type on solder joint reliability. In this study, accelerated thermal cycling (ATC) tests were performed. The test boards had organic solder preservative (OSP) surface finish and were populated with SMT leadfree components. The solder paste used in the SMT process was Sn-4Ag-0.5Cu in a water-soluble formulation. The components assembled included a PBGA and a CSP. Four lead-free solder ball metallurgies (Sn-Ag, Sn-Cu, Sn-Ag-Cu and Sn-Ag-Cu-Bi) have been evaluated on the PBGA. Weibull plots have been produced and failure analysis has been performed. The data suggest that the lead-free alloys have thermal fatigue resistance as good or better than eutectic tin-lead and that there is a significant package dependence on the reliability of lead-free solder joints.

Paper deals with results of solderability testing of Printed Circuit Boards (PCBs). For measurement was used one of the solderability evaluation methods known as the wetting balance test. The wetting balance test measures the wetting forces imposed by the molten solder on the test surface as it is dipped into and held in the solder bath as a function of time. Paper will present the comparison of results of printed circuit board solderability test with preheating in agreement with IPC/EIA J-STD-003A, with IEC 60068-2-54 and with hot air double-sided preheating. This comparison will be also made for several lead-free surface finish types. It was tested printed circuit board coupons with surface finishing of galvanic tin, immersion tin, ENIG (electroless nickel immersion gold), OSP (organic solderability preservative) and pure copper.