P-rich Ni Layer Research Articles

Interfacial reactions of Cu-containing lead-free solders with Au/NiP metallization

Cu-containing solder alloys have been used to identify their interfacial reactions with electroless NiP. As-reflowed, AuSn4 intermetallic compounds (IMCs) are formed in the Sn-Cu and Sn-Ag-Cu solders, but in the cases of Sn-Ag-Cu-In, In-Sn-Au IMCs are formed and are uniformly distributed in the solder. Different types of IMCs such as high-Cu (>30 at.%), medium-Cu (30-15 at.%), and low-Cu (<15 at.%) containing IMCs are formed at the interface. High-Cu and medium-Cu containing ternary intermetallic compounds (TIMCs) are found in the Sn-Cu and Sn-Ag-Cu solder joints, respectively. Medium-Cu containing quaternary intermetallic compounds (QIMCs) are found in the Sn-Ag-Cu-In joints. Initially, TIMCs and QIMCs have higher growth rates, resulting in the entrapment of some Pb-rich phase in the high-Cu containing TIMCs and some In-Sn-Au phase in the QIMCs. High-Cu containing TIMCs have a lower growth rate and consume less of the NiP layer. The spalling of medium-Cu containing TIMCs in the Sn-Ag-Cu solder increases both the growth rate of TIMCs and the consumption rate of the NiP layer. Low-Cu containing QIMCs in the Sn-Ag-Cu-In solder are stable on P-rich Ni and reduce the dissolution rate of the NiP layer. Consumption of the NiP layer can be reduced by adding Cu or In, because of the changes of the interfacial IMCs phases, which are stable and adhere well to the P-rich Ni layer during reflow.

Interfacial reactions of Sn-3.5% Ag and Sn-3.5% Ag-0.5% Cu solder with electroless Ni/Au metallization during multiple reflow cycles

The increasing industry awareness of lead-free activities has prompted original equipment manufacturers and suppliers to investigate lead-free solder systems in detail. The reliability of lead-free solders has been studied a lot recently, but the knowledge of it is still incomplete and many issues related to them are under heavy debate. In this study, the interfacial reactions of Sn-3.5Ag and Sn-3.5Ag-0.5Cu (wt.%) solders with Cu/Ni(P)/Au ball grid array (BGA) pad metallization were systematically investigated after multiple reflows. The peak reflow temperature was fixed at 260°C. It was found that relatively high consumption of Ni(P) was observed in the case of Sn-3.5%Ag solder alloys during multiple reflow cycles. A white layer of P rich Ni-Sn compound was observed above the dark Ni3P layer for Sn-3.5%Ag solder after several reflows. It was noticed that the mean thickness of the intermetallics and the dark P-rich Ni layer at the interface was decreased just by adding 0.5% Cu in Sn-3.5%Ag solder alloy with less overall interfacial reaction at the solder joint.

Effect of 9 wt.% in addition to Sn3.5Ag0.5Cu solder on the interfacial reaction with the Au/NiP metallization on Cu pads

Sn-based lead-free solders have a high melting temperature, which often cause excessive interfacial reactions at the interface. A small amount of In is added to reduce the melting temperature and to change the intermetallic compound (IMC) phases. Sn3.5Ag0.5Cu and Sn3.5Ag0.5Cu9In lead-free solder alloys have been used to identify its interfacial reactions with 2-metal layer flexible substrates. In this paper we investigate the effect of 9 at.% In addition to Sn3.5Ag0.5Cu solder during extended reflow. During reflow, Au diffuses rapidly in the molten Sn Ag Cu solder and forms AuSn 4 IMC but in the case of In-containing solder, In Sn Au IMCs form and are uniformly distributed in the solder. Some In Sn Au IMCs have been entrapped in the Sn Cu Ni In quaternary intermetallic compounds (QIMCs) due to lower diffusion rate of Au in the In-containing solder. Initially Sn Cu Ni ternary intermetallic compounds (TIMCs) and Sn Cu Ni In QIMCs form at the interface, which have higher growth rate and consume more of the NiP layer. Low-Cu QIMCs are found in the In-containing solder after 30 min reflow which are more stable in the P-rich Ni layer and significantly reduce the dissolution rate of the NiP layer. The spalling of Sn Cu Ni TIMCs in the Sn Ag Cu solder increases the diffusion rate of Sn atoms and as a consequence both the TIMCs growth rate and dissolution rate of the NiP layer also increases. In-containing solder have lower growth rate of the QIMCs and lower dissolution rate of the NiP layer than the Sn Ag Cu solder. Consumption of the NiP layer can be reduced by adding In, because of the formation of QIMCs at the interface, QIMCs are stable and are well adhering to the P-rich Ni layer during reflow.

