Low Temperature Solder Paste Research Articles

Addressing Challenges in Dual Sided SiP Thermal Budget

System-in-Package (SiP) has been adopted as the mainstream advanced packaging technology in many consumer applications including 5G smartphones, tablets, wireless earbuds, smart wearables, etc. Heterogeneous integration, smaller form factor, better electrical performance, lower cost of assembly and faster time to market are the key differentiators of SiP as compared to conventional packaging techniques. In applications such as 5G mobile RF Front-End (RFFE) and Antenna-in-package (AiP) modules, dual sided SiP is the popular approach which brings even higher level of integration and smaller form factor. However thermal warpage control is a significant challenge during two times of solder reflow processes for the front and back sides of the package, especially if one side is with underfill and mold materials which have significant CTE mismatch to the substrate and dies, causing delamination failures and yield losses. This study examines the novel solution of reducing thermal budget with the use of a low temperature solder paste material based on SnBiAg alloy in ultra-fine pitch Type 6 solder powders. It allows a minimal peak reflow temperature of only 170 °C that effectively reduces warpage of dual sided SiP substrates. Solder alloy and paste properties and solder joint microstructures are examined in detail for tests including fine-pitch printing and component soldering, thermal aging test on different substrate finishes, thermal cycling test and mechanical drop test for reliability performance.

A Novel Lead-Free Low-Temperature Solder Paste for Wafer-Level Package Application

An In-containing low-temperature solder paste (DFLT) has been developed and successfully used in mobile phone board-stacking application. It is now being tried for main-board application, targeting at using lower-temperature reflow profiles but delivering at least the comparable reliability to the mainstream SAC305. The customized drop-shock packages and the daisy-chained WLP256 assemblies were reflowed under a variety of reflow profiles, in which peak temperatures ranged from 200o C up to 240o C. SAC305 has been used as the control leg reflowed under a traditional 240o C peak temperature lead-free reflow profile. The joint morphology changed with the reflow profiles. Under the 200o C peak reflow, hybrid joints were formed in which the mixing zone, dominated by DFLT, was present at the PCB side while the SAC305 ball at the chip side maintained the original morphology. Increasing the reflow peak temperature led to the fully-merged SAC305 ball with DFLT and, finally the formation of the homogeneous joint. Based on the reflow profile, DFLT has shown the drop-shock performance (g-force>6000g) comparable to or substantially better than SAC305, in which 220o C peak reflow allows DFLT having the drop-shock performance more than 90% better than SAC305. Temperature cycling test (TCT) has been done with the profile of -40 to 125o C and 20 minutes dwelling time. Similar to drop-shock, depending on the reflow profiles DFLT has the TCT performance outperforming SAC305. Reflow under a peak temperature of 210o C results in best TCT performance among all profiles. Although the reflow profiles impacts the performance ranking, the failure modes remain the similar for all the assemblies in drop-shock and TCT, respectively.

Microstructure changes in Sn-Bi solder joints reinforced with Zn@Sn particles in thermal cycling and thermomigration

BackgroundIn the field of electronic packaging, Sn-58Bi solder paste is a promising low-temperature option for step soldering. However, the tendency for the Bi-rich phase to coarsen and segregate poses a significant challenge to the long-term reliability of the solder joints. MethodsThe Sn coated Zn (Zn@Sn) core-shell particles were prepared through a chemical plating process and incorporated into the Sn-58Bi low-temperature solder paste to enhance its reliability. The microstructural and mechanical characteristics of the solder joints were evaluated under thermal cycling and thermomigration conditions to assess their long-term service reliability. Significant findingsThe addition of Zn lowers the nucleation energy and promotes the refinement of the eutectic structure, avoiding a significant reduction in shear strength after thermal cycling. In thermomigration, the heat transfer Q* of Bi is calculated to be 22.5 kJ/mol. The Zn-rich phases hardly move and are evenly distributed within the solder joints, acting like a barrier to prevent the Bi phase diffusing from the hot end to the cold end.

