Printed Circuit Board Surface Finish Research Articles

The Effect of Bismuth on Intermetallics Growth between Lead-Free Solders and Electroless Nickel Immersion Silver (ENIMAG) Surface Finish

Surface finish is coating layer plated on a bare copper board of printed circuit board (PCB). Among PCB surface finishes, Electroless Nickel/Immersion Gold (ENIG) finish is a top choice among electronic packaging manufacturer due to its excellent properties for PCB. However, the use of gold element in ENIG is very high cost and the black pad issue have not been resolved. Thus, by introducing an Electroless Nickel/Immersion Silver (ENImAg) as alternative surface finish hopefully can reduce the cost and offer better properties. The aim of this study is to investigate the effect of bismuth on interfacial reaction during reflow soldering between Sn-2.5Ag (SA25), Sn-3.4Ag-4.8Bi (SAB3448) and ENIMAG surface finish. Solder balls with sizes of 500μm diameters were used. The characteristics of intermetallic compound (IMC) were analyzed by using scanning electron microscopy (SEM), optical microscope and energy dispersive x-ray (EDX). After reflow soldering, the result revealed that only the irregular circle-shape of (Cu,Ni)6Sn5IMC layer was formed at the interface and change to an irregular rod-like shape meanwhile the irregular needle-shape (Ni,Cu)3Sn4was appeared after aging treatment. The result also indicated that, the grain size and thickness of IMC for SAB3448 is smaller and thinner compared to the SA25. The IMC thickness is proportional to the aging duration and IMC morphology for both solder are became thicker, larger and coarser after isothermal aging. No bismuth particle has been detected on SAB3448 solder during top surface examination. In addition, the Bi has been observed can reduce the grain size and the growth rate of IMC. Keywords: ENIMAG, reflow soldering, lead-free solder, intermetallic compound, bismuth

Printed Circuit Board Surface Finish and Effects of Chloride Contamination, Electric Field, and Humidity on Corrosion Reliability

Corrosion reliability is a serious issue today for electronic devices, components, and printed circuit boards (PCBs) due to factors such as miniaturization, globalized manufacturing practices which can lead to process-related residues, and global usage effects such as bias voltage and unpredictable user environments. The investigation reported in this paper focuses on understanding the synergistic effect of such parameters, namely contamination, humidity, PCB surface finish, pitch distance, and potential bias on leakage current under different humidity levels, and electrochemical migration probability under condensing conditions. Leakage currents were measured on interdigitated comb test patterns with three different types of surface finish typically used in the electronics industry, namely gold, copper, and tin. Susceptibility to electrochemical migration was studied under droplet conditions. The level of base leakage current (BLC) was similar for the different surface finishes and NaCl contamination levels up to relative humidity (RH) of 65%. A significant increase in leakage current was found for comb patterns contaminated with NaCl above 70% to 75% RH, close to the deliquescent RH of NaCl. Droplet tests on Cu comb patterns with varying pitch size showed that the initial BLC before dendrite formation increased with increasing NaCl contamination level, whereas electrochemical migration and the frequency of dendrite formation increased with bias voltage. The effect of different surface finishes on leakage current under humid conditions was not very prominent.

Deposition of Electroless Nickel Boron as Printed Circuit Board Surface Finish

Electroless Nickel Boron had been appointed as potential coating in several applications in industry like aeronatics, petrochemical industry, electronics and firearms due to its desirable physical and mechanical properties such as high wear resistance and high hardness including provides uniformity in coating thickness. However, in semiconductor sector, the usage of Nickel Boron as coating layer still insufficient due to lack of study in term of its potential as coating on printed circuit board. This study aims to investigate the mechanical and physical properties of electroless Nickel Boron as potential printed circuit board coating layer. The study conducted by cutting Copper substrates to 50mm x 7mm x 1.5mm and then subjected to surface pre-treatment before soak in Nickel Boron plating bath solution that contain of different concentration of Sodium Borohydrate (NaBH4), 0.4, 0.6,0.8,1.0 and 1.2 g/l. Surface roughness was evaluated using 3D Roughness Reconstruction software. For mechanical properties, the hardness test was conducted by using Vickers Hardness Test MMT-X7 Matsuzawa and surface structure was evaluate using ProgRes C3 IM7200 Optical Microscope and Field Emission Scanning Electron Microscopy (FESEM). It was found that the surface roughness and hardness resistance were affected by Sodium Borohydrate (NaBH4) that correlate with the surface microstructure.

