Solder paste – the gray goo that comes in a jar, cartridge, or syringe – is applied to circuit boards to attach components and create electrical connections. It comes with specific storage and handling instructions. It requires refrigeration. You can’t leave it open too long, or it goes bad. You must carefully dial in your process settings to optimize its performance.
But why? What is this gray goo made of? How is it manufactured?
Solder Paste Basics
Solder paste is made from solder powder – tiny spheres of alloyed metal, combined with a flux medium – rosins, resins, and other chemicals designed to promote cleaning and prevent oxidation.
So, to make solder paste, you must first create an alloy. Then you must turn that alloy into tiny spheres. Next, you need a flux medium with the right chemical composition. Finally, you combine the tiny spheres with the flux medium in a carefully controlled environment.
The exact method by which all this is done varies from one manufacturer to another, and the exact details are often proprietary and closely guarded. That said, we can still give you a general outline of the process.
Alloying and Atomization
Pre-atomization processes include alloying, assaying, and casting the metal that will be fed to the atomizer. One atomization method involves pouring a controlled stream of molten solder onto a rotating disc.
As the liquid hits the disc, it is deflected into droplets that are formed by surface tension into individual spheres before they freeze, as shown in the diagram below. Atomizing parameters include melt temperature, disc speed, chamber environment and other variables that influence key particle characteristics such as size, shape, and oxide level.
Atomization operations are often optimized to produce a particular powder size, with spheres outside the desired range considered byproducts of the process. Spherical shapes are ideal, but sometimes irregular shapes like dog bones or tails are formed, or multiple spheres clump together. These undesired geometries can affect a solder paste’s rheology and print performance and are removed in subsequent processing.
Oxidation is controlled by manipulating levels of inert gases in the atomization environment.
Solder Paste Powder Sizes/Types
Solder pastes are classified based on the size range of the solder powder (tiny spheres) they contain. Powder sizes range from Type 1 to Type 10, with Types 3-6 the most commonly used currently in electronics manufacturing. Type 4 is the most popular size at present, with a particle size range of 20-38 µm. It is capable of printing 0201s and microBGAs.
How small is this? For reference, the thickness of a human hair is about 100 µm and a white blood cell is about 10-20 µm in diameter. So, these particles are very small indeed!
Type 5 powder, with 15-25 µm particles, is suited to finer applications like QFNs and 01005 devices. Types 6 and smaller venture into ultra-miniaturization, necessary for cutting-edge technologies.
Powder Size Classification and Distribution
After atomization, the powder is refined and sorted by size. Methods for doing this include air classifying and sieving.
The air classification process segregates the solder powder into different classes based on mass. The powder is blown through a stream of gas where the target-sized particles are concentrated. The smaller sized particles are carried away in the flowing gas and the heavier particles fall into a collection area.
This process permits rapid winnowing of the powder to where the most desirably sized particles are heavily concentrated. Air classifying makes the subsequent sieving operation much more efficient and effective.
Sieving sorts the classified powder using large, vibrating sieves with progressively finer screens. The screens correlate to the mesh size that defines the “type” of powder as designated by JEDEC and J-STD-005. The table in the previous section shows the classifications, both in terms of mesh size for the screens and resulting particle sizes.
Spheres categorized as Type 3, or T3, will fall through a 325-mesh screen but not through a 500-mesh screen, hence the term -325/+500. This equates to particle sizes of 25 to 45µm; 80% of the particles must meet this size requirement. Likewise, Type 4 paste will fall through a 400-mesh screen but not a 635-mesh screen, equating to particle sizes of 20 to 38µm.
There is size overlap between the T3 and T4 classes; therefore, T4 solder paste can typically offer a slight edge in fine feature printing without introducing reflow concerns.
Particle size drops off quickly at Type 5 to 10µm to 25µm, however, and while the solder paste’s fine feature print capability is dramatically improved, the effects of the surface oxides can start to factor in; shelf life may be affected, and the potential for reflow coalescence problems are increased.
Solder Paste Flux
Flux is a chemical agent that facilitates soldering by cleaning the metal surfaces, enhancing the flow of solder, and preventing oxidation during the reflow process.
The flux used in solder paste is a combination of various chemicals, including activators, rheology modifiers, solvents, and plasticizers. The specific composition of the flux depends on the intended application and required properties, such as activation temperature, viscosity, and cleaning requirements.
The Final Step: Combining Powder with Flux
The final step in making solder paste involved combining solder alloy powder and flux in specific ratios to ensure the paste is uniform and homogenous. The typical ratio is about 50/50 flux to alloy by volume or 10/90 by weight, but this can vary based on the application’s specific requirements.
Solder paste manufacturers use proprietary blending processes involving mixers that are carefully calibrated to ensure stable temperatures and consistent mixing. Every part of the process is carefully measured, monitored, and kept consistent from one batch to another, ensuring the paste produced is uniform and meets all the required specifications.
After mixing, the paste is packaged into jars, cartridges or, syringes depending on how it will be applied to the circuit boards.
The final product then undergoes rigorous testing to ensure it meets all relevant standards. This includes tests for viscosity, slump, solder balling, wetting, and shelf life, among others.
We’ve Mentioned Oxidation a Lot. What is That About?
Oxidation is a natural chemical process that occurs between substances and the oxygen in air. You see oxidation all the time – it’s what makes sliced apples turn brown when left to sit, or what makes rust form on an old car. It also happens when the metal in a solder alloy comes in contact with air.
The more contact a solder alloy has with air, the more propensity it has for oxidation. Oxidation can also be accelerated by moisture, warmer temperatures, and heating. When solder alloys react with oxygen, they form an oxide layer on their surface. This oxide layer is less conductive and less mechanically stable than the pure alloy, which means it makes it difficult to form good electrical connections.
Because of this, one of the main goals when creating and using solder paste is to minimize oxidation as much as possible. In fact, one of the key roles of the flux medium is to remove oxides and prevent oxidation. It does this via chemical interactions as well as by creating a barrier between the metal and the air.
This propensity for metals to react with oxygen is why solder paste requires careful handling and storage. It is also why the manufacturing process for the paste must be carefully controlled. Often, the powder formation occurs in an environment where the oxygen has been replaced with nitrogen – a gas that does not react with metal.
The propensity for oxidation is why some manufacturers use nitrogen atmospheres during reflow. And it is also why finer solder pastes are more delicate – when the powder size gets smaller, there is a greater ratio of surface area to metal mass. Since oxidation happens on the surface, more surface area means more chances for oxidation.
Final Words
There you have it! The gray goo is made by creating tiny metal alloy spheres and combining them with a chemical paste optimized to mitigate oxidation and promote strong electrical connections. Oxidation is the enemy of both the process and the final product – and is the reason for the storage and handling instructions.