Re‐Evaluation of a Dual‐Feed Linear Polarized 2‐by‐2 Circular Patch Array Antenna for 60‐GHz‐Band Digital Beamforming Applications

In this letter, re‐evaluation results of a 60‐GHz‐band digital beamforming antenna using a dual‐feed linear polarized 2 × 2 circular patch array antenna is reported. The extension from our previous report is providing new measurement data under the condition of all intermediate frequency (IF) signal usage by re‐fabrication, although we reported imperfect measurement results using the IF signals only 5 ch out of 8 ch in the previous report due to the fabrication error. A low temperature solder paste was newly used to prevent the warp of the multilayered substrate. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Monolithic integration of MEMS thermal flow sensor and its driving circuit onto flexible Cu on polyimide film

A thermal sensor integrated with its driving circuit by using Cu on polyimide (COP) as a substrate was developed. A heater, working as the thermal flow sensor and constant temperature circuit (CTC), working as its driving feedback circuit, were successfully integrated on the COP substrate by photolithography, metal deposition, and sacrificial selective etching in the Cu layer. The Cu layer works to form a cavity that acts thermal isolation for the heater and electrical feedthroughs in the CTC. The electrical components, i.e., resistors, capacitors, and operational amplifiers, were mounted on the electrical pads using low-temperature solder paste of Sn–Bi at the CTC area. The areas of the thermal sensor and CTC were 11 × 23 mm and 16 × 24 mm, respectively. The area of the thermal sensor was inserted into a tube with a diameter of 5 mm acting as the flow sensor. The sensor outputs increased along with the airflow rate up to 10.0 L/min, and the calibration curve as the airflow-rate sensor was experimentally derived. The fabricated sensor outputs were not affected by variations in the airflow temperature and only depended on the airflow rate thanks to the added temperature compensation function, and a response time of 0.095 s was obtained.

Enabling paper-based flexible circuits with aluminium and copper conductors

Implementing electronics systems on paper is an important area of flexible circuit technologies. One of the approaches is to print conductive inks onto paper substrates, on which silicon-based surface mount device components are mounted. However, one of the problems is that the printed conductors have unneglectable resistivity. In this paper, we present paper-based flexible circuits, using copper and aluminium conductors that are laminated onto paper substrates using a high-speed roll-to-roll method. Edge roughness inspections and repeated two-point bending tests are carried out to evaluate the manufactured flexible circuits. Three surface mount techniques are used to assemble standard surface mount device components onto the flexible circuits, including an isotropic conductive adhesive, an anisotropic conductive adhesive, and a low-temperature solder paste. Several characterizations are performed to the surface mount techniques, including contact resistance measurements, component bonding strength tests, assembled circuit bending tests, and scanning electron microscopy. The results of the characterizations suggest that flexible circuits made from Cu with paper substrate achieve satisfactory results for mechanical reliability, all surface mount techniques, and have the potential to be used on automatic component assembly lines. In order to test whether such flexible circuits and surface mount techniques can be used in implementing electronics systems, passive NFC tags with relative humidity sensing functionality are made, which are interrogated by an NFC equipped mobile phone.

Soldering surface mount components on screen-printed Ag patterns on paper and polyimide substrates for hybrid printed electronics

The development of printed electronics on flexible substrates is increasing rapidly, where the main benefits are the large area and low cost. However, the functionality and performance of printed circuits cannot compete with standard silicon based microprocessors or integrated circuits, though the functionality and performance of printed circuits are increasing. Therefore, in this work we investigate the possibilities of using Sn42/Bi57.6/Ag0.4 low-temperature solder paste together with a reflow solder oven and hot air solder iron to mount regular surface mount device (SMD) components on screen-printed silver tracks. It was found that it is possible to solder standard Si SMD packages onto screen-printed Ag ink tracks on paper substrate; however, the component bonding strength to polyimide was not satisfactory. The resistance of the solder joints was found to be no more than 240 mΩ. The bond strength was found to be higher using the manual hot air solder iron than the reflow solder oven. Bending tests show that the bonding strength is higher for the hot air soldered components. Reference samples on FR-4 based copper printed circuit board show a bond strength ∼10 times higher but we conclude that it is possible to solder regular Si SMD components onto Ag-printed conductors on paper substrate with good results. The process could be used to fabricate hybrid printed electronics in a standard solder process line.

Low Cost Semi Automated Assembly Unit for Small Size Back Contact Modules and Low Cost Interconnection Approach

We present our low cost assembly unit to manufacture back contact solar modules based on the conductive backsheet (CBS) approach. This in house developed apparatus was built to assemble test modules containing one up to four 6 inch back contact solar cells. The system is a retrofit of a commercially available CNC system which is equipped with a cell grabber and a manual dispensing system (by Nordson). The total cost of the setup was roughly 4000 € excluding the dispenser unit. Using this equipment we assembled several small size modules containing one and four Zebra cells, which are low cost 6 inch IBC solar cells developed at ISC Konstanz [1,2]. The contact between copper backsheet and back contact cell of the one cell modules we present here is formed by low temperature solder paste (LTSP). First cell to module (CTM) loss evaluations and reliability results suggest that this material could be a viable alternative to electrically conductive adhesive (ECA) which is currently the most commonly used material for this purpose.

