It is essential to decrease RC delay in order to increase the IC device speed. Cu is now adopted to decrease the resistivity of interconnects. Several low k materials have been studied to decrease the dielectric constant. Ag is one of the candidates after Cu as a low resisitivity interconnect metal although it has not yet been adopted in the actual products. It is also studied to decrease the effective dielectric constant values by considering the device configuration such as the cap layer only on the metal. Co-W-P is reported as one of the cap material candidates for Cu. Ni-B is proposed herein as the cap material for Cu and Ag. The study results confirmed that an Ni-B layer can be selectively deposited on Ag metal by the electroless plating method, using DMAB (dimethylamine borane) as the reducing agent; that a deposition rate of 150 nm/min and a B content of 3.2 at% can be obtained under conditions of pH 10 and 353 K; that an Ni-B layer with a 3.2 at%B content provides a harrier effect that prevents Ag and Cu diffusion through thermal processes (in this case, Ni-B remained crystalline before and after a thermal process); and that an Ni-B layer with a 13.5 at%B content does not provide a barrier effect that would prevent Cu diffusion through thermal processes (in this case, the Ni-B structure changed from amorphous to crystalline after the thermal process). Damascene interconnects with Ag metal and a Ni-B cap layer were also formed as part of the trial effort.

Ni-B electroless plating as cap layer for Ag multi-level metallization : Special issue on materials-related issues for Cu interconnects used in ultra high speed large scaled integrated Si devices

The surface finish is an essential step in printed circuit boards design because it provides a solderable surface for electronic components. The purpose of this study to investigate the effects of different surface finishes during the soldering and ageing process.,The solder joints of Sn-4.0Ag-0.5Cu/Cu and Sn-4.0Ag-0.5Cu/electroless nickel/immersion silver (ENImAg) were investigated in terms of intermetallic (IMC) thickness, morphology and shear strength. The microstructure and compositions of solder joints are observed, and analyzed by using scanning electron microscopy (SEM-EDX) and optical microscope (OM).,Compounds of Cu6Sn5 and (Cu, Ni)6Sn5 IMC were formed in SAC405/Cu and SAC405/ENImAg, respectively, as-reflowed. When the sample was exposed to ageing, new layers of Cu3Sn and (Ni, Cu)3Sn5 were observed at the interface. Analogous growth in the thickness of the IMC layer and increased grains size commensurate with ageing time. The results equally revealed an increase in shear strength of SAC405/ENImAg because of the thin layer of IMC and surface finish used compared to SAC405/Cu. Hence, a ductile fracture was observed at the bulk solder. Overall, the ENImAg surface finish showed excellent performance of solder joints than that of bare Cu.,The novel surface finish (ENImAg) has been developed and optimized. This alternative lead-free surface finish solved the challenges in electroless nickel/immersion gold and reduced cost without affecting the performance.

The formation of intermetallic layer structure of SAC405/Cu and SAC405/ENImAg solder joint interfaces

The selection of surface finish on printed circuit board is very important in soldering process because it can prevent the copper from oxidation and provide solderable surface. It is also will influence the cost, manufacturing, shelf life, surface quality, final product reliability and environment.Electroless and immersion plating process has now become an important and popular surface finish in electronic industry due to simple and low cost process. The application of electroless plating are widely used in electronic industry, however it also was used in various industries such as aerospace, automotive, oil and gas, chemical processing where the need on the complex shape, homogeneous and uniform layer are required. Besides, every surface finish has their strength and weakness. Immersion silver is one of the surface finishes frequently used on the printed circuit board due to their characteristics such as good solderability and wettability, easy assembly and low cost. Both of these two processes were acts as barrier layer between copper substrate and solder balls. This review was discussed about electroless nickel and immersion silver plated on copper substrate, coating process and parameters involved. The topic also covered include the morphology and thickness of the intermetallics formed during soldering as well as the isothermal aging process. Keyword – Electroless plating, Electroless nickel, Immersion silver, Intermetallic compound, Reflow soldering

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Electroless and Immersion Plating Process towards Structures and IMC Formation

