Why do I need Surface Mount Adhesives?
Surface mount adhesives (SMAs)—sometimes called chip bonders—are generally used to attach components to printed circuited boards before soldering. They can also be used to fix components or repair boards. An SMA may seem secondary to your work, but the type and dispensing mechanism chosen will make a big difference in the end results. Here’s a closer look at the importance of SMAs.
What’s in my SMA?
So, what’s in a typical SMA? We know it’s far more than Elmer’s glue:
- An SMA needs to provide fast bonding, and components should fully adhere to a PCB in seconds.
- The adhesive needs to have the capacity to hold the components in place while the adhesive is curing; this is referred to as “green strength.”
- The attributes of the bonding material in the SMA need to be formulated so that specific sized dots are distributed from the applicator.
- The SMA needs to be durable enough to resist peeling or flaking over time as the PCB is used.
Now that’s a tall order for glue! SMAs were first introduced in 1984 for commercial use by a company named Henkel. Henkel saw the issues PCB designers and assemblers faced and were first to market with the product. Competitors have since brought similar products to market, but Henkel remains a global leader.
Generally, these adhesives are comprised of linear polymers, such as polyurethane and polyamide, synthesized from oil. The polymers serve as the adhesive base with additional materials to give them added qualities. For example, a conductive epoxy contains polymers and metal oxides that provide conduction.
The construction or synthesis of the polymer is what gives it its stickiness, final form after extrusion from an application, and part of its curing and thermal properties. Companies that have been perfecting petro-based adhesive formulations are now searching for ways to obtain the same bonding results using more sustainable and environmentally friendly sourced materials.
Differences Between SMAs and Dispensers
Let’s talk dots. When sticking components to a PCB board before soldering, you want the most accurate positioning and deposition of adhesive as possible. Too much adhesive can result in a poor solder, and too little risks the event of components falling off or not being soldered in the correct place.
Generally, SMAs are shimmed adhesives, which means they are formulated to be more hydrophobic than other adhesives. Its hydrophobicity gives it better structural integrity allowing it to be laid with the highest precision. A shimmed adhesive leaves the dot size and height up to the applicator. The amount of pressure and time of extrusion control the height of the dot, while the size of the dispensing needle and its proximity to the application surface control the width.
Manual applicator needles—sometimes called syringes—are available, but high precision applications require the use of automated dispensers. Different types of dispensing needles are attached to containers of SMAs. You can see what type of applications each needle and SMA formulation is best for on the product specification pages within TestEquity. In addition, some SMA and needle combinations are designed to produce high dots, while others produce dots with a lower profile.
Some other differences between SMAs include: their curing temperature (some will cure at room temperature while others need applied heat); their curing speed; their conductivity, green strength, and wet strength. For example, you want an SMA with high green strength if you are using it to hold PCB components to the underside of a board.
SMAs come in two colors, red and orange. Why red and orange? Red is green’s complementary color. Using red or orange, which has red in it, makes it easy to see. Visualizing the SMA on the board during and after assembly helps PCB inspectors identify issues with the PCB assembly process. If the SMA was clear, green, or difficult to make out, potential assembly flaws might not be discovered and could impose significant costs.
Having two colors is beneficial for multi-step assembly processes. Using one color for each step aids inspectors in identifying which step of the process has gone awry so it can be fixed. The different colors can also be used to delineate regions, components, or purposes for the SMA.
Where to get your SMAs
TestEquity offers a wide variety of SMAs as well as manual and automated dispensing equipment. If you are unsure about which product is best suited to your needs, TestEquity’s knowledgeable staff can help you choose from the most respected SMA brands in this industry. Visit the website today to see in-stock SMAs, or inquire for more information.