Fine‐pitch surface component mounting on screen‐printed fabric circuits

A novel method of bonding surface mount components to screen-printed fabric circuits is proposed. The method is achieved by applying the same technique of screen-printing conductive ink patterns of fabric circuit for printing a layer of low-temperature solder paste onto uncured conductive ink pads, and placing the circuit with mounted components inside an oven for a one-step curing process. A screen-printing frame with 42 μm mesh opening is used for both conductive ink and solder paste layers. Thus, a 0.2 mm resolution for printed traces and spaces is achieved, which meets the demanding geometrical requirements of most integrated circuit chips with fine-pitch and tiny footprints. Measurement results show the printed tracks after curing exhibit excellent conductivity while retaining good flexibility. Examination under the microscope shows the components are well-bonded to their on-fabric pads. The proposed method for surface component mounting and one-step curing process for screen-printed fabric circuits can potentially streamline and expedite the twin process of circuit fabrication and component mounting for future smart textile applications.

Experimental Study on Low Temperature Sn-Bi Solder Paste

For lack of existing low temperature solder paste , developed a low-temperature Sn-Bi solder paste, focused on finding the optimal formula , at the same time on the flux of the solder paste, is printing performance and wettability can be tested through performance test to find the optimal flux formula.

Thermal oxidative degradation of a composition based on a polyester resin and tetramethylammonium chloride

Thermal oxidative degradation of a composition based on a polyester resin modified with rosin and tetramethylammonium chloride and its use as a flux for low-temperature solder pastes were studied.

Reaction of nanocrystalline mechanically alloyed CuSn alloy with GaInSn eutectic

An extended solid solution of Cu20wt%Sn with lattice parameter 0.3715 nm and grain sizes 7 nm, was obtained by mechanical alloying. A DSC trace of Cu20wt%Sn showed an exothermic effect at 643 K due to the crystalline grain growth and the formation of an equilibrium solid solution. The reaction of Cu20wt%Sn with a GaInSn liquid alloy was studied at room temperature by measuring peak intensities of CuGa 2, the intermetallic compound formed during the reaction, as a function of time. The reaction rate increased when crystalline grain size D decreased. Mechanical alloying process used to control the reactivity of nanocrystalline solid solutions as components in low temperature solder pastes.

Reaction of nanocrystalline mechanically alloyed Cu-Sn alloy with Ga-In-Sn eutectic

Abstract An extended solid solution of Cu20wt%Sn with lattice parameter 0.3715 nm and grain sizes 7 nm, was obtained by mechanical alloying. A DSC trace of Cu20wt%Sn showed an exothermic effect at 643 K due to the crystalline grain growth and the formation of an equilibrium solid solution. The reaction of Cu20wt%Sn with a GaInSn liquid alloy was studied at room temperature by measuring peak intensities of CuGa 2 , the intermetallic compound formed during the reaction, as a function of time. The reaction rate increased when crystalline grain size D decreased. Mechanical alloying process used to control the reactivity of nanocrystalline solid solutions as components in low temperature solder pastes.

Reaction of nanocrystalline Cu-Sn alloy with Ga-In-Sn eutectic

An extended solid solution of Cu-20%wtSn with a lattice parameter of 0.3715 nm and a crystalline grain sizes D ranging from 30-7 nm was obtained by mechanical alloying. The reaction rate for the formation of CuGa 2 due to the interaction of Cu-20%wtSn with a Ga-In-Sn liquid at room temperature increased significantly when D decreased to 7 nm. A mechanical alloying process was used to control the reactivity of the nanocrystalline solid solutions, used as components in the low temperature solder pastes.

Thermocouple arrays for temperature measurements in vivo

I have read with interest the letters submitted to your journal about our recent technical note (ter Haar and Dunn, 1984) by Drs. Hand and Dickinson (1984) and Gronlund et al (1984). We had hoped that, by publication of this note, we could bring to public attention the ease with which multi-junction thermocouple arrays (of any number of junctions) may be made using low-temperature solder paste.