Joining soldered joints

C12-0091. To make a good solder joint, the liquid metal solder must adhere well to the metal surfaces being joined. Flux is used to clean the metal surfaces. What types of substances must flux remove ... 

Soldering is a technology that has been used extensively for utilitarian and decorative purposes for thousands of years. In the last 100 years, soldering has been raised from art to science for joining electrical assemblies. By the late twentieth century, the tin-lead solder joint and properties of its constituent materials had been studied well enough that it could be modeled for purposes of predicting reliability. 

There are three important constituents of a solder joint the two materials or surfaces to be joined and the solder itself Each has its own set of attributes and variables that contribute to the ease and quality of soldering. A delicate balance of material conditions and process parameters determine the solder joint appearance, resultant strength, and reliabihty of the soldered assembly. The composition of the solder, the surface finish on lead, pad or plated through-hole (PTH), environmental factors, chemical implications, and thermal conditions aU impact the soldering process. Each will be discussed. 

Solder is the cement that joins lead to pad, imparts the mechanical robustness required for a reliable assembly, and also possesses the electrical conduction needed for circuitry. Generally composed of an alloy of metals, it is chosen to melt at a temperature compatible with other materials associated with the soldered assembly. Once molten, the solder must wet the component lead and bond pad. Upon solidification, the resultant solder joint must provide bond strength to survive differences in thermal expansion rates of the associated component assembly. There must be compatibility of the solder alloy and related materials to requirements for assembly and service at elevated operating temperatures as well as resistance to mechanical shock and vibration. 

Brighteners. Sometimes Ag is added to solder to improve wetting, making the solder joint smooth and shiny. When not used as a scavenger, the addition of Ag should be avoided as it is not usually needed for most electronic joining applications with Sn-Pb. 

There are three important Au-Sn intermetallics AuSn, AuSn2, and AuSri4, Eutectic points are at about 85 wt percent Sn (215°C) and 20 percent Sn (280°C). Gold can also form a ternary compound when joined with Sn-Pb solder, exhibiting a eutectic at about 175°C. This can radically alter reflow characteristics and solder-joint performance. Au, like Pb, is poorly soluble in Sn, which means that it dissolves in hot Sn, Pb, or Sn-Pb solder and precipitates out of solution during the liquid-to-solid phase change of the solder. When it does, it forms brittle, platelike crystals that appear needle-like in cross-section. " ... 

Selection of the flux type depends on the solder alloy, metal finishes on the board and components, condition of the surfaces to be joined, the type of soldering process selected, required solder-joint attributes, and the intended final use of the assembly. 

In order to predict the mechaiucal properties of the solder joint, it is necessary to understand the various structures and manner in which their properties effect those of the whole joint. The solder joint is comprised of the solder alloy, the substrate materials that are being joined together, and the interfaces between the substrate materials and the solder that is typically comprised of one or more intermetallic compound... 

IMC) layers. In some cases, the base materials have a solderable finish to which the solder has been joined. Then, there is a solder/protective finish interface that typically also generates an IMC layer. There is also an interface between the substrate material and the solderable finish. A solderable finish is covered with a protective finish before soldering. The protective finish is dissolved into the molten solder during the assembly process. The dissolved protective finish can impact the mechanical properties of the solder, the extent of which depends upon the thickness of the coating and the volume of the solder. In summary, the mechanical performance of the solder joint is a function of the mechanical properties of the solder, the base materials (including solderable finishes, if present), and the mutual interfaces. 

To achieve proper solder joints, the molten metal must be in direct contact with the metal surface to be joined. For bonding to occur, some form of interaction must take place at the interface. In the case of Sn-Pb solders bonding to Cu, Sn must diffuse into the Cu surface, forming the intermetallic compounds CusSn and CueSus with the bulk of the solder alloy bonded to this layer. Solder-to-solder bonding results from alloy reacting at the liquid interfaces. 

Although solder joints are found with various geometries, for the sake of discussion a simple geometry consisting of two essentially planar metalhzations joined by solder was selected to illustrate the concept. Whereas the metallizations may be only 10 pm in thickness, solder reactions generally do not consume the metalhzation material (exceptions include thin, passiva-... 

FIG. 1 A sketch (not to scale) of a typical solder joint in microelectronics— two planar terminal metallization pads joined by the solder bulk or matrix, with intermetallic compounds formed at the metallization/solder matrix interfaces. The vertical dimension can vary from approximately 100 pm to... 

As early as a few thousand years ago, metals were joined together by utilizing low-melting alloys. Beginning with the electronic age, solder joints served two functions to conduct electricity, and to create a joint with sufficient mechanical integrity. With the continuous reduction in solder joint dimensions required in microelectronics, the ability to adequately wet the solder surfaces has become increasingly important. Therefore, there is an increased reliance on inerting to assure functionality and sound solder joints. 

One of the simplest and most useful tests performed on reflow-attached flip chips to determine the adequacy of the solder joints (i.e., C4) is a tensile pull test (Fig. 31). This is done by adhesively attaching a metal stud to the back of a joined chip and pulling the joints in tension at a slow strain rate (approximately 1.0x10 sec ). The pull force is measured during the test using an appropriate load cell. The pull strength is a useful parameter, but the failure mode is a very important indicator of joint quahty. Planar failure at the solder joint interfaces is indicative of a weak and unacceptable interface condition. 

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