Rigid-flexible interconnection realized by laser soldering through polyimide

Reliability of laser soldering using low melting temperature eutectic Sn Bi solder and electroless Ni-electroless Pd-immersion Au-finished Cu pad

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.

To obtain the interfacial behaviors and the joint strength of Sn–Bi solid solution solder, Sn–2.5Bi and Sn–5Bi solders were selected to be investigated in this work with pure Sn/Cu solder joints as a reference. The results showed that the growth behaviors of intermetallic compound (IMC) at Sn–Bi solder joints were different under reflow soldering and isothermal aging, which was related with the presence of Bi atoms and the diffusion rate of Cu atoms. Furthermore, the Cu6Sn5 particles in solder matrix and the Bi precipitation could change the diffusion path of Cu atoms, which caused the change on the distribution and size of Cu6Sn5 in pure Sn, Sn–2.5Bi and Sn–5Bi solder matrix after isothermal aging. The size and distribution of Cu6Sn5 in solder matrix finally affected the shear strength of solder joints. The shear strength of Sn–2.5Bi/Cu solder joints increased compared with pure Sn/Cu solder joints, and there was no obvious change with the aging times for Sn–2.5Bi/Cu solder joints due to the effect of the reinforcement of Bi atoms. However, the shear strength of Sn–5Bi/Cu solder joints increased firstly and then decreased with the prolongation of aging times. This phenomenon was attributed to the segregation of Bi and the large amounts of Cu6Sn5 particles distributed in solder matrix during the aging condition. Besides, the IMC thickness in Sn–5Bi/Cu solder joint played the important role on the shear strength due to the brittle nature of IMC. In pure Sn solder matrix, the size of Cu6Sn5 particles were larger than that in Sn–Bi solder matrix, which deteriorated the shear strength on pure Sn solder. From the observation on the fracture morphologies, Sn/Cu and Sn–2.5Bi/Cu solder joints showed the ductile fracture occurred in the solder matrix, while Sn–5Bi/Cu solder joints exhibited a mixed fracture.

Laser soldering is becoming more popular for soldering pin through-hole (PTH) components onto printed circuit boards (PCBs). Solder volume influences soldering joint reliability. However, it is difficult to accurately measure the effect of solder volume during the laser soldering process. The finite volume method (FVM) is used to simulate the molten solder flow in laser soldering to connect the through-hole capacitor onto the PCB in ansysfluent software. The simulation results reveal the effects of different SAC305 solder volumes on joining quality in terms of fillet shape formation, pressure, and velocity. The preferred solder volume is 1.517 mm3 with a filling time of 1.7 s, a temperature of 642 K, a hole diameter of 1.0 mm, a lead diameter of 0.5 mm and a PCB thickness of 1.20 mm, produces a fillet shape similar to a real sample of PTH laser soldering joint from the industry. This ideal volume was chosen with a suitable pressure and velocity to create a nice concave fillet shape and minimize voids during soldering. The pressure and velocity increase when the solder volume increases causing solidified solder to melt. The discrepancy between numerical and experimental values of fillet height is comparable, which inferred that the numerical model was well-validated. The good consistency between both results proves that the finite volume model will effectively predict the optimum solder volume. This opens up new opportunities for the optimization of the laser soldering parameters in the application of electronic manufacturing.

