E6. inspection of powder - The simplest method for checking solder powders used in solder paste is to spread a thin layer of the powder on a glass slide and then examine this layer with a microscope. With this approach, the size distribution of the particles and their shape can be monitored in a qualitative way.  

To gain a quantitative breakdown of the portion of the particles in various size ranges, the standard approach is to run a sample from the lot into a stack of small screens with progressively smaller screen openings from top to bottom. With the largest screen opening on the top, the particles larger than the opening are trapped on the screen and smaller particles fall through it to the next screen. The same thing happens at the next screen, which has slightly smaller screen openings. The largest of the particles are trapped; the balance pass through the screen. This process can be repeated with as many different size screens as desired, all placed in one vertical stack. The amount of powder left on each screen indicates the portion of the lot that is in that particular size range.  

E7. mixing solder paste - is a batch operation. Solder powder, flux, plasticizers, tackifiers, thickeners, or thinners, are blended in mixers designed for the high density (because of tin and lead content) and high viscosity of the paste. Some of the mixers described in section 11G5 for stiff, viscous materials can be used for solder paste. Fig. 13E7 illustrates a machine particularly suitable for mixing solder paste. Machines which pass the paste between parallel rollers may also be employed as part of the mixing operational sequence.

Fig. 13E7 A dual shaft mixer designed for solder paste. One shaft rotates at high speed and disperses the ingredients; the other shaft rotates at low speed and moves and blends the mixture. ( Courtesy Myers Engineering, Inc. )

E8. inspection of paste - A number of tests can be made on the solder paste to verify its properties: The ability of the paste to be dispensed - its rheology - is controlled most commonly by measuring the viscosity for which several viscosity-measuring instruments are available. The most prominent variety uses a rotatable spindle into which a small diameter rod with a cross piece (“T-bar”) is inserted. The T-bar is lowered into a container of just-remixed solder paste that is at a specified and uniform temperature. The T-bar follows a helical path so that the bar is always meeting resistance from the paste as the bar rotates. The instrument measures the resistance to the rotation and translates this into a digital viscosity reading.  

The metal content of the paste is normally checked by weighing a sample of paste, heating it to melt and coalesce the solder into one wafer, washing away the flux, and weighing the resultant metal wafer. The ratio of the two weights indicates the percentage of metal.  

Flux conformance to specifications is determined by immersing the paste in a suitable solvent, filtering out the metal powder, evaporating the solvent and performing various analytical tests on the residue.  

Fineness of grind of the metal particles in the paste is measured with a gauge based on those in use in the paint industry to measure paint pigments. A sample of solder paste is placed on the gauge, which is a hardened steel block having two tapered grooves in the surface. The grooves range in depth at the deep end of about 185 microns (0.007 in) to zero at the other end. The paste is placed at the deep end of the grooves and a scraping blade draws it along the length of the channel so that it remains on the gauge only in the grooves. The depth of the grooves at the point where the line of paste in the grooves ends indicates the size of the finest metal particles in the paste.  

Tackiness testing - verifies that the paste has the necessary tackiness to hold surface mounted devices placed on a circuit board until solder reflow takes place. A motorized commercial testing device is used. A sample of paste is placed on the surface of a glass slide which is then stored for a length of time equivalent to that involved in production conditions. The slide is then placed on the work surface of the testing device. A probe in the device descends into the paste at a controlled rate with a specified amount of force. The probe is then withdrawn and the pulling force needed to withdraw it is measured. The magnitude of this force gives a quantitative indication of the holding power of the paste for mounted devices. Another device, sometimes used, measures the shear resistance of the paste, and therefore its resistance to the movement of devices on the board before solder reflow.  

Slump tests - measure the increase in area from gravitational forces of a deposit of solder paste after the solder has been applied to a surface. Standardized test patterns of paste are applied to a surface, by screening or stenciling, and their dimensions are then observed and, if desired, measured for a change in spread. Excessive slump causes problems in holding the mounted components and can also predict solder ball and other problems in reflow soldering.  

Performance tests 3 - Several tests can be used to verify that the solder paste performs satisfactorily when used on the components to be soldered: 1) compatibility tests verify that the solder paste is suitable for the joint surface materials under the expected production conditions. A small amount of paste (50% or less of the expected production amount) is placed on the joint surface, which is then heated to reflow the solder. The solidified spot of solder is then inspected. If good wetting is evident, the materials are compatible. 2) solder ball test . A small spot of solder paste is screened onto a ceramic test surface. The ceramic is heated on a hot plate sufficiently to reflow the solder. The spot of solder is then examined. If the solder forms one large spot, it is ideal. If there are more than three separate spots (solder balls), the paste is not acceptable. The flux area surrounding the solder spots should also be examined. Black particles in the flux indicate unreduced fine solder powder and the paste is not suitable for critical applications.