Fillet formation

FIGURE 7A.10 Variation of power number with impeller speed for two-phase (gas-liquid) and three-phase (gas-liquid-solid) stirred reactors. Two phase (solid-liquid) system. A, fillet formation B, disappearance of fillets C, off-bottom suspension of solids D, recirculation of mixture. Three phase (gas-liquid-sohd). A, no dispersion of gas solid settled on bottom B, gas dispersed beginning of solid suspension C, gas dispersed off-bottom suspension of solids D, recirculation of mixture. (Reproduced from Rewatkar et al. 1991 with permission from American Chemical Society. 1991, American Chemical Society.)... 

FIGURE 44.2 Solder fillet formation (a) Cross-section of an SMT solder joint (b) a PTH solder joint in cross-section. In the case of the SMT solder joint, solder has wet to the pad and lead. The low wetting angles and good fillet formations are indications of good solder wetting. The PTH solder joint shows good fillet formation on the secondary side of the PWB. At the top, the primary side, the fillets are not as well formed, probably due to lower temperature at the top side of the board. (Courtesy of Hewlett-Packard.)... 

Figure 1 compares ENIG boards soldered with SnAgCu and SnCuNi, which are lower in voiding and have good fillet formation. 

During first-pass soldering at 260°C, no sign of topside hole fill was observed. When the pot temperature was raised to 274°C, acceptable hole fill was achieved with a dwell time of ten seconds. Good hole fill and fillet formation were seen on all PDIP-16 leads. With board preheat for first-pass PDIP-16 soldering, acceptable hole fill could be achieved at lower dwell times. 

Second-pass (rework) soldering found that a pot temperature of 274°C was needed to achieve close to 75% hole fill. Acceptable hole fill and fillet formation could be seen at dwell times of about 30 sec. at 274°C without board preheat. Tables 2 through 4 compare the results of SnPb and SnAgCu soldering of the PDIP component, showing pot temperatures and dwell times, and highlighting increased pot temperatiu es and dwell times needed for SnAgCu solder without board preheat. 

Solder wettability is the capability of an alloy in the molten state to spread over the solid surface of another metal, usually metallization on a terminal pad. It is the precursor of solderability, which describes the solder s ability to form a solder joint on a PCB. Joint formation includes such aspects as fillet formation between a vertical surface and a horizontal surface, and capillary flow of the solder as required to fiU holes or gaps. 

Fillet formation Wicks up lead, full fillet at lead heel and toe Does not wick up a lead, partial fillet at lead heel and toe... 

FIG. 23 Photograph of through-hole incomplete pad wetting and incomplete fillet formation. 

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