Five halogen-free (HF) dummy plastic ball grid array (PBGA) components with daisy-chains and Sn4.0Ag0.5Cu (SAC405) Pb-free solder balls were assembled on a HF high density interconnection (HDI) printed circuit board (PCB) using Sn1.0Ag0.5Cu (SAC105) and Sn3.0Ag0.5Cu (SAC305) Pb-free solder pastes, respectively. The above compositions were in weight percent. The assemblies were then experienced to moisture sensitive level testing with three times reflow at a peak temperature of 260 °C; no delamination was found present in both the component and PCB laminates. The microstructure showed that the utilization of SAC105 solder paste was beneficial in refining the Ag3Sn intermetallic compound (IMC) within the solder joint and the intermetallic layers formed at various interfaces with different Ni contents and thicknesses due to different metal finishes. The IMC spalling was found at the BGA-side interface within the solder joints formed with SAC105 solder paste but not discovered within the ones made of SAC305 solder paste. The pull strength of the solder joint formed with SAC305 solder paste was always higher than that made from SAC105 no matter on Cu or electroless Ni. Moreover, the fracture was found at the interface between the Cu foil and epoxy in the halogen-free test device. Numerical analysis showed that the thickness of IMC layer dominated the pull strength of the solder joint because the Z-axial normal stress applied to the solder joints formed with Cu and electroless Ni were 752.0 and 816.6 MPa, respectively, and a thicker IMC layer was beneficial to provide a higher pull strength of solder joint.

The Sn-3.0Ag-0.5Cu (SAC305) solder, widely used in the electronics industry, exhibits excellent wettability and mechanical stability. However, its relatively high melting point of 217 °C can cause thermal damage and interfacial delamination during repeated reflow processes. As an alternative, the Sn-58Bi solder, with a low melting point of 139 °C, has attracted significant attention. Sn58Bi solder is cost-effective and shows favorable mechanical properties and wettability. Nevertheless, its high Bi content (58 wt%) leads to brittleness and the formation of coarse Bi-rich phases, which degrade joint reliability. To overcome these limitations, a hybrid solder structure combining Sn58Bi and SAC305 has been proposed, allowing the integration of the low processing temperature of Sn58Bi with the mechanical robustness of SAC305. However, conventional reflow soldering still involves a high thermal load and long processing time, which result in excessive energy consumption and accelerate intermetallic compound (IMC) growth and warpage. To address these issues, this study adopts intense pulsed light (IPL) soldering, which utilizes the photothermal effect of a Xe flash lamp as an alternative process. The IPL process enables localized heating within a few seconds to induce solder melting and bonding, allowing the formation of a thin IMC layer with precise temperature control. In this study, Sn58Bi solder paste was printed on Cu pads, and SAC305 solder balls were mounted on top to form a hybrid joint structure. The joints were then fabricated using both reflow and IPL soldering processes, and their microstructural and mechanical properties were compared according to the process conditions. As a result, the IPL process produced a thinner IMC layer than reflow soldering and, although it exhibited slightly lower shear strength, the increased shear displacement indicated improved ductility of the joints.

The Sn–Bi solder paste is commonly used in electronic assembly and packaging, but its brittleness causes poor reliability in shock environments. In this study, the mechanical reliability of Sn–Bi solder paste and Sn–Bi composite solder paste with thermosetting epoxy (TSEP Sn–Bi) was investigated with the board level drop test. The crack characterizations of solder joints were evaluated using a stereomicroscope after the dye and pull test. The microstructure characterization and failure types of solder joints were analyzed by a scanning electron microscope (SEM), and an energy dispersive spectrometer (EDS) was employed to investigate the initial phase identification and elemental analysis. Compared with Sn–Bi solder paste, the results show that the TSEP Sn–Bi solder pastes reduced the proportion of the complete failure and partial failure of the solder joints during the drop test. The microstructure observation of the crack path showed that the Sn–Bi/TSEP Sn–Bi solder joints were reinforced through the cured epoxy resin. The number of drops of the Sn–Bi/TSEP Sn–Bi-x (x = 3, 5, 7) solder joints had 1.55, 2.57, and over 3.00 times that of Sn–Bi/Sn–Bi solder joints after the board level drop test.

