Flux Amount Control for Consistent Process Quality

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.

It is important that the right amount of flux is applied on the right spot with a defined spread. Flux must not penetrate into SMD areas. Selective solder flux may not be compatible with solder paste residues. Flux that is not activated may also affect reliability in the long term because of potential electro migration.

The application of flux is done by drop jet systems. Many high-volume applications require a high throughput. Short cycle-times can be achieved with high-frequency drop jets mounted on robots that can accelerate and decelerate quickly. Any clogging of the drop jet may result in solder defects like bridging or open joints. Spraying therefore becomes critical, and the flux supply may also be affected by outer noise such as clogging, flux pollution, pressure, temperature and other changes. Controlling the amount of flux may require a closed-loop system to guarantee a consistent process quality. This study evaluates the flux application process and tries to monitor the impact of drifts and changes. A method is shown where the process can be robust without being sensitive to noise. Statistical tools are used to prove repeatability in a selective soldering process and between different machines.

Feasibility of Soldering Fine Pitch THT Components

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.

The method of soldering THT components depends on the number of components to alloy and available time on the machine. For low volume products a sequential method (point to point soldering) is most likely the most cost effective way. Available methods, apart from hand soldering, are robot, laser or selective soldering using a mini wave nozzle. For high volume assemblies a simultaneous soldering method is preferred because of the shorter cycle times. There are three main ways that THT components could be soldered. The first is in a reflow oven (using Pin in Paste technology). The second would be in a traditional wave solder machine using pallets that cover the small SMD’s. Final solution could be a selective soldering application which would involve dipping the complete assembly on to a dedicated nozzle plate.

Effective Methods to Get Volatile Compounds Out of Reflow Process

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. Pick and place machines can handle more components and to meet this high through put some SMD lines are equipped with dual lane conveyors, doubling solder paste consumption. With the introduction of pin in paste to solder through hole components contamination of the oven increased due to dripping of the paste.  

The iNemi Roadmap identified seven key metrics for the reflow process:

Temperature delta performance
Inerting capability
Cooling rates
Flux management
Cost of operation
Traceability
Changeover time
The current flux collection systems need to focus on improvements to minimize maintenance downtimes. Flux management and cost of operation will benefit from an efficient oven cleaning method. The filter and condensation systems that were successful running in SnPb processes have to be reviewed and new technology is introduced to have a more efficient removal of solder paste, board and component gasses. 

Position Accuracy Machines for Selective Soldering Fine Pitch Components

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.

The materials that are used to make the solder connections require higher temperatures. Due to the introduction of lead-free alloys, the boards need more heat to get the barrels filled with solder. This not only affects the properties of the flux and components, but the operation temperatures of solder machines become higher. A nitrogen tunnel wave solder machine requires a temperature control in the tunnel to prevent overheating. Advanced systems are available that insert cold nitrogen. The closed tunnel wave soldering process has a wide process window and is not sensitive to small changes in environmental conditions. The same counts for wave solder machines that have nitrogen blanket systems over the wave. Improved preheaters will bring sufficient heat in the assembly and exhaust systems are adequate enough to maintain required process conditions. The nitrogen will improve the soldering and minimize dross amounts at these elevated solder temperatures.

Selective soldering is a different process. Compared to wave soldering there are additional process parameters that are affected by the higher temperatures. Solder joints have to be made close to SMD pads or components. An off-set of 0.5 mm may result in solder skips or re-melting SMD components. The higher temperature may cause warpage of the board, which  also affects the position accuracy of the solder nozzle. All materials will expand at higher temperatures, but not all expansion coefficients of the materials used are equal. This not only introduces stress, but also may create off-sets.

Achieving Repeatable, Consistent Control Over the Selective Production Process

Selective soldering machines are available in many different configurations, primarily because the assemblies are very different, not only in design, width, mass, but also in volume and in the number of solder joints.

Nevertheless, they all require the same three sub-processes of fluxing, preheating and soldering. These processes can have different methods, but in the end all main parameters are identical to create a solder connection: the amount of flux per area, preheat temperature, solder temperature, and contact time.

Selective soldering with a minimum flux amount

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.

Heat is required to activate the flux and make the solder flow into the barrels. If there is not enough thermal energy in the assembly the solder will solidify in the barrel during the process and there may not be a sufficient hole fill (defined in the IPC-A-610E chapter 7.3.5).

To improve the hole filling properties and to clean the metal surfaces a flux is applied before the soldering. Critical in this process is that the flux is applied on the right place (cleaning the metal and supporting the solder to penetrate into the barrels) and that the flux is activated. For activation of the flux a specified temperature must be achieved for a certain time. This activation temperature and time are flux specific and should be defined by the flux supplier, since they are the only to be familiar with the ingredients that are used in the flux chemistry.

Reliability Threats Fine Pitch Through Hole Soldering

How assemblies should be soldered depends on several conditions. Apart from the selection between reflow and liquid soldering there are certain factors that defines the soldering method. The preferred method to solder a mixed assembly with SMD and through hole components is pin in paste (also called intrusive) reflow soldering. This process requires a dedicated stencil to print the solder paste.

If liquid soldering is selected there are more methods. In case of some prototyping hand soldering or better selective soldering with a small dedicated nozzle (fountain) is a fast method that doesn’t come with additional tools.

In case of high volume processing fine pitch through hole mixed with SMD components there are 3 processes:

Reflow pin in paste
Wave soldering with pallets
Selective soldering
In hand soldering a consistent quality and process robustness fails and machine soldering is a must for many industries. Automotive also focusses on no rework. All different soldering methods have their challenges soldering the small fine pitch THT components especially avoiding rework. 

Oxygen Doping or Closed Loop Controlled Nitrogen in Reflow Oven

INTRODUCTION

Nitrogen is a chemical element with the symbol N and has the atomic number 7. Nitrogen gas is transparent, odorless and is diatomic. Diatomic molecules contain two atoms; for this reason Nitrogen gas is also called N2 factor. Tombstoning will however disappear at a higher PPM level. So the process window will be defined by solder defects (Lower Specification Limit) and spreading properties of the solder paste (Upper Specification Limit).

In order to meet these requirements the supplied Nitrogen should have a ROL (Residual Oxygen Level) of 20 PPM or lower.

Design Improvements for Selective Soldering Assemblies

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. Many defects can be eliminated in the design phase of the assemblies when specific rules for a robust selective soldering process are applied. This includes material selection as well as board design-related properties. This paper details methods of defect prevention through the application of design rules that are made for selective soldering processes using different methods of soldering. These rules includes recommendations for handling the board (placement accuracy, warpage, etc.), dimensions of pads, distances to surrounding SMDs or other components, improving heat transfer to the board by designing special via holes or modified pad structure, and more. The rules are identical for leaded or lead-free applications, although lead-free is more difficult due to the alloy’s higher melting point, increased copper leaching, solder contamination, and the greater difficulty in achieving sufficient hole fill.

Reliable Soldering for High and Mixed Volume Selective Soldering Processes

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.

Selective soldering is very effective method to solder the THT components. There are two mainstream selective solder applications: 
•    Drag soldering (point to point)
•    Dip soldering (stamp)

Both applications have a dropjet flux device to apply very small amounts of flux on the areas that are soldered. Despite these small amounts the remaining flux residues might cause electro-migration if not de-activated completely during the assembly process. This drives engineers to implement different flux formulations to avoid claims. Special in the automotive applications there are examples in the field that bridges and corrosion are caused by the growth of dendrites due to flux residues in combination with temperature, humidity and a current. The solder joints can be much more reliable when an inert flux is used in combination with a controlled soldering process