Fluxes Surface finish

Ion Bombardment. A third method which improved the surface finish was ion bombardment of the coating surface during the deposition process. This was accomplished by negatively pulse biasing the vibrating pan to approximately 400 volts for 50 microseconds every 2 milliseconds. This produced a 5-10 milliamp current of ions. The effect of the ion flux is to both increase the coating rate and improve surface finish. 

The intrinsic solderability performance of Pb-free solders is being improved by two means. First, new flux formulations are available that more effectively reduce the surface tension of the solder. Second, alternative surface finishes can be specified for the component I/Os and/or circuit boards that improve wetting and spreading activity exhibited by the Pb-free alloys. 

The impact of changing fluxes This is especially useful for lead-free assembly development, given the myriad of surface finishes the assembly engineer is now faced with. 

The importance of the previous information cannot be over stated as the industry transitions to Pb-free assembly. A test that provides everything in the previous list as its output can improve a process. It can also be used to improve a supplier or disqualify a suppher, select a flux or a surface finish, and/or the best combination of both to minimize defects. 

Unlike the PCB world where the basis metals used are inherently solderable, such as copper or nickel, the component world deals in basis metals that are non-solderable such as stainless steel or brass (solderable with fluxes not typically found in the electronics industry), marginally solderable (such as Alloy 42—a nickel iron alloy), to easily solderable (such as C194 copper alloy). This fact needs to be realized when designing with or choosing a metal or alloy for a given component and the simple question, Will this part solder if the surface finish application or quality is marginal needs to be asked. If the answer is no, as when using stainless... 

Certain no-clean fluxes are not compatible with each other. Their chemical interaction could result in corrosion on the PCA. To avoid corrosion issues, solder paste flux, wave solder flux, and repair flux should be tested separately and in combination for SIR and ECM with the board surface finish. 

In terms of printability, tack, slump, and solder balling, there is no clear and consistent difference between the Sn-Pb and lead-free solder pastes (Ref 59-60), because these performances depend on the solder paste formulation, not directly on the solder alloy. Very clear and consistent differences have been observed, however, in wettability between the tin-lead and lead-free solder pastes. In general, the wettability of lead-free solder paste is not as good as the tin-lead solder paste. For example, lead-free solder paste exhibits very limited spreading on OSP during reflow, and exposed corners after reflow are quite common, unless overprint or round corner pads are used. The difference in wettability between OSP and ENIG surface finishes, which is already evident for the tin-lead solder, becomes even more pronounced for lead-free solders. This has been observed with a variety of solder paste and flux formulations from a number of vendors. 

The surface finishes selected were Immersion Silver, Immersion Tin, two HASL (lead-free) and four Organic Surface Protections (OSPs). Four solder alloys were planned for testing, but two were eliminated due to cost, leaving SACX and Sn/Cu/Ni. Five fluxes were evaluated, two of which were chosen because they were commonly used in other experiments. 

The primary raw materials used in the surface finishing process include surface cleaning agents, metals for hot-dip deposition, fluxes, chemicals for plating and anodizing baths, and treatment reagents for process effluents. The use of effluent chemicals is required to reduce hexavalent chromium to trivalent chromium to allow for the precipitation of heavy metal content and to ionize the cyanide content. 

However, mainstream SMT soldering applications using eutectic Bi-Sn solder and conventional fluxes and surface finishes have not been reported. This may be due to the fact that the wetting characteristics of the Bi-Sn solder are generally not as favorable as Sn-Pb solder with existing flux materials and surface metallizations. 

---