The ball-grid array (BGA) is widely used to reduce component size and it had advantages such as high I/O pins and fine pitch. Typical Sn-Ag-Cu (SAC) solder alloys are used for formation of BGA because SAC solder has excellent characteristics among lead-free solders. However, the electronic components assembled by SAC solder were easily damaged by heat during manufacture process because SAC solder had high melting point of 220 °C. To prevent these thermal damages, SAC305 BGA component assembled by Sn-58Bi solder paste has been studied because Sn-58Bi solder had low melting point of 139 °C. In generally, Sn-58Bi solder was improved by additional elements or polymer such as epoxy because Sn-58Bi had a brittle property. However, the epoxy Sn-58Bi solder did not guaranteed high environmental reliability such as high-temperature high-humidity (HTHH) test. Thus, we evaluated the shear strength of solder joints assembled by SAC305 BGA components with Sn-58Bi solder paste and epoxy Sn-58Bi solder paste. The shear strength of solder joints was evaluated by die shear test after HTHH test at the 85 °C/85% RH conditions. The Cu6Sn5 intermetallic-compound (IMC) at the interface of solder joints was observed by scanning electron microscope (SEM). The IMC thickness of Sn-58Bi solder joints was smaller than that of epoxy Sn-58Bi solder. The shear strength was improved up to 20% by epoxy addition. The shear strength of epoxy Sn-58Bi solder joints dramatically decreased after HTHH test for 100 h.

Sn-Bi solder with different Bi content can realize a low-to-medium-to-high soldering process. To obtain the effect of Bi content in Sn-Bi solder on the microstructure of solder, interfacial behaviors in solder joints with Cu and the joints strength, five Sn-Bi solders including Sn-5Bi and Sn-15Bi solid solution, Sn-30Bi and Sn-45Bi hypoeutectic and Sn-58Bi eutectic were selected in this work. The microstructure, interfacial reaction under soldering and subsequent aging and the shear properties of Sn-Bi solder joints were studied. Bi content in Sn-Bi solder had an obvious effect on the microstructure and the distribution of Bi phases. Solid solution Sn-Bi solder was composed of the β-Sn phases embedded with fine Bi particles, while hypoeutectic Sn-Bi solder was composed of the primary β-Sn phases and Sn-Bi eutectic structure from networked Sn and Bi phases, and eutectic Sn-Bi solder was mainly composed of a eutectic structure from short striped Sn and Bi phases. During soldering with Cu, the increase on Bi content in Sn-Bi solder slightly increased the interfacial Cu6Sn5 intermetallic compound (IMC)thickness, gradually flattened the IMC morphology, and promoted the accumulation of more Bi atoms to interfacial Cu6Sn5 IMC. During the subsequent aging, the growth rate of the IMC layer at the interface of Sn-Bi solder/Cu rapidly increased from solid solution Sn-Bi solder to hypoeutectic Sn-Bi solder, and then slightly decreased for Sn-58Bi solder joints. The accumulation of Bi atoms at the interface promoted the rapid growth of interfacial Cu6Sn5 IMC layer in hypoeutectic or eutectic Sn-Bi solder through blocking the formation of Cu6Sn5 in solder matrix and the transition from Cu6Sn5 to Cu3Sn. Ball shear tests on Sn-Bi as-soldered joints showed that the increase of Bi content in Sn-Bi deteriorated the shear strength of solder joints. The addition of Bi into Sn solder was also inclined to produce brittle morphology with interfacial fracture, which suggests that the addition of Bi increased the shear resistance strength of Sn-Bi solder.