Evaluation of ENEPIG and Immersion Silver Surface Finishes Under Drop Loading

The effect of printed circuit board surface finish on the drop loading reliability of ball grid array (BGA) solder interconnects has been examined. The finishes examined include electroless nickel/electroless palladium/immersion gold (ENEPIG) and immersion silver (ImAg). For the ENEPIG finish, the effect of the Pd plating layer thickness was evaluated by testing two different thicknesses: 0.05 μm and 0.15 μm. BGA components were assembled onto the boards using either eutectic Sn-Pb or Sn-3.0Ag-0.5Cu (SAC305) solder. Prior to testing, the assembled boards were aged at 100°C for 24 h or 500 h. The boards were then subjected to multiple 1500-g drop tests. Failure analysis indicated the primary failure site for the BGAs to be the solder balls at the board-side solder interface. Cratering of the board laminate under the solder-attached pads was also observed. In all cases, isothermal aging reduced the number of drops to failure. The components soldered onto the boards with the 0.15-μm-Pd ENEPIG finish with the SAC305 solder had the highest characteristic life, at 234 drops to failure, compared with the other finish–solder combinations.

Reflow Optimization Process: Thermal Stress Using Numerical Analysis and Intermetallic Spallation in Backwards Compatibility Solder Joints

This study applies the reflow optimization process to investigate the phenomenon of spalling in aerospace backward compatibility solder joints when utilized with a Ni substrate [electroless nickel immersion gold, printed circuit board surface finish, and ball grid array (BGA) pads]. The backward compatibility assembly comprised a lead-free tin–silver–copper (96.5Sn3.0Ag0.5Cu/SAC 305 alloy) solder ball assembled with tin–lead paste. The soldering of the lead-free BGA was conducted using two reflow temperature profiles and two conveyor speeds under a nitrogen atmosphere in a full convection reflow oven. The optimized reflow profile has peak temperatures ranging from 237.06 to 237.09°C for 68.94–69.36 s. Scanning electron microscope reveals intermetallic compound formation with maximum thicknesses which are lower than 12 μm as per aerospace requirement. Intermetallic compound spalling of solder ball interface components was not observed. However, spalling between printed circuit board and solder bulk was noted. Nevertheless, at both reflow temperature profiles, the composition and phase distribution of the lead-free BGA ball and tin–lead solder paste were fairly uniform across all joints. This work also presents a finite element-based simulation of backward compatibility assembly in reflow process. A growing number of manufacturers are changing their components to lead-free types without notifying customers. If an aerospace production line is still running a tin–lead-based process, understanding how these lead-free components are processed with tin–lead solder becomes essential. This paper will serve as a reference for manufacturing engineers, particularly those involved in surface mount technology application.

Effect of Isothermal Aging on the Long-Term Reliability of Fine-Pitch Sn–Ag–Cu and Sn–Ag Solder Interconnects With and Without Board-Side Ni Surface Finish

The combined effects on long-term reliability of isothermal aging and chemically balanced or unbalanced surface finish have been investigated for fine-pitch ball grid array packages with Sn–3.0Ag–0.5Cu (SAC305) (wt.%) and Sn–3.5Ag (SnAg) (wt.%) solder ball interconnects. Two different printed circuit board surface finishes were selected to compare the effects of chemically balanced and unbalanced structure interconnects with and without board-side Ni surface finish. NiAu/solder/Cu and NiAu/solder/NiAu interconnects were isothermally aged and thermally cycled to evaluate long-term thermal fatigue reliability. Weibull plots of the combined effects of each aging condition and each surface finish revealed lifetime for NiAu/SAC305/Cu was reduced by approximately 40% by aging at 150°C; less degradation was observed for NiAu/SAC305/NiAu. Further reduction of characteristic life-cycle number was observed for NiAu/SnAg/NiAu joints. Microstructure was studied, focusing on its evolution near the board and package-side interfaces. Different mechanisms of aging were apparent under the different joint configurations. Their effects on the fatigue life of solder joints are discussed.