The soldering of BGA packages is well known process. Despite the increasing complexity of PCBs and diversity of BGA package footprints, further optimization and improvement of soldering is required. In this paper, the influence of heat flow direction during reflow soldering on the structure of ball joint was described. The SAC305 solder alloy was used for ball joint investigation under the no-clean solder flux condition. Tested samples were made from FR4 material with ENIG as a surface finish. The heating was generated by infrared heaters placed on the top, bottom and both sides of the sample. The substantial part of this work was focused on the characterization the solder joint structure with the respect to the intermetallic compounds formation. SEM and optical microscope images of cross-sectioned solder joints were used to characterize the quality of the boundary formation. Shear test method was used to evaluate the impact of intermetallic layer and inner solder joint structure on the mechanical properties of joints. This research helps to determine the influence of the direction of heat flux, i.e. direction of heating of the infrared heater used in the reflow soldering, for the formation of the solder ball joint on BGA package (primarily for rework).

Influence of heating direction on BGA solder balls structure

Purpose




This paper aims to study the effect of aging and cooling rate on the reliability of the solder joint using electroless nickel boron (EN-Boron) as a surface finish in the electronic packaging area.




Design/methodology/approach




EN-Boron was plated on a Cu substrate through electroless plating method. This process was followed by reflow soldering of Sn–3.0Ag–0.5Cu solder alloy on metallized Cu substrate to form a joining. Then, the specimens were cooled using different cooling mediums such as air (slow cooling) with 15.7 °C/min and water (fast cooling) with 110.5 °C/min. After that, the specimens were subjected to isothermal aging at 150°C for 0, 250 and 1,000 h. Finally, they went through a lap shear test following ASTM D1002. Optical microscope and scanning electron microscopy were used for intermetallic compound (IMC) characterization. The type of IMC formed was confirmed by field emission scanning electron microscope-energy-dispersive X-ray spectroscopy (FESEM-EDX).




Findings




The results showed that the IMC type changed from the combination of Ni3Sn4 and (Ni, Cu)3Sn4 after reflow soldering into fully (Ni, Cu)3Sn4 when aged for 1,000 h. The formation of (Ni, Cu)3Sn4 and Cu3Sn underneath the IMC layer played a role in reducing the shear strength of joining. Overall, water cooling was reported to provide higher shear strength of solder joint compared to air cooling medium.




Originality/value




The shear strength when using EN-Boron as the surface finish is comparable to the surface finish conventionally used.

Intermetallic growth and shear strength of SAC305/EN-Boron

The formation of intermetallic compounds (IMCs) on solder pads is strongly affected by the type of PCB surface finish since different finishes may lead to several different interfacial reactions and IMCs formation. Since the joint reliability is also determined by the type of the interfacial IMC layer between the solder and substrate, understanding how such intermetallic compounds form and grow during soldering and subsequent thermal ageing is essential. In this paper, experimental results of the effect of three different surface finishes namely: electroless nickel (phosphorus)/immersion gold, electroless nickel (boron)/ immersion gold and bare copper (for comparison) on the formation and growth of interfacial reactions during soldering and thermal ageing with Sn-3Ag-0.5Cu and Sn-4Ag-0.5Cu solders are presented. Several techniques of materials characterization including optical, image analysis, scanning electron microscopy and energy dispersive X-ray analysis were used to examine and quantify the intermetallics in terms of composition, thickness and morphology. The results showed that after soldering on Ni-Au finishes the reaction layer was found to consist of only one layer of (Cu, Ni) 6 Sn 5 with a needle-shape morphology. In addition, the results from SEM with energy dispersive x-ray (EDX) have revealed that isothermal aging at 150°C has caused the thickening and coarsening of IMCs as well as changing them into more spherical shape. During soldering on Ni(B)/Au finish it was observed that closer control of the reflow temperature was very important compared to soldering on Ni(P)/Au finish. Selective etching of solder joints also revealed that the morphology of the intermetallic formed is different across a single solder joint from the centre to the outside edge.

Siti Rabiatull Aisha

Papers

Interfacial reactions of SAC305 and SAC405 solders on electroless Ni(P)/immersion Au and electroless Ni(B)/immersion Au finishes