Wetting, solder requirements

FIGURE 42.15 This shows a test for some surface mount components that pass the five second dip and look test, but clearly show an issue with surface oxides prior to reaching a very acceptable final wetting force. The supplier of these parts, using dip and look testing, certified them to meet solderability requirements but the end... 

The process window for lead-free soldering requires higher peak reflow temperatures and longer times above liquidus. Soldering in a nitrogen environment improves wetting and reduces oxidation of the flux residues and the solder. Due to the higher reflow temperature. 

The next critical component for this material was the development of the flux and activator system for the paste. This proved to be a key factor in the success of this project. As the wettability of lead-free alloys is lower than eutectic Sn-Pb, a suitable flux system was critical to the successful introduction of the MD s conversion to lead-free. In addition to the flux system having the properties to promote wetting, the requirements for screen printing, stencil life, tackiness, and so on were also important when evaluating a solder paste system. Flux technology is discussed in... 

The contact between the aluminium layers and the ceramic substrate requires a joining material which will wet both metal and ceramic, and solders such as the conventional Pb-Sn alloy have been used which are molten during the annealing process. The contact between the solder and the aluminium layer is frequently unsatisfactoty because of the intervention of the AI2O3 layer, and a practical solution appears to be to place drree layers of metal clrromium in contact widr the aluminium, copper in contact with the clrromium, and gold between the copper layer and the solder. 

In order for soldering to be successful many requirements have to be met. One of these is that the surface has to be wetted by the solder (figure 11.7.3)... 

Normally a flux is used to remove oxides from surfaces when brazing or soldering, and a gas-oxide torch also requires the use of a flux. However, if the flame is oxygen-rich, as the flux is boiled or burnt off, the excess oxygen from the torch will oxidize the metal s surface before the solder can wet the surface. In a reducing flame, as the flux is boiled or burnt off, the reducing flame will maintain the pure metal until the solder can flow over and wet the surface. 

The stick solder formulation (Table 13.5) can be made in several ways. The components can be mixed as a hot melt and cast into small-diameter tubes. The components can also be ground to 60-mesh powder and then compressed into void-free rods. To apply, the area to be bonded is heated so that when the stick solder comes into contact with the surface, it melts and wets the substrate. An elevated-temperature cure is then required to crosslink the adhesive. 

The lack of a tme thickness measnrement control is a key issue to OEMs that require control metrics. If the OSP coating is too thin, the PCBs will have inadequate protection. If the coating is too thick, there is a possibility for poor solder wetting and poor PTH filling. The OSP coating is invisible to the eye, so there is no way to inspect for unprotected copper if the OSP coating was not applied correctly. 

A minimum temperature of 230°C is required to ensure adequate wetting and spreading by the Pb-free solder on circuit board pads as well as on component leads and terminations. Temperatures exceeding 245°C increase the likelihood of thermal damage to larger, plastic-molded packages (e.g., BGA and QFP devices). When temperatures exceed 260°C, there is the potential for thermal degradation of passive chip components (chip capacitors, indnctors, or filters) as well as to circuit board structures (e.g., vias) and laminate materials. In the case of Sn-Pb eutectic solder that melts at 183°C, the minimum process temperature of 215°C ensures adequate solderability, which is 15°C lower than that of a nominal Pb-free process. The available Sn-Pb process window is 215°C to 260°C, or a AT equal to 45°C, rather than the AT of 30°C for the Sn-Ag-Cu Pb-free solders (T di = 217°C). 

With the exception of tin-lead surface finish—HASL—there is no requirement to prodnce a positive meniscus for meeting class 3 acceptability. The solder should wet the hole completely, but it does not have to show positive meniscus. 

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. 

This is the most basic of requirements. The solder has to be in contact with the materials to be joined. The contact area of the solder is not wholly important as long as the solder is in point contact with the surface to be soldered when it reaches hquidus. Surface tension effects and metallurgical wetting will complete the spreading of solder contact. 

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