Study on the properties of epoxy-based Sn[sbnd]58Bi solder joints

첨단 전자기기에 사용되는 전자부품의 크기와 접속피치가 감소하면서 리플로우 공정 후 플럭스 잔사의 세정이 어려워짐에 따라 무세정 솔더 페이스트에 대한 요구가 증가하고 있다. 본 연구에서는 SAC305 솔더분말과 에폭시 레진을 주성분으로 하는 경화성 플럭스를 혼합하여 제조한 에폭시 경화형 솔더 페이스트에 대하여 리플로우 공정성, 플럭스 잔사의 부식성, 솔더볼 및 보드레벨 BGA 패키지 솔더 접합부의 기계적 거동을 기존 로진계 솔더 페이스트와 비교하여 평가하였다. 에폭시 경화형 솔더 페이스트는 솔더 접합부 주변에 경화물 필렛을 형성한 것으로 보아 플럭싱 작용에 의해 솔더 접합부가 형성된 이후에 경화반응이 진행되는 것을 확인할 수 있었으며, 동판에 대한 젖음성 시험을 통해 기존상용 솔더 페이스트 정도의 납퍼짐성을 갖는 것을 알 수 있었다. 리플로우 후 동판에 대한 고온 고습 시험을 통해 에폭시 경화형 솔더 페이스트는 동판 부식을 전혀 발생시키지 않는 것으로 나타났는데, 이는 FT-IR 분석결과 에폭시 경화반응을 통해 단단히 고정된 결과로 생각되었다. 볼전단, 볼당김 및 다이전단 시험 결과, 솔더 접합부 주변에 형성된 경화물 필렛은 솔더 표면과 접착본딩을 형성하며, 다이전단강도를 15~40% 정도 향상시키는 것으로 보아 에폭시 경화형 솔더 페이스트는 플럭스 잔사 세정공정의 생략과 함께 솔더 접합부 보강효과를 통해 패키지 신뢰성 향상에도 기여할 수 있을 것으로 생각되었다. With difficulties during the cleaning of reflow flux residues due to the decrease of the part size and interconnection pitch in the advanced electronic devices, the need for the no-clean solder paste is increasing. In this study, an epoxy curable solder paste was made with SAC305 solder powder and the curable flux of which the main ingredient is epoxy resin and its reflow solderability, flux residue corrosivity and solder joint mechanical properties was investigated with comparison to the commercial rosin type solder paste. The fillet shape of the cured product around the reflowed solder joint revealed that the curing reaction occurred following the fluxing reaction and solder joint formation. The copper plate solderability test result also revealed that the wettability of the epoxy curable solder paste was comparable to those of the commercial rosin type solder pastes. In the highly accelerated temperature and humidity test, the cured product residue of the curable solder paste showed no corrosion of copper plate. From FT-IR analysis, it was considered to be resulted from the formation of tight bond through epoxy curing reaction. Ball shear, ball pull and die shear tests revealed that the adhesive bonding was formed with the solder surface and the increase of die shear strength of about 15~40% was achieved. It was considered that the epoxy curable solder paste could contribute to the improvement of the package reliability as well as the removal of the flux residue cleaning process.

In the last decade, the Ultra-Thin HDI (High Density Interconnect) technology has dominated the mobile/wearable products designs, yet the standard test vehicles used for qualifying materials, components and connectors provided by IPC (Institute of Printed Circuits), ANSI (American National Standard Institute) and JEDEC (Joint Electron Device Engineering Council aka Solid State Technology Association), are obsolete and OEM’s (Original Equipment Manufacturer) have stopped using them. These test vehicles do not reflect the form factors, density and interconnect methods used by the industry. In addition, they do not capture failure modes of the fine pitch BGA (Ball Grid Array) technology and as a result don’t provide upstream Users adequate data for development. Microvias and dielectrics are the key, along with finer lines and spaces, in obtaining the density of Ultra-Thin HDI boards. Continuous reduction in PCB dimensions and feature sizes challenges both fabrication process and test vehicles. HDP User Group’s paper gives a brief introduction to the modern Ultra-Thin PCB (Printed Circuit Board) fabrication process. It discusses typical board level reliability tests and presents a generic reliability test vehicle design which is based on current Ultra-Thin HDI technology. With the proposed test vehicles’ design of modularity and scalability it is easy to modify to meet the next generation of Ultra-Thin HDI PCB requirements. The study indicates that the proposed TV design covers HDI test requirements and most of the current tests are critical when evaluating modern Ultra-Thin HDI circuit board reliability. The HDP User Group project also conducted a study on the application and reliability of a very fine pitch WLCSP (Wafer Level Chip Scale Package) on this Ultra-Thin HDI test vehicle. A 0.3mm pitch WLCSP daisy chain, commercially available solder paste, flux and underfill were used for this study. The details of the assembly process including solder paste printing, dip flux, component placement, reflow and underfill dispensing are discussed. The impact of the test vehicle OSP (Organic Surface Protection) surface finish on the assembly process and result is addressed as well. JEDEC standard air-to-air thermal cycling and drop tests are performed for reliability evaluation purposes. The reliability data of solder paste printing vs. dip flux, underfill vs. non-underfill are compared and evaluated. All results show that the WLCSP application process and materials on the ultra-thin test vehicle are robust and reliable.

