Powder Sizing
The screens used in the sizing process are tightly controlled by the number of holes per square inch. The number of holes corresponds with the powder size described in the powder specification. For example, to achieve a 140-mesh powder, a screen with 140 holes per square inch is used. If the powder is to be a 325 mesh, a screen with 325 holes per square inch would be the correct size to separate the powder and collect the product. As previously described, the powder from the furnaces would be passed over the 140-hole screen and all the powder that passed through would be collected as product. The powder that stayed on top of the screen would be removed from further processing, as it does not meet the size requirement.

When the powder size is defined by two or more sizes, the screening setup becomes more complex. As an example, a powder that is identified as ­140 +325 will have to be screened over two screens. The top screen contains 140 holes per square inch, the bottom one will have 325 holes per square inch. Now as the powder is vibrated over the top screen, the powder too big to go through the top screen is removed. The powder that passes through the top screen will then pass over the bottom 325-hole screen. The powder that stays on the bottom screen is the product. It is the ­140 (went through the 140-hole screen), +325 (stayed on top of the 325-hole screen) powder.

The process would be the same for a fine powder. If the powder is described as ­325 +22 micron, the powder between the two screen sizes would be collected and finish processed as the product.

Finishing the Process
There are several steps that must still be accomplished before the powders are ready for delivery to a customer or for a subsequent process such as paste or tape manufacturing.

At the screening process, a sieve analysis is performed. This is a function of the quality process where several screens with different size holes are used at one time. By measuring the amount of powder that is retained on each screen it is possible to determine if the finished lot of powder meets all of the requirements of the various external specifications for the powder. This analysis will eventually be reported on a material certification issued by the manufacturer. If the sieve analysis shows the powder meets the manufacturing specifications, it is moved on for further processing. If it does not meet the requirements, it is reprocessed through the screens until the requirements are met.

Fig. 5 - The screens are set up so that the powder that falls through the first screen but not through the second screen is the desired powder size. The ovesize and undersize particles are collected for reprecessing.

The next step in processing the powder is blending of the "lot" of finished powder. This is done to ensure the powder particle sizes are homogeneous. There are many types of blenders, but those most often used are V-blenders, cone blenders, and double-cone blenders, so named because of their shapes. To ensure the best mix of the particle sizes, tumbling the powder from the narrow end of a V or cone into the wide end of these shapes is essential. Blending is accomplished in 30 to 60 minutes, depending on the weight of material in the blender. It is generally accepted that less than 30 minutes is insufficient time to get a well-blended, homogeneous mix of the powder particles, and more than 60 minutes can actually begin to overblend the powder, causing it to segregate. As a further note about blending, most manufacturers recommend the purchaser reblend the solder and braze powders prior to their use. It is possible for the powders to segregate somewhat during shipping in their containers, so each should be blended in a small V or cone blender before being applied to the components to be brazed or soldered.

To complete the powder manufacturing process, it is necessary to analyze the chemistry and size of the powder and issue material certifications based on the analysis. There are many pieces of equipment that can be used to perform the over-checks. For analysis of the chemistry, there are various spectroscopy methods including ICP or AA analysis machines. There are wet and dry methods for checking the powder size; oxygen, nitrogen, or helium analyzers for measuring the levels of gases; and furnaces for testing the melting and flow characteristics of the braze and solder alloys. All of these tests are completed to ensure the quality of the braze and solder alloy powders before they leave the manufacturer.

The last operation prior to shipment is to package the powders in containers that keep them clean and dry. The proper identification of the powder should be on the labels, and other information might include specification number, lot number, product warnings, and storage information. Containers vary from manufacturer to manufacturer, but all are designed to keep the braze and solder alloy powders at the same quality level as when manufactured.

Alloy Compositions
Braze and solder alloys start with a base metal to which other metals are added to improve or change the properties of the starting material. The original braze and solder metals of copper, silver, and gold were used as early as 4400 B.C.1 The metals were readily available and could be melted in charcoal furnaces - the technology of the day. Today, there is a wider range of metals used for the basis of braze and solder alloy powders. These include copper, gold, nickel, iron, silver, and tin. To these base metals, additional metal components are added depending on what is needed to meet service conditions of the brazed or solder components, or for functionality of the braze or solder joint design. The materials listed in Table 2 represent some of the metals currently found in braze or solder alloy powders.

At present, there are hundreds of braze and solder alloys available. Also, alloys can be designed to meet the special requirements of a component to be joined by working with a manufacturer of alloy powders. The only limit is the imagination.

Interpreting Powder Mesh Analysis
It can be of great advantage to the success of a brazing or solder operation to understand the mesh size of the powder as stated on the material certification issued by the manufacturer.

Powder sizing is simple if you remember that either the powder particles fall through the holes of a screen or they stay on top. The size designations on certifications or specifications are based on screens having a specific number of holes per square inch, so no conversion is needed to understand the analysis received with a delivery of powder. Dissection of an example of a typical mesh size will help to demystify alloy powder sizing.

A specification that reads:

0.0% minimum +120
90% minimum ­140
55% maximum ­325
5% maximum 22 micron

Is interpreted as the following:

• 0.0% means that all the powder particles must be small enough to pass through a screen with 120 holes per square inch. No powder is to remain on top of the screen
• Ninety percent of the powder has to be small enough to fall through a screen with 140 holes per square inch, or only 10% of particles large enough to stay on top of the screen are allowed.
• The powder is further identified by the amount of fine powder that can be included in the coarse mesh powder. In the example, only 55% of particles small enough to pass through the 325-hole screen are allowed in the product. The balance of the powder must be larger or remain on top of the 325 screen.
• The last size controls how fine the powder can be. Of the 55% allowed to be smaller than the 325 screen, only 5% can be smaller than 22 microns. Unlike the other sizes that are based on the holes per square inch, very fine powders are described based on the actual measured size of the particles, which, in this case, is 22 microns or 0.0008 in.

In general, if the mesh specification has a plus (+) sign, it is referring to powder that is larger than the screen size. A minus (­) sign refers to powder smaller than the screen size. Don't let the pluses and minuses cause confusion in understanding the powder size. Just think of the powder as staying on or falling through a screen. Then, whether the powder size is listed as 90% minimum ­140 or 10% maximum +140, there won't be any confusion about the actual size of the powder.

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