Qualitek Solder Powder
Solder powders are the key component of all solder pastes. Pastes made with the same flux but with powders that differ in particle sizes often have different rheological/physical properties resulting in differences in screen/stencil printing, component placement and production yields. The powder constituent of solder paste, in fact, affects all stages of surface mount assembly from stability and printability of the paste, to the quality of the final fillet.
Good solder powder must have the following properties: a “clean dry sand” appearance, a gray-silver color vs. a charcoal-gray, which indicates presence of oxidation and good flow behavior go to functional test. Other important features include high powder density close to the theoretical density of the alloy and no internal voids or external satellites (microspheres attached to a single sphere).
The trained eye of a technician often evaluates some of those properties. Others are governed by formal test procedures. For example, powder flow can be tested according to ASTM B-213, and the presence of voids can be checked by microscopic examination of cross section of individual particles. The user can identify the powder by both composition and type. Alloy composition refers to the main alloying elements; type to the range of particle (mesh) sizes.
Small variations in composition and the level of impurities can affect wetting properties of the solder paste: oxidation behavior, melting temperatures, flow within the joint and joint strength. The ideal solder composition will feature tight tolerances of alloy composition.
The J-STD-006 (Joint Industry Standard) places requirements on the content of alloying elements. For alloys with 5% of less of an element, its content must not vary by more tan 0.2% by weight. For elements contents higher than 5% the acceptable variation from the nominal is extended to 0.5%. Because of possible deleterious effects on the solder limits also are placed on the presence of 14 impurity elements. For instance, aluminum cadmium and arsenic impurities induce dewetting, and sulfur blamed for joint grittiness. However, even when the alloy’s composition is within the industry specification small variations in elemental content can affect joint aesthetics. One joint can turn out smooth and shimmy while another will display a dull an grainy appearance. To deal with these issues, paste manufacturers often impose in-house specifications that feature strict composition tolerances.
Powder can be spherical or irregular. According to J-Std-006, powder is spherical if 90% of the particles have a length-to-width ratio of less that 1.5. For most applications a high degree of “sphericity” is preferred, the major advantage being lower surface area, which minimizes oxidation, and better load acceptance (less tendency for clogging and interlocking), which assists dispensability and release through the stencil aperture.
Particle roundness influences paste viscosity and the tendency to shear. Spheres offer less resistance to viscous flow compared with particles of irregular shape. Accordingly, paste made from the same flux and spherical powder will have lower viscosity than those of the same weight percent and particle size range but of irregular shape. One possible advantage with pastes of the latter appearance is that they are less likely to shear thin when screen/stencil printed at high speed and constant squeegee motion. Interlocking of the powder reduces paste flowout. Reduction of shear thinning is important because it will prevent slumping and smearing which can result in solder bridging and solder balling.
Powders are divided into six particle type with the finer powder represented by higher numbers. Table 1 also includes the customary designation of powders by mesh sizes of the sieves used to sort the particles according to ASTM B-214 Test Method 1.