The Mechanical Property Consideration to Achieve High-Reliability Lead-Free Solder Joints
The basic solder mechanical properties include stress and strain, creep resistance, and fatigue resistance. Shear strength is also an important factor in the reliability of solder joints because most solder joints are subjected to shear stress when used. Creep is another factor in solder joint reliability testing. Creep is the plastic deformation that occurs when temperature and stress are constant. This time-dependent deformation can happen at any temperature above absolute zero. However, creep is only manifested at "active" temperatures. Fatigue refers to the decrease in reliability, leading to the failure of solder joints under alternating stress. Fatigue cracking usually starts from several small cracks and enlarges under the cyclic action of stress, resulting in decreased solder joint reliability and failure of component soldering. Solders in electronic packaging and assembly are typically subject to low-cycle fatigue and high stress. Thermal fatigue is another test mode used to characterize solder. It is compulsory to test and analyze the shear strength, pull strength, creep, and other conditions of the solder joints to achieve high-reliability solder joints.
Shear strength is an important factor that reflects the reliability of solder joints. The following figures represent the shear strength of Sn3.5Ag and Sn4.0Ag0.5Cu alloys on pads that have undergone the OSP process and Electroless Nickel Immersion Gold (ENIG) process.
As shown in the figures above, the shear strength of the alloys on the pads with the ENIG surface treatment process is better than the pads with the OSP surface treatment process. The shear strengths of SAC alloy and SA alloy are similar. Shear strength decreases with aging.
The thickness of the IMC layer in the solder joint has a critical impact on the solder joint reliability. Both thick or thin IMC layers will negatively affect the stability and service life of the solder joints. The following figures show the effects of shear strength and shear speed on the IMC layer thickness.
It can be seen from the above figure that the faster the shear velocity, the greater the effect on the IMC layer thickness. However, alloy composition has a minute effect on the IMC layer thickness.
The thickness of the IMC layer influences the reliability of solder joints. The impacts of tensile strength and tensile speed on the IMC layer thickness are shown below.
The shear strength and pull strength have a significant impact on the IMC layer thickness. An experiment was conducted to detect the influences of shear strength and pull strength on the IMC layer thickness for two different alloys and surface treatment processes.
The intersection curves of SAC alloy and SnPb alloy representing creep are shown in the figures below.
For better solder joint reliability, it should control the shear strength and tensile strength, as well as the thickness of the IMC layer, to make it meet the solder reliability requirements. When using electronic devices, the ambient temperature changes spontaneously. The power cycle of the chip influences the surrounding temperature. The thermal expansion coefficients of the chip and substrate are different. Therefore, thermal stress is generated in the solder joints and causes fatigue. Meanwhile, the melting point of the solder is relatively low compared with the ambient temperature. The solder joints will have obvious viscosity degradation, resulting in creep and solder joint fracture. The external failure mode is manifested as electrical signal transmission distortion, including poor electrical contacts, short circuits, and open circuits.
Fatigue and creep have interaction under certain conditions. Creep accelerates the formation and extension of cracks. The damage caused by cyclic cracking promotes the progress of creep, significantly shortening the cycle life. However, electronic products in the aerospace field are usually under harsher temperature cycle conditions. The fatigue and creep of solder joints have become an inherent danger of electronic product failure. Therefore, controlling the fatigue and creep of solder joints has become an important topic in electronic packaging.
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