Reflow soldering processes are common for surface-mount device (SMD) components. If there are only low quantities of pieces, like typically needed for prototyping or small production runs, a full convection system does not really pay off financially. Full convection soldering is a gentle process with a high repeatability, which is why it is one of the most used processes in the industry. Similarly, high quality should be kept in mind when choosing a soldering system for the small quantities needed for prototyping purposes. A procedure such as hot air soldering is worth considering and is a great idea when working on rework applications because it is possible to selectively heat up the SMD-pads. But in general, it is not a precise soldering process, and not necessarily suitable for some temperature-sensitive components. This also applies to the infrared heater.
In the this article, another reflow soldering process using vapor phase soldering will be the focus. It is one the gentlest processes in electronics production and way more precise than other procedures such as hot air soldering. Another advantage is that even an industrial inline vapor phase soldering system has a smaller energy and space requirement compared to a full convection system.
What are prototyping challenges associated with the soldering process
Already in the schematic and layout design, the question arises how to contact the components with the circuit board. When it comes to prototyping or custom-made circuit boards there are high expectations to meet in terms of quality of the soldering process. It is important to guarantee a uniform temperature in the soldering environment and a reliable compliance with the temperature profile.
A reliable circuit board is key in prototyping because it will get challenged in many ways. Often a circuit board is soldered to check the schematic and layout design. An inaccuracy in the soldering process can cause some unnecessary delay in troubleshooting and commissioning. Speaking of delay, of course one wants to minimize the time spent with post-processing.
Common problems are the tombstone effect or solder wicking. Their occurrence can be reduced by using a gentle soldering process that follows a certain temperature profile or by adjusting the amount of soldering paste that is used. But if those effects occur, it requires a lot of time to be spent to correct them. Also, many (benchmark) tests are carried out on the board. This could be tests regarding the power, or reaction to temperature or vibration. Of course, a presentable appearance is desirable too, in case the circuit board is shown to a customer.
How is the vapor phase soldering process performed?
Inside the system there is a liquid medium, known as Galden. This medium boils at a certain temperature. Galden LS230 boils at exactly 230°C (446°F), which limits the maximum temperature that can be reached within the process. When the Galden is boiling, a vapor arises that is heavier than air. The circuit board is then exposed to this vapor. The medium condenses at the surface of the assembly. The vapor is displacing nearly all the oxygen, this prevents oxidation. While the Galden is condensing, it transfers the thermic energy evenly into the assembly. This heats up the PCB, the components and the solder paste that is applied between them. The solder paste will melt as soon as the PCB reaches the appropriate temperature, which depends on the solder paste that is used. Typically, this happens around 180°C (356°F). When the temperature decreases there is still a lot of vapors in the process chamber. It is important to keep the system closed until the vapor condensates and the Galden is liquid again. A powerful cooling system is key to reduce the waiting period between several soldering cycles. As soon as the temperature drops significantly the solder will solidify.
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Figure 1: Typical temperature profile using Galden in vapor phase soldering. (Source: PCB Arts)
What is important to understand for good results?
A well-performed soldering process must guarantee a uniform temperature in the soldering environment and reliable compliance with the temperature profile. Before the vapor phase soldering machine is soldering, the engineer must set such a profile for the process. This profile determines at what time which temperature should be reached within the soldering process. It is crucial to adjust this profile because higher thermic masses require a longer period of high temperatures to make sure they are completely soldered. Vapor phase soldering in general has the highest heat transfer coefficient of all procedures mentioned, which is crucial if PCBs with high thermic masses are soldered. Also, some components like LEDs are really temperature sensitive and require a well-controlled soldering process, which a vapor phase soldering system can offer.
What vapor phase soldering systems are suitable for prototyping?
In the prototyping area several vapor phase soldering systems are available. They differ in terms of support of highly dynamic temperature profiles or conservation of the high priced medium Galden. Some use air cooling, others have an open or closed water-cooling circuit. The performance of the cooling elements also determines the dynamic of the temperature profiles that can be applied. This can also get supported by an automatic lift which controls the distance between the PCB and the boiling Galden. Within the soldering process, the lift can increase the distance between the boiling Galden and the PCB and thus also decrease the temperature used to solder the PCB.
Advanced vapor phase soldering systems such as the PCB Arts’ Vapor Phase One support dynamic and rapidly rising temperature profiles. Also, the system includes an anti-condensation mode. By keeping the temperature of the PCB at 120°C after finishing the soldering process, the condensation of Galden on the PCB is minimized. Beside allowing the circuit board to be removed dry, consumption of Galden is minimized.
Based in Germany, PCB Arts takes a new approach towards innovation by opening the inner workings of the soldering system by providing an open hardware and open software product. Not only can engineers get a more profound knowledge about the product they want to purchase. It also makes repairing and fixing bugs much easier. Some occurring bugs maybe can be fixed even without consulting the customer service or a pricey repair service. Another advantage is the temperature profiles that can be adjusted individually. There are three premade profiles, that come with the soldering system. But in case the PCB requires a special kind of temperature process, those temperature profiles can easily get set by the engineer.
Among available soldering procedures for prototyping and small production, vapor phase soldering offers a precision and a gentle soldering process to protect sensitive, high-cost components. By using this approach with an open-source solution, engineers can gain a deeper understanding of the process and more easily adapt the system to their individual needs.
|Anna Metzenroth is a marketing and PR manager at PCB Arts, a start-up that designs, creates and develops electronics and hardware. Anna is a prospective B.Sc. of Industrial, electrical engineering at Friedrich-Alexander-University in Erlangen-Nuremberg.|
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