By Dientje Fortuna

Most brazing and soldering operations start with the application of an alloy powder to the parts. This can be in the form of a paste, slurry, spray, tape, or preform, all of which begin as a powder. How the powders are manufactured can impact the cost, quality, and success of soldered and brazed components. Understanding how the powders are manufactured and sized can help one make decisions on the alloy powder to use and how best to apply it to the parts to be soldered or brazed.

A number of manufacturing processes are used to produce metal powders. These processes include sintering, crushing, spray drying, cladding, and atomization. While all of these methods produce excellent forms of powder, atomization produces powders that meet the extensive requirements of the brazing and soldering processes - Fig. 1.

Defining the Atomization Process
Atomization of molten metal is similar to the atomization of perfume. A liquid stream of perfume is hit with air to break up the liquid into millions of tiny drops that deposit on the skin as a fine mist. With metal powder production, the process is essentially the same. The molten metal stream is hit by a jet stream of air or other material and broken up into tiny drops, each of which contains all of the elements of the molten metal. The droplets cool and shrink to form powder particles.

The jet of air, as the way to break up the stream of liquid metal, can be replaced with other materials such as oil, water, or gases like argon, nitrogen, or helium. The powder manufacturing process itself can be accomplished in air, under vacuum, or under cover of a shielding gas such as those mentioned above. In every case, a high-quality braze or solder alloy powder is produced.

Choosing the method to manufacture a solder or braze alloy powder depends on the type of metals that make up the alloy powder. As you can imagine, the process to make a titanium-beryllium braze alloy is different than that for making a palladium-nickel braze alloy or a copper-tin-lead solder alloy. The temperatures required to melt the metals and the oxidation levels or properties of the finished powders are examples of some of the reasons one process is chosen over another to make these powders.

Table 1 shows the base metal alloy powders and the various atomization processes used to manufacture them.

How Does It All Start?
It begins with a recipe.

The recipe, which is actually the processing or job card, is based on internal or external chemistry and size specifications for the solder or braze alloy powder. It details the exact amount of each raw material component needed to produce the powder. An example of chemistry for a recipe might be something like the following:

60% Nickel
14% Nickel Boron
10% Ferrochrome
8% Silicon
5% Iron
3% Chromium

Of course, the quality of the powder actually starts with the purchase of raw materials. Knowing the reliability of the supplier and the quality level of the raw materials will provide consistent product realization and ensure the braze and solder metal powders function as expected and meet the requirements of the industry specifications. As added insurance, most raw materials are retested prior to use in the production of the alloy powders.

Atomization Process
The atomization process is controlled from start to finish through detailed job instructions, control checks at each step in the process, and extensively trained operators. Production controls can include accurate weights on the raw materials and monitoring of the melting temperature, gas pressures, water temperatures, pour time, and gas levels. Also important is the training and skill level of the production operators. Effective training can take 6 to 12 months for a new operator to become proficient at the melting/atomizing process. Essential to the success of the atomization of powder alloys is the documentation of the processes and practices of production.

Step one of the atomization process is the melting of the raw materials or melt stock. If using raw material, the exact amount of each component is weighed and combined in a container that will hold the total weight. This is referred to as a "furnace charge." It is possible, and sometimes desirable, to purchase melt stock that is premelted and contains most or all of the elements of a braze or solder alloy. This becomes the furnace charge that is melted and turned into alloy powder. A refractory furnace is used because it can withstand the heat required to melt metal alloys. The melt temperature can vary from 71°C (160°F) for low-temperature solder alloys to almost 1650°C (3000°F) for high-temperature braze alloys. The heat source for melting can be induction (electric) or gas and is introduced so that the entire furnace charge is heated uniformly. The furnace operator carefully monitors the melting process using temperature measurements and visual exams until the furnace charge has become completely molten and the temperature necessary for all the elements to become homogeneous has been achieved. Often, reactive elements such as rhenium, hafnium, yttrium, zirconium, and rare earths are added very late in the melting process to keep them from being dissolved rather than melted.

Figure 2 is a schematic of a typical induction furnace used for gas atomization of nickel-, cobalt-, and copper-based alloy powders. Similar furnaces are used for all types of alloy powders.

Once the metals are liquified and ready to pour, the furnace is tipped and the molten stream is poured into a chamber where the atomization takes place - Fig. 3. This is a rather dramatic operation as the hot liquid metal is hit with a stream of air, oil, or gas and broken up into millions of tiny droplets - Fig. 4. The droplets are then cooled in air, gas, or water to form powder particles.