Measuring the effect of substrate and component finishes on the reliability of isotropic electrically conductive adhesive joints

PurposeAs the transition away from lead‐containing solders gathers momentum, isotropic conducting adhesives (ICAs) are being considered as possible replacements for conventional SnPb solder in a range of applications. Consequently, the reliability of ICA joints is under scrutiny. The purpose of this paper is to report the effect of printed circuit board (PCB) and component finishes on the reliability of ICA joints.Design/methodology/approachPrevious work by the authors identified a suitable test regime to generate relevant reliability data. In the present work, those tests are employed to investigate whether the finishes on the components and/or PCBs have any effect on the reliability of the ICA joints after exposure to damp heat conditions.FindingsThe effect of different finishes is found to be very adhesive material dependent. Two adhesives are studied, and for one material the joint reliability is relatively unaffected by changes in component or PCB surface finish. However, for the second material, and components with a high‐tin content‐plated finish, the joints display a less stable resistance. The surface finish on the PCB is found to have a smaller effect on joint reliability than the component finish, with results dependent on adhesive material type. Performance with one material exhibited little difference in reliability irrespective of the PCB surface finish. For the second material, the joint reliability performance with components having the electroless nickel/immersion gold finish, is not as good as that with components having the immersion tin or silver finishes.Originality/valueThe paper shows that surface finish is an important factor in determining the conductivity of ICA joints during exposure to the 85°C/85%RH environment. Systems containing tin are more prone to lose conductivity and, conversely, noble metal systems are more immune to degradation. This is a major concern as the industry is showing many signs that the component termination of choice will be pure tin.

Drop impact reliability testing for lead-free and lead-based soldered IC packages

Board-level drop impact testing is a useful way to characterize the drop durability of the different soldered assemblies onto the printed circuit board (PCB). The characterization process is critical to the lead-free (Pb-free) solders that are replacing lead-based (Pb-based) solders. In this study, drop impact solder joint reliability for plastic ball grid array (PBGA), very-thin quad flat no-lead (VQFN) and plastic quad flat pack (PQFP) packages was investigated for Pb-based (62Sn–36Pb–2Ag) and Pb-free (Sn–4Ag–0.5Cu) soldered assemblies onto different PCB surface finishes of OSP (organic solderability preservative) and ENIG (electroless nickel immersion gold). The Pb-free solder joints on ENIG finish revealed weaker drop reliability performance than the OSP finish. The formation of the brittle intermetallic compound (IMC) Cu–Ni–Sn has led to detrimental interfacial fracture of the PBGA solder joints. For both Pb-based and Pb-free solders onto OSP coated copper pad, the formation of Cu 6Sn 5 IMC resulted in different failure sites and modes. The failures migrated to the PCB copper traces and resin layers instead. The VQFN package is the most resistant to drop impact failures due to its small size and weight. The compliant leads of the PQFP are more resistant to drop failures compared to the PBGA solder joints.

Failure behavior of small outline J Lead/Sn–X (X = AgCu or Pb) solder joints under thermomechanical fatigue test

A failure analysis of thermomechanical fatigued small outline J (SOJ) lead/Sn–X (X = AgCu and Pb) solder joints over different printed circuit board surface finishes underwent pull test has been conducted. Experimental results showed that the dimple-fracture feature was due to the occurrence of fracture in the solder matrix and the intergranular-fracture feature was due to the occurrence of fracture in the IMC layer. The growth rate of the IMC layer at the SOJ/SnAgCu interfaces is much faster than that of the IMC layer at the SOJ/SnPb interfaces. The increase in the thickness of IMC (Cu 6Sn 5) and the fatigue of the SnPb solder matrix upon thermal cycling resulted in the decline of the strength of the SOJ/SnAgCu/(Ni/Au) and SOJ/SnPb/(Ni/Au) solder joints, respectively. After a pull test, the existence of different phases and the coverage variation of these phases on the fracture surface disclosed not only the distribution of stress but also the thermal fatigue properties of bulk solder alloy and IMC between the SOJ lead and solder.

