As technology improves and products become smaller and smaller, the argument for double-sided board applications grows. Using a double-sided board in your finished application allows you to produce more complex circuits while saving space, offering an array of benefits for high-tech applications and electronics. Challenges to double-sided board implementation include placement questions, solder processing challenges, and heat dissipation.
Planning out a double-sided assembly can seem overwhelming at first, but taking it one step at a time can help show that while the process may be more involved and complex, it’s not substantially different from handling a single-sided board assembly. One of the most important aspects to consider is how you’ll address the reflow process.
Take process control into account first. Make sure you have the information that you need through tests and inspections that can monitor both sides of the board simultaneously. This lets you know possible failure points of your assembly process and which areas deserve the most focus. Allow for enough space at the edges of the board to be transported on the conveyor system or use panels with frames. Flexible bottom supports that can be adjusted to accommodate components on the bottom of the board may be a good choice as well. Ensure that your boards won’t warp or twist as they’re being reflowed, which could negatively impact solder joint reliability. Also, this may go without saying, but designing your board so that the heaviest components are all located on one side of the board—where they can be placed after the first reflow and won’t need to hang at the mercy of the solder’s surface tension—is a key part of the layout process.
Further, clearance can be a concern. Screen printers often use individual board supports that can damage your board, and the pressure of a board being clamped down with a solder screen pressed onto it will transmit directly through existing parts and can result in a hairline crack that may be overlooked.
When thinking of your reflow process for a double-sided assembly, you need to settle on how you’re going to fix your components to the underside of your board. For a number of assemblies, there’s no need for an additional step to the process. If you’re careful about the layout of your components and their respective weights, the surface tension of the solder may be enough to hold your components in place. The question is how you’re going to keep your components fixed in place throughout the secondary reflow process. You may need to take extra steps to ensure that your components stay held in place throughout the second reflow stage. There are a variety of options available to you, each with their own associated benefits and drawbacks.
For example, consider gluing the components to hold them in place or utilizing a Loctite paste to temporarily give components support as they undergo the reflow process. This is a workable solution but can lead to increased costs, additional process steps, and specialized equipment. You may also consider using a hierarchical alloy system with two different alloys with different melting points used to affix the components to the board. This can lead to complications, including damage to the components due to the higher reflow temperature of the high melting point alloy or a shift of components in operation due to the low-temperature threshold of the low melting point alloy.
Finally, you could consider a system to blow cool gas across the bottom side of the assembly throughout a secondary reflow process, ensuring the solder joints on the bottom of the assembly remain below a liquidus temperature. However, this could introduce potential stresses on the board due to the temperature differential between sides. If you’re looking to prevent component fall-off beyond ensuring that the weight of the components doesn’t exceed the surface tension of your solder, you’ll need to consider these questions.
The density of components makes double-sided assemblies an attractive choice for a number of high-tech applications. For some electronics, the size of a given circuit makes double-sided assemblies a necessity. If you’re worried about tackling your first double-sided assembly for a product, it’s important to be prepared and know what issues you might face. While double-sided assembly does add complications to the PCBA process, in many cases, the benefits outweigh any issues you might face. As with any SMT issue, having the information that you need is the key. Make sure you’re aware of the pros and cons of double-sided assembly before jumping into a new project as well as potential snags you may encounter when building out your production steps.
Mike Fiorilla is a writer at Manncorp Inc.
This column was originally published in the May 2019 issue of SMT007 Magazine.