Screening of Powders
The specialization of the atomizing equipment can help to optimize the range of the droplet sizes that are formed from the exploded metal stream of alloy. But, even under the best conditions, the sizes of the powder particles created from this process range from 5 to 600 microns (0.0002 to 0.023 in.). From this wide range of particle sizes, the desired particles, defined by internal and external specifications, must be separated and collected to create the end product that will be delivered to the brazing and soldering customers. The "product" could be as little as 20% or as much as 50% of the powder produced in the atomizing process. This is determined by how tight the particle size specifications are for the powder, and will be discussed later. Sizing of powders is accomplished by screening, and various types of screening equipment are employed. It is not unusual for two different types of equipment to be used to produce one braze or solder powder.

Screening is a method utilized to separate powders into size fractions in order to collect and deliver the product as defined by the specifications. Just as the name suggests, the process uses screens similar to those on windows and doors. They are made of steel or nylon materials woven to have a specific size hole and are based on a specific number of holes per square inch - Fig. 5. For efficiency, the screening process is designed to size the powder with the least amount of screening time. The powders are passed over the screens with a rotating or vibrating motion and, depending on the sizes of the particles, they go through the holes of the screen or stay on top (too big to pass through). The powders are then collected based on where they stayed during the process.

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.

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.

Using This Knowledge
From a cost standpoint, there are several ways to capitalize on the understanding of powder production and the resulting alloy powders. One area to review would be how the method of manufacturing the alloy powders impacts the cost of these materials. As an example, when using a copper-based braze alloy, look for a manufacturer that atomizes in air or water, as these are lower cost processes. Most copper brazing applications do not require the higher cost, gas atomization process, so there is no reason to pay extra for a lower oxide copper powder. On the other hand, if the brazing process involves a base metal that oxidizes readily, such as those that contain aluminum or titanium, it may be more cost effective to use a gas-atomized copper powder even though the initial cost is higher. The dollars that could be saved in the processing of these difficult-to-braze components will more than make up for the higher purchase price.

Another cost consideration is with the mesh size of the alloy powders. When manufacturing powders, the greater the yield, or the more of the "as-atomized" powder that can be used to produce the final product, the lower the cost. For many brazing and soldering applications, no other consideration is necessary beyond that of the lowest cost filler metal. However, there are other applications where the method to apply the filler metal makes it impossible to use the "optimized yield" alloy mesh size. As an example, applications where the powder is sprayed or dusted onto the component could be negatively impacted by a powder that has a high percentage of fine particles as it will tend to clump or clog during these types of applications. The use of the lower priced powder could cost more in lost production time than the upfront cost of the higher priced, restricted mesh size powder.

Understanding the powder size used in a paste can help select the right application equipment for applying the alloy paste. If a coarser powder is used to manufacture the paste, there could be particles as large as 0.006 in. (0.15 mm). But, if the part to be alloyed needs a fine bead of alloy, a typical way to achieve this would be to use a small-diameter needle so only a thin bead of alloy paste is applied. The small-diameter needles measuring from 0.004 to 0.010 in. (0.1 to 0.25 mm) would exclude a lot of the powder particles from passing through. Most often, the larger particles will clog up the entrance to the needle, causing a reaction on the part of the application technician to increase the pressure. The larger particles will yield under the pressure and move aside, resulting in a messy burst of paste from the needle that must be cleaned up. Time and alloy paste are wasted. As if this waste is not bad enough, the larger particles become stuck in the syringe or cartridge and pile up as the smaller particles are extruded. Eventually the syringe or cartridge and the remaining braze or solder material are discarded when no more alloy paste can be pushed from the container.

In a similar situation where a paste or slurry is mixed by the end user, knowing the powder mesh size can help to make the right decision on the amount of binder to add. If a process has been established based on a certain ratio of binder and powder that results in an acceptable viscosity, the process can be upset when the mesh size of a new powder lot is not examined prior to use. To engineering, the most important issue is the ratio by weight between the binder and the alloy powder in order to ensure a specific amount of solder or braze alloy in the joint. Unfortunately, in the real world, the operator has to have a paste viscosity that will extrude through the needle onto the part. When a new powder lot has a higher percentage of fine powder, more binder may be necessary to achieve an acceptable viscosity. The additional surface area of the finer powder particles will yield a thicker, more viscous paste if the same amount of binder is used, which won't go through the needle. This situation is usually fixed by adding enough additional binder so the viscosity is the same as usual. What is rarely measured, unfortunately, is the drop in powder weight this new mix of paste or slurry has. The risk is voids and rework because of the lower alloy weight in the joint.

Using the information provided with each powder certification can help control costs and make it easier to achieve processing goals. Be proactive and talk to the braze and solder manufacturers about specific processes and alternatives they may have to offer that will not only meet the specifications required, but might offer lower costs or higher yields in the brazing or soldering process. There is nothing to loose and a lot to gain with just a little education about the alloy powders being used. u

DIENTJE FORTUNA Deni.Fortuna@ sulzer.com is Braze Product Manager, Sulzer Metco (US) Inc., Troy, Mich.