| In the field of electronic packaging,the reliability of solder joints is regarded as an important factor which determines the service life of components.Generally,the reliability of solder joints is measured by the type and thickness of the solder joint interfacial IMC and the shear strength of the solder joint.The traditional Sn-based solder and Cu substrate often react quickly to form an excessively thick brittle IMC layer,which reduces the reliability of the solder joint.Currently,a Ni(P)electroless plating layer is inserted between the solder and the substrate to suppress the growth of the interface IMC.However,the P in the Ni(P)plating layer will complicate the interface reaction.The Ni(P)plating layer will form a brittle Ni3P layer or Ni2SnP layer during the aging of the solder joint.After that,the interface of the solder joint will be embrittled and the interfacial IMC layer will be peeled off,reducing the reliability of the solder joint.In this experiment,it is attempted to add nano Cu particles to the Ni(P)electroless plating to form a Ni(P)-Cu nano composite plating.This nano composite plating is soldered with Sn58Bi lead-free solder and solid-state aging at constant temperature.It is studied that the effect of this composite plating on IMC growth at solder joint interface and tensile shear strength of the solder joint.In order to obtain the most appropriate thickness of the Ni(P)electroless plating during the soldering reaction with the Sn58Bi solder,different thickness Ni(P)plating were obtained on the Cu substrate by controlling the plating time.After that,the substrates were soldering with the Sn58Bi solder.And the solder joints were solid aging at 115? for different time.It was found that the substrate surface roughness was positively correlated with the wettability,and both had a similar quadratic function relationship with the plating thickness.When the plating thickness was 0.5?m,the substrate surface roughness was the lowest and the solder had the worst wettability on the substrate.During the solid aging process,the thicker the Ni(P)plating layer,the later the Ni(P)plating layer transforms into the Ni2SnP layer,and the lower the growth rate of the interface IMC.When the thickness of the plating reaches 1.5 ?m,the Ni(P)plating is so thick that the interfacial reaction is dominated by the Sn-Ni interaction,and the depletion of the Ni(P)plating is low even after aging for 240 h.As the thickness of the Ni(P)plating increased,the morphology of the interface IMC in the solder joints changed from pebble-type to columnar-type after solid aging for 240 h,and finally formed a block-type.When the thickness of the plating layer is 0.1 ?m,the columnar-type IMC formed at the interface will promote the rapid growth of the solder joint interfacial IMC.It can be concluded that in Sn58Bi solder joint,the appropriate thickness of the Ni(P)plating layer should be ranged from 0.5 ?m to 1.0 ?m.After the appropriate thickness of the Ni(P)plating was obtained,nano-Cu particles with a content of 0.8 g/L were added into the plating solution during the plating process,and the stirring speed was controlled to 220 rpm using an electric stirrer.Ni(P)-Cu nano-composite plating and Sn58Bi solder were soldering and solid aging.A Ni(P)plating with the same plating time was used for comparison.The results show that the addition of nano-Cu particles in the plating solution reduces the volume of the unit cell on the surface of the plating and makes the unit cell dense,and at the same time reduces the deposition rate of the Ni(P)plating.After solid aging of the solder joint,Cu in the plating layer participates in the interface reaction between Ni-Sn and promotes the rapid formation of planar(Cu,Ni)6Sn5 at the interface.In the top view,the(Cu,Ni)6Sn5 crystal at the solder joint interface The grains have a close-packed morphology.Atomic diffusion can only pass through irregular grain boundaries,and the diffusion rate is low.Therefore,even after the Ni(P)-Cu plating is transformed into the Ni2SnP layer,the solder joints are effectively suppressed.And the Cu substrate is protected during solid aging.In order to study the effect of nano Cu particles added into the Ni(P)plating on the shear behavior of the solder joints,tensile shear tests were performed on the solder joints at a tensile rate of 1 mm/min.After shear test,observe the fracture morphology and fracture interface structure of the solder joint,and record the shear strength of the solder joint.The results show that in Sn58Bi/Ni(P)solder joints,the fracture position of the solder joint gradually moves from the solder to the interior of the IMC as the aging time increases.The solder joint fracture mode is gradually dominated by the brittle fracture of the IMC.As a result,the shear strength of the solder joint is also reduced.When the solder joint after solid aging for 120 h,the solder joint shear strength is only a quarter of that just after soldering.The Sn58Bi/Ni(P)-Cu solder joint has the opposite shear behavior compared with the Sn58Bi/Ni(P)solder joint.As the aging time increases,the fracture position of the solder joint gradually moves from the IMC to the solder.The ductile fracture in the solder dominates shear mode.The shear strength of the solder joint is basically stable at 41 MPa. |
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