Comparative Study of the Dissolution Kinetics of Electrolytic Ni and Electroless NiP Layers by Molten Sn3.5Ag Solder Alloy

Sn-based, Pb-free solders with high a Sn content and high melting temperature often cause excessive interfacial reactions at interfaces. Sn-3.5Ag solder alloy has been used to identify its interfacial reactions with two-metal layer flexile substrates. In this paper the dissolution kinetics of Sn3.5Ag solder on the electrolytic Ni and electroless NiP layer are investigated. It is found that during 1 min reflow the electrolytic Ni layer dissolves much less than the electroless NiP layer due to the formation of Ni3Sn and Ni3Sn2 intermetallic compounds (IMCs) on the electrolytic Ni layer. The faster nucleation of Ni3Sn4 IMC on the NiP layer is proposed as the main reason for the higher initial dissolution rate of the electroless NiP layer. A P-rich Ni layer is formed underneath the Ni3Sn4 IMC due to the solder-assisted reactions. This P-rich Ni layer acts as a good diffusion barrier layer, which decreases the dissolution rate of the NiP layer as compared to that of the Ni layer, but weakens the interface of solder joints and reduces the ball shear load and reliability. Below a certain thickness, the P-rich Ni layer breaks and an increase in the diffusion of Sn atoms through the fractured P-rich Ni layer occurs that increases the growth rate of IMCs again, and thus the dissolution rate of the NiP layer becomes higher again than for the Ni layer. It is found that a 3μm thick NiP layer cannot protect the Cu layer for more than 120 min reflow at 250°C. An electrolytic Ni∕solder system has a relatively higher shear load, a lower dissolution rate of the Ni layer, and is more protective for the Cu layer during extended times of reflow.

Interfacial reactions on electrolytic Ni and electroless Ni(P) metallization with Sn–In–Ag–Cu solder

In this study, interfacial reactions of electrolytic Ni and electroless Ni(P) metallization of the ball-grid-array (BGA) substrate with the molten Sn–9In–3.5Ag–0.5Cu (wt.%) solder alloy were investigated, focusing on identification of the intermetallic compound (IMC) phases and the IMC growth rates at various reflowing times. A fixed volume of BGA solder ball (760 μm diameter) was used on a substrate metallization pad with a diameter of 650 μm. The peak reflow temperature was fixed at 240 °C while the reflow time was varied between 1 and 120 min. The intermetallic compounds thickness was lower on the electroless Ni(P)/solder interface. The consumption of the electroless Ni(P) in Sn–9In–3.5Ag–0.5Cu was also lower than that of the electrolytic Ni. The appearance of a P-rich Ni layer at the interface played a major role in inhibiting the overall interfacial reaction in the soldering reaction with the electroless Ni(P) metallization. The stable IMC initially formed at the interface was the (Cu, Ni) 6 (Sn, In) 5 phase. During further reflowing, the (Cu, Ni) 3 (Sn, In) 4 IMC started forming because of the limited Cu content in the solder. These bright low-Cu IMCs appeared earlier at the interface of the Ni/solder system than the Ni(P)/solder system. Bulk of the Sn–9In–3.5Ag–0.5Cu solder contained Cu 6(Sn, In) 5, Au–In–Sn and Au–In–Sn precipitates embedded in the Sn-rich matrix.

