Wetting, solder types

FIG. 74 Cross sections depicting wetting angles of Pb-Sn (type A) and lead-free solders (types B-D) as a function of board finish. (From Ref. 33.)... 

Surface finish is defined as the outerlayer metallization on a component or substrate being soldered. Both the type and quality of the surface finish have a major impact on surface sol-derability and on process soldering ability. Here, solderability is defined as the ability of the surface to be wetted by a solder, and the forming of an intermetalhc layer between the solder... 

Solderability Lead-free solder does not wet as well to solderable surfaces as leaded solder does, so pad size, solder quantity, and flux type and quantity must be taken into account. 

Surface Finishes. Tin-lead solder alloys (e.g., 63/37) are the most popular alloy used for surface finishes on PCBs. Other surface finishes are rapidly finding their way onto the PCBs. IPC-6012 lists more than 20 different surface finishes that are now in use for PCBs. Lead-free alloys are also appearing on PCBs. It is important to understand that the composition and type of the surface finish influence solderability. The procurement documentation must state specifications for surface finish. Methods available for analyzing the alloy composition on the plated PCB include wet analysis, atomic absorption, and x-ray fluorescence (XRF). XRF is popular because of the ease of obtaining the alloy composition and thickness nondestructively. 

When soldering wires to terminals, the lead outline must be visible and good wetting between the wire and terminal must be evident to be acceptable. The wire insulation gap can be near zero if the insulation has not melted into the solder joint. Slight melting of the insulation is acceptable. If the insulation gap is too large and allows potential shorting of wire to an electrically noncommon conductor, the joint is unacceptable. If the wire insulation is severely burned and the melt by-product intrudes into the solder joint, the joint is unacceptable For round post terminals, the contact of wrap must be 180° for Class 3 and 90° for Classes 1 and 2. Most of the other terminal types, such as pierced terminals, require 90° of wire wrap (see Fig. 52.33). 

Pinch and Sjostak utilized this type of equipment to characterize solderability of various Cu pastes on ceramic circuit boards. This type of test is particularly useful in distinguishing minor performance variations in materials that typically exhibit good solderabUity. There are numerous suppliers for this type of wetting balance test apparatus. ... 

Rework for lead-free solders has been found to be more difficult, because the lead-free solder alloys typically do not wet or wick as easily as the Sn-Pb solder due to their difference in wettability. This can be easily seen with QFP packages. In spite of these differences, successful rework methods (both manual and semi-automatic) have been developed (Ref 74-75) with lead-free solders (Sn-Ag-Cu, or Sn-Ag), for many different types of components. Most of the rework equipment for tin-lead can still be used for lead-free solder. For area array packages, it is helpful to use a rework system with split vision and temperature profiling features. The soldering parameters must be adjusted to accommodate the higher melting temperature and reduced wettability of the lead-free solder. The other precautions for tin-lead rework (such as board baking) still apply to lead-free rework. 

De-wetting often is due to a lack of flux activity. This rarely occurs with water-washable type pastes because these pastes are highly activated. Lower-activity solder pastes tend to create this on more... 

Flux is sometimes thought of as a catalyst that lowers the surface tension between the molten solder and a metal surface [98]. In reality, the chemistry of flux interactions at oxide surfaces can be very complicated and involve acid-base, oxidation-reduction, and coordination-type and adsorption-type reactions discussed in later sections [102-104]. Spalik prefers to think of most fluxes used for electronic soldering as substances that react as Bronsted-Lowry acids with metallic oxides to form their respective salts and water, and that the salts serve as surfactants that promote solder wetting. 

Although fundamental rehability aspects depend on such factors as the type and stabiUty of microstructures of the interconnection systems, the ultimate product rehabihty depends on several additional factors imposed by various constituent parts of an assembly. These include the chip carrier and its surface finish, component structure, lead materials and finishes, and their combined effects on the bulk solder interconnection. For example, electroless nickel/itnmersion gold (ENIG) is a popular board finish due to its long shelf fife and excellent wetting characteristics with eutectic Sn Pb solders. However, several solderabiUty and reliability concerns that are primarily due to complex interfacial reaction products that occasionally cause unexpected failures of eutectic Sn Pb and Pb-free solder joints have surfaced. The major aspects are discussed in the following sections. 

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