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58 SMT007 MAGAZINE I MAY 2019 First, can water contamination cause failure? Absolutely! Rain- water contains much higher elec- trochemically active content than the deionized water used for clean- ing PCB assemblies and can lead to corrosion and dendritic growth. If discovered before introduction of a board being clamped down with a solder screen pressed onto it will transmit directly through existing parts and can result in a hair- line 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 sec- ondary 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 compo- nents to hold them in place or utilizing a Loc- tite paste to temporarily give components sup- port 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 hier - archical 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 compo - nents due to the higher reflow temperature of the high melting point alloy or a shift of com- ponents in operation due to the low-tempera- ture threshold of the low melting point alloy. Finally, you could consider a system to blow cool gas across the bottom side of the assem- bly throughout a secondary reflow process, ensuring the solder joints on the bottom of the assembly remain below a liquidus tempera- ture. 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 ensur- ing 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 dou- ble-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 pre - pared and know what issues you might face. While double-sided assembly does add compli- cations to the PCBA process, in many cases, the benefits outweigh any issues you might face. As with any SMT issue, having the informa- tion 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. SMT007 Mike Fiorilla is a writer at Manncorp Inc. To read past columns or contact Fiorilla, click here. to the service environment, cleaning may be possible, and the assemblies may be able to be salvaged. Exposure to rainwater in the service environment is a sure path to failure unless the assembly has been ruggedized against such exposures as part of the design. To find out more what Jason Fullerton, customer techni - cal support engineer at the Assembly Division of MacDer- mid Alpha Electronics Solutions, has to say, click here. Tips & Tricks: Water Contamination and Flux Expiry

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