Effect of reflow time on interfacial reaction and shear strength of Sn–0.7Cu solder/Cu and electroless Ni–P BGA joints

The interfacial reaction and shear strength between the eutectic Sn–0.7wt.% Cu solder and two different kinds of ball grid array (BGA) substrates (Cu and Au/Ni–P/Cu) during reflow at 255°C for up to 30min were studied. With the solder joints between the Sn–0.7Cu solder and Cu substrates, a Cu6Sn5 intermetallic compound (IMC) was formed at the interface. In the case of the electroless Ni–P substrate, the IMC formed at the interface was mainly (Cu, Ni)6Sn5. Also, a P-rich Ni (Ni3P) layer was observed as a by-product of the Cu–Ni–Sn reaction. The Ni3P layer was between the (Cu, Ni)6Sn5 IMC and the electroless Ni–P layer. The thickness of the Cu6Sn5, (Cu, Ni)6Sn5 and Ni3P layer was found to increase with the reaction time. The growth of the Cu6Sn5 IMC layer for Cu substrate was faster than that of the (Cu, Ni)6Sn5 IMC layer for electroless Ni–P substrate. The Ni content in the (Cu, Ni)6Sn5 IMC layer formed at the interface increased with increasing reaction time. In the ball shear tests, the shear strength value did not change much as a function of reflow time. In all samples, the fracture mainly occurred in the bulk solder. However, for the Sn–0.7Cu/electroless Ni–P joint reflowed for 10min, the fracture occurred in ductile and slightly brittle mode.

Growth Kinetics of IMC Formed between Sn-3.5Ag-0.75Cu BGA Solder and Electroless Ni-P/Cu Substrate by Solid-State Isothermal Aging

The growth kinetics of intermetallic compound (IMC) layers formed between Sn-3.5Ag-0.75Cu BGA (ball grid array) solder and electroless Ni-P/Cu substrate by solid state isothermal aging were examined at temperatures between 70 and 170°C for 0 to 100 days. In the solder joints between the solder ball and electroless Ni-P/Cu pads, the IMC layer was (Cu,Ni)6Sn5. Also, a P-rich Ni layer formed at the interface between (Cu,Ni)6Sn5 and original Ni-P deposit layer because of the phosphorous accumulation. These IMC layer thicknesses increased linearly with the square root of aging time and the growth was faster for higher aging temperatures. On the contrary, the shear strength decreased with the increasing temperature and time. The growth of IMC layer was mainly controlled by diffusion-controlled mechanism over the temperature range studied. The apparent activation energy calculated for the growth of the (Cu,Ni)6Sn5 IMC was 69.75 kJ/mol.

Growth kinetics of Ni 3Sn 4 and Ni 3P layer between Sn–3.5Ag solder and electroless Ni–P substrate

Interfacial reactions and growth kinetics of intermetallic compound (IMC) layers formed between Sn–3.5Ag solder and electroless Ni–P substrate were investigated at temperature between 100 and 170 °C for up to 60 days. The IMC formed at the interface was mainly Ni 3Sn 4. Also, a P-rich Ni (Ni 3P) layer was observed as a by-product of the Ni–Sn reaction. The Ni 3P layer was between the Ni 3Sn 4 IMC and the electroless Ni–P deposit layer. The thickness results fitted a power-law relationship with exponent of 0.4 for the Ni 3Sn 4 layer. The interface between electroless Ni–P and Ni 3P was planar and the thickness of the Ni 3P layer also was found to increase linearly with the square root of aging time. The thickness of the Ni 3Sn 4 and Ni 3P layer reached about 3 and 2 μm after 60 days of aging at 170 °C, respectively. The activation energy for the growth of the Ni 3Sn 4 IMC equals 49 kJ/mol.