Effects of different printed-circuit-board surface finishes on the formation and growth of intermetallics at thermomechanically fatigued, small outline J leads/Sn-Ag-Cu interfaces

The effects of printed-circuit-board (PCB) surface finish and thermomechanical fatigue (TMF) on the formation and growth of intermetallic compounds (IMCs) between small outline J (SOJ) leads and Sn-3.0Ag-0.5Cu solder were investigated. The thickness of the IMC layer formed initially at the as-soldered SOJ/Sn-Ag-Cu interface over a Ni/Au PCB surface finish was about 1.7 times of that over the organic solderability preservative (OSP) PCB surface finish. The parabolic TMF-cycle dependence clearly suggests that the growth processes are controlled primarily by solid-state diffusion. The diffusion coefficient for the growth of the total IMC layer at the SOJ/Sn-Ag-Cu interface over the Ni/Au PCB surface finish is the same as that over the OSP PCB surface finish, and thus, the total IMC layer at the SOJ/Sn-Ag-Cu interface over the Ni/Au PCB surface finish is thicker than that over the OSP PCB surface finish. Using the Cu-Ni-Sn ternary isotherm, the anomalous phenomenon that the presence of Ni retards the growth of the Cu3Sn layer while increasing the initial growth of the Cu6Sn5 layer can be addressed.

Effects of different printed circuit board surface finishes on the formation and growth of intermetallics at thermomechanically fatigued small outline J leads/Sn–Pb interfaces

The effects of printed circuit board (PCB) surface finish and thermomechanical fatigue (TMF) on the formation and growth of intermetallic compounds (IMCs) between Small Outline J (SOJ) leads and Sn–37Pb solder were investigated. The thickness of the IMC layer formed initially at as-soldered SOJ/Sn–Pb interface over Ni/Au PCB surface finish was about 1.3 times of that over OSP PCB surface finish. The parabolic TMF cycle dependence clearly suggests that the growth processes are controlled primarily by solid-state diffusion. The diffusion coefficient for the growth of total IMC layer at SOJ/Sn–Pb interface over Ni/Au PCB surface finish is the same as that over OSP PCB surface finish and, thus, the total IMC layer at the SOJ/Sn–Pb interface over Ni/Au PCB surface finish is thicker than that over OSP PCB surface finish. Using the Cu–Ni–Sn ternary isotherm, the anomalous phenomenon that the presence of Ni retards the growth of the Cu 3Sn layer while increasing the initial growth of the Cu 6Sn 5 layer can be addressed.

Effect of reflow profile on wetting and intermetallic formation between Sn/Ag/Cu solder components and printed circuit boards

The wetting performance and intermetallic formation of a Sn/Ag/Cu alloy on printed circuit board (PCB) surfaces and on component terminations were studied in this work. Two different PCB surface finishes, immersion gold over electroless nickel (Ni/Au) and an organic solderability preservative (OSP), were studied. Chip components with Sn/Pb coating and a gull‐wing type component with 100% Sn coating were used in these experiments. Different reflow profiles were tested, and the dependence of the wetting performance, intermetallic layer thickness and the microstructure of the solder joints on the reflow profile were investigated.It was found that reflow process conditions did not significantly influence the spreading or intermetallic formation on either of the surfaces. Neither the wetting onto the component nor the general microstructure of the solder joints varied significantly with the reflow profile. When a Sn/Pb ‐coated component was used, the content and size of Pb‐rich phases in the solder joint increased with a longer time above liquidus or a higher reflow peak temperature.

Interfacial reactions between Ni substrate and the component Bi in solders

The reactions between Ni and liquid Bi at 300, 360, 420, and 480°C were studied. Bismuth is an important element in many electronic solders, while Ni is used in many printed circuit board surface finishes. It was found that the only intermetallic compound formed was NiBi3. The other intermetallic compound NiBi, which is therm odynamically stable at these temperatures, did not form. Reaction at 300°C produced a thick reaction zone, which is a two-phase mixture of NiBi3 needles dispersed in Bi matrix. The thickness of the reaction zone increased rapidly with reaction time, reaching 400 µm after 360 min. Reactions at 360 and 420°C produced very thin reaction zones, and the major interaction was the dissolution of Ni into liquid Bi. Reaction at 480°C produced extremely thin reaction zone, and the dissolution of Ni into liquid Bi was very fast and was the major interaction. It is proposed that the formation of the reaction zone is controlled by two factors: the solubility limit and the diffusivity of Ni in liquid Bi. Small diffusivity and small solubility limit, i.e., lower temperature, tend to favor the formation of a thick reaction zone. In addition to the NiBi3 formed within the reaction zone, NiBi3 also formed outside the reaction zone in the form of long needles with hexagonal cross section. The dissolution rate of Ni into Bi is comparable to that of Ni into Sn at the same temperature, and is much slower than the dissolution rates for Au, Ag, Cu, and Pd into Sn.