Flux is one of the most critical parameters in the selective soldering process. Flux deposition on the board needs to be carefully controlled. It should have the right balance between solderability and reliability. Flux has a major impact on barrel filling and defect production in challenging thermal applications. Robust flux design for selective soldering is therefore a critical factor. Partnership between flux and machine designers is a key component for success.
Vitronics Soltec Technical Papers
One of the major trends in circuit board assembly is the drive to smaller components and pitches. Where the focus is on SMD (Surface Mount Devices) and also THT (Through Hole Technology) the designers have the intention to go smaller and smaller. The result is less space on the boards but with increased functionality. The Roadmaps from IPC and iNemi mention a minimum pitch of 40 mils (1.00 mm) in the near future. The physical properties of the lead-free alloy, flux and solder mask give the engineer problems to set-up a consistent and robust soldering process for these smaller devices.
Although reflow ovens may not have been dramatically changed during the last decade the reflow process changes step by step. With the introduction of lead-free soldering not only operation temperatures increased, but also the chemistry of the solder paste was modified to meet the higher thermal requirements. Miniaturization is a second factor that impacts the reflow process. The density on the assembly is increasing where solder paste deposit volumes decreases due to smaller pad and component dimensions.
The drive towards fine pitch technology also affects the soldering processes. Selective soldering is a reliable soldering process for THT (through hole) connectors and offers a wide process window for designers. THT connectors can be soldered on the top and bottom side of boards, board in board, PCB’s to metal shields or housing out of plastic or aluminum are today’s state of the art.
Selective soldering is a process with more than one hundred different parameters that may impact soldering performance. Some conditions change over time, e.g. machine temperature, humidity, contamination, wear of parts or settings after maintenance. Other factors in the process include the materials used, component wettability, solder mask surface energy, board material Tg and Td values, solder oxidation and composition.
During production, conditions may begin to drift. In order to avoid solder defects, statistical process control or SPC is the best method of identifying unexpected changes in the process. When using this statistical technique, however, it is important that the machine have the tools to measure these essential process parameters and if necessary control them to maintain a robust soldering process. Apart from machine parameters, the materials have a big impact. A robust selective soldering process should have a wide process window that is able to handle variations in material quality.
In this paper, critical process parameters are discussed as well as methods that can be used to widen the process window. Additionally, process robustness is evaluated.
Three ingredients are required to make a good solder joint: solder, clean metal surfaces to connect and heat. In a selective soldering process all three have a big influence on the final result. The solder properties define not only the hole filling, but have an impact on the appearance and shininess of the solder joint as well.
Miniaturization continues in all kind of electronic assemblies and thus also in the different soldering processes. Not only consumer electronic assemblies become smaller, but also in other areas like automotive, industrial and medical parts reduce in size where functionality increases. No matter if products are assembled in high or small volumes soldering technology has to deal with finer pitch and higher accuracy of the soldering process. Small electronic assemblies contain mainly SMD components that are soldered with solder paste in a reflow oven. Traditional through hole soldering is replaced by surface mount devices nevertheless there are still some reasons to have through hole components instead of SMD’s. The main two reasons are strength (reliability) and heat resistance. Some components like flex cables, LCD displays and other plastic parts just cannot withstand the high reflow temperatures of ~ 250 ºC for a longer time. In order to meet the high temperatures of reflow special high temperature resistant plastics are required; making the parts more expensive.
This paper will discuss how this small through hole components can be soldered when assembled together with SMD’s, either in a reflow oven or in a wave or selective soldering process.
An inert atmosphere opens the reflow process window. Reflow ovens that have a Nitrogen environment may have better yields because of the lack of oxidation during the process. The wetting properties are improved due to the absence of Oxygen. The drawback of a low Oxygen level is that there is a potential risk for tombstoning. Due to the surface tension this defect is more likely to happen in a Nitrogen oven than in air. To overcome this risk a controlled Oxygen level of 500 – 1000 PPM is preferred. It is not the objective of this paper to define the most efficient Oxygen level, but rather, the method to keep this level consistent during different process conditions.
Two concepts are used in the market to keep the Oxygen level in the reflow process constant. One method purges Oxygen into a Nitrogen environment. Depending on the Oxygen PPM level, more or less Oxygen will be doped into the system. The second concept controls the Nitrogen supply to maintain the Oxygen level required. If the Oxygen level goes up, more Nitrogen will be purged into the oven. When there are no boards entering the oven the Nitrogen supply can be reduced and still maintain the Oxygen level. Pro and cons of both concepts are compared.
Soldering requirements for PCB assembly have become ever more critical. The automotive industry tends to eliminate repair of soldering defects, which makes it even more important to understand the soldering process and material characteristics in order to avoid excessive waste and costs. Many designs have their roots in wave soldering and defects can be dramatically reduced when some simple improvements are made to enhance compatibility with selective soldering applications.
Although the number of Through Hole (THT) connections the electronics assemblies is decreasing due to miniaturization, selective soldering is still a growing market. More and more Surface Mount Devices (SMD) technology is being used and reflow soldering is becoming the main stream solder method. Nevertheless there will remain components that require through hole connections for strength or because they simply can’t withstand the high temperatures of a lead-free reflow process.