The assembly of miniature components on Printed Circuit Boards (PCBs) poses numerous challenges to each process step. The 0201 components are miniature components which are 0.020” in length and 0.010” in width. The assembly of 0201s using process settings that are currently in-place may result in defects including bridging, tombstoning and solderballing. The objectives of this research are to evaluate two no-clean eutectic solder pastes, determine ‘optimum’ print parameters, and develop good reflow profiles to eliminate defects that are commonly observed during the assembly of 0201 components. The two no-clean solder pastes are evaluated and the ‘optimum’ print parameters were determined through a full factorial experiment. Statistical tests, such as the paired t-test and the calculation of the coefficient of variation of the volume of the solder paste deposited, helped to evaluate the two solder pastes. A comprehensive literature review was conducted to arrive at the initial reflow parameters. The solder paste printing and reflow processes were modified as per the findings and were implemented to assemble several PCBs (test vehicles) with 0201s on them. The assemblies were inspected for defects, both visually and using a 10X microscope. In addition, the assemblies were also inspected using automated optical inspection equipment. No defects were observed during these inspections. Consequently, it can be concluded that the recommended changes to the printing and reflow soldering processes have resulted in the successful development of a viable ‘drop in’ process for the assembly of 0201 components.

Sn-58Bi eutectic solder is the most recommended low temperature Pb-free solder but is also limited from the interfacial embrittlement of Bi segregation. Since the quaternary Sn-38Bi-1.5Sb-0.7Ag solder provides a similar melting point as Sn-58Bi eutectic, this paper systematically investigated the properties of this solder from wettability, bulk tensile properties, interfacial microstructure in solder joints with a Cu substrate, interfacial evolution in joints during isothermal aging and the shear strength on ball solder joints with effect of aging conditions. The results were also compared with Sn-58Bi solder. The wettability of solder alloys was evaluated with wetting balance testing, and the quaternary Sn-38Bi-1.5Sb-0.7Ag solder had a better wettability than Sn-58Bi solder on the wetting time. Tensile tests on bulk solder alloys indicated that the quaternary Sn-38Bi-1.5Sb-0.7Ag solder had a higher tensile strength and similar elongation compared with Sn-58Bi solder due to the finely distributed SnSb and Ag3Sn intermetallics in the solder matrix. The tensile strength of solder decreased with a decrease in the strain rate and with an increase in temperature, while the elongation of solder was independent of the temperature and strain rate. When soldering with a Cu substrate, a thin Cu6Sn5 intermetallic compound (IMC) is produced at the interface in the solder joint. Measurement on IMC thickness showed that the quaternary Sn-38Bi-1.5Sb-0.7Ag had a lower IMC growth rate during the following isothermal aging. Ball shear test on solder joints illustrated that the quaternary Sn-38Bi-1.5Sb-0.7Ag solder joints had higher shear strength than Sn-58Bi solder joints. Compared with the serious deterioration on shear strength of Sn-58Bi joints from isothermal aging, the quaternary Sn-38Bi-1.5Sb-0.7Ag solder joints presented a superior high temperature stability. Therefore, the quaternary Sn-38Bi-1.5Sb-0.7Ag solder provides better performances and the possibility to replace Sn-58Bi solder to realize low temperature soldering.

8 - SC soldering processes