Growth of an intermetallic compound layer with Sn-3.5Ag-5Bi on Cu and Ni-P/Cu during aging treatment

Growth kinetics of intermetallic compound (IMC) layers formed between the Sn-3.5Ag-5Bi solder and the Cu and electroless Ni-P substrates were investigated at temperatures ranging from 70°C to 200°C for 0–60 days. With the solder joints between the Sn-Ag-Bi solder and Cu substrates, the IMC layer consisted of two phases: the Cu6Sn5 (η phase) adjacent to the solder and the Cu3Sn (e phase) adjacent to the Cu substrate. In the case of the electroless Ni-P substrate, the IMC formed at the interface was mainly Ni3Sn4, and a P-rich Ni (Ni3P) layer was also observed as a by-product of the Ni-Sn reaction, which was between the Ni3Sn4 IMC and the electroless Ni-P deposit layer. With all the intermetallic layers, time exponent (n) was approximately 0.5, suggesting a diffusion-controlled mechanism over the temperature range studied. The interface between electroless Ni-P and Ni3P was planar, and the time exponent for the Ni3P layer growth was also 0.5. The Ni3P layer thickness reached about 2.5 µm after 60 days of aging at 170°C. The activation energies for the growth of the total Cu-Sn compound layer (Cu6Sn5 + Cu3Sn) and the Ni3Sn4 IMC were 88.6 kJ/mol and 52.85 kJ/mol, respectively.

Effect of reaction time and P content on mechanical strength of the interface formed between eutectic Sn–Ag solder and Au/electroless Ni(P)/Cu bond pad

In this work, shear strengths of the solder joints for Sn–Ag eutectic alloy with the Au/electroless Ni(P)/Cu bond pad were measured for three different electroless Ni(P) layers. Sn–Ag eutectic solder alloy was kept in molten condition (240 °C) on the Au/electroless Ni(P)/Cu bond pad for different time periods ranging from 0.5 min to 180 min to render the ultimate interfacial reaction and the consecutive shear strength. After the shear test, fracture surfaces were investigated by scanning electron microscopy equipped with energy dispersed x ray. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces. It was found that formation of crystalline phosphorous-rich Ni layer at the solder interface of Au/electroless Ni(P)/Cu bond pad with Sn–Ag eutectic alloy deteriorates the mechanical strength of the joints significantly. It was also noticed that such weak P-rich Ni layer appears quickly for high-P content electroless Ni(P) layer. However, when this P-rich Ni layer disappears from a prolonged reaction, the shear strength increases again.

Comparative study of the dissolution kinetics of electrolytic Ni and electroless Ni–P by the molten Sn3.5Ag0.5Cu solder alloy

Lead-free solders have high Sn content and high melting temperature, which often cause excessive interfacial reactions at the interface. Sn3.5Ag0.5Cu lead-free solder alloy has been used to identify its interfacial reactions with two-metal layer flexible substrates. In this paper we investigate the dissolution kinetics of Sn3.5Ag0.5Cu solder on electrolytic Ni/electroless NiP layer. It is found that during 1 min of reflow electroless NiP layer dissolves slightly lower than the electrolytic Ni due to the barrier layer formation between the intermetallic compounds (IMCs) and electroless NiP layer. Faster nucleation of IMCs on the electrolytic Ni layer is proposed as the main reason for higher initial dissolution. The appearance of P-rich Ni layer acts as a diffusion barrier layer between the solder and electroless NiP layer, which decreases the dissolution rate and IMCs growth rate than that of the electrolytic Ni layer, but weaken the interface and reduces the ball shear strength and reliability. After acquiring certain thickness P-rich Ni layer breaks and increases the diffusion rate of Sn and as a consequence both the IMCs growth rate and dissolution rate also increases. It is found that 3 μm thick electroless NiP layer cannot protect the Cu layer for more than 120 min at 250 °C. In electrolytic Ni shear strength does not change significantly and lower dissolution rate and more protective for Cu layer during long time molten reaction.

Interfacial reaction of Pb-Sn solder and Sn-Ag solder with electroless Ni deposit during reflow

The interfaces between electroless Ni-P deposit and Pb-Sn solder and Sn-Ag solder were formed by reflowing for different time periods to examine their microstructures and microchemistry. It was found that the Pb-Sn solder interface is more stable than the Sn-Ag solder interface. Sn-Ag solder reacts quickly with the electroless Ni-P deposit and forms nonadherent Ni-Sn intermetallic compounds (IMCs). Pb-Sn solder reacts slowly and forms adherent Ni-Sn IMC. A P-rich Ni layer, revealed as a dark layer under scanning electron microscopy (SEM), is formed on the electroless Ni-P deposit due to the solder reaction. For short reflow times, this P-rich Ni layer consists of only Ni3P compound, but during prolonged reflow, new crystals of Ni2P, Ni5P4, and NiP2 are also found to be formed from the amorphous electroless Ni-P layer.

Studies on the Interfacial Reaction of Screen-Printed Sn-37Pb,Sn-3.5Ag and Sn-3.8Ag-0.7Cu Solder Bumps on Ni/Au and OSP finished PCB

In this study, three solders, Sn-37Pb, Sn-3.5Ag, and Sn-3.8Ag-0.7Cu were screen printed on both electroless Ni/Au and OSP metal finished micro-via PCBs (Printed Circuit Boards). The interfacial reaction between PCB metal pad finish materials and solder materials, and its effects on the solder bump joint mechanical reliability were investigated. The lead free solders formed a large amount of intermetallic compounds (IMC) than Sn-37Pb on both electroless Ni/Au and OSP (Organic Solderabilty Preservatives) finished PCBs during solder reflows because of the higher Sn content and higher reflow temperature. For OSP finish, scallop-like <TEX>$Cu_{6}$</TEX> /<TEX>$Sn_{5}$</TEX> and planar <TEX>$Cu_3$</TEX>Sn intermetallic compounds (IMC) were formed, and fracture occurred 100% within the solder regardless of reflow numbers and solder materials. Bump shear strength of lead free solders showed higher value than that of Sn-37Pb solder, because lead free solders are usually harder than eutectic Sn-37Pb solder. For Ni/Au finish, polygonal shaped <TEX>$Ni_3$</TEX><TEX>$Sn_4$</TEX> IMC and P-rich Ni layer were formed, and a brittle fracture at the Ni-Sn IMC layer or the interface between Ni-Sn intermetallic and P-rich Ni layer was observed after several reflows. Therefore, bump shear strength values of the Ni/Au finish are relatively lower than those of OSP finish. Especially, spalled IMCs at Sn-3.5Ag interface was observed after several reflow times. And, for the Sn-3.8Ag-0.7Cu solder case, the ternary Sn-Ni-Cu IMCs were observed. As a result, it was found that OSP finished PCB was a better choice for solders on PCB in terms of flip chip mechanical reliability.

Effect of Isothermal Aging on Ball Shear Strength in BGA Joints with Sn-3.5Ag-0.75Cu Solder

The ball shear strength of BGA solder joints during isothermal aging was studied with Sn–3.5Ag–0.75Cu solder on three different pads (Cu, electroless Ni–P/Cu, immersion Au/Ni–P/Cu) at temperature between 70 and 170°C for times ranging from 1 to 100 days. The reliability of solder ball attachment was characterized by mechanical ball shear tests. As a whole, the shear strength of BGA joints decreased with increasing temperature and time. The shear strength for both the immersion Au/Ni–P/Cu and electroless Ni–P/Cu pads was consistently higher than that of the Cu pad for all isothermal aging conditions. The fracture surface showed various characteristics depending on aging temperature, time, and the types of BGA pad. The P-rich Ni layer formed at the interface between (Cu, Ni)6Sn5 and Ni–P deposits layer after aging, but fracture at this interface was not the dominant site for immersion Au/Ni–P/Cu and electroless Ni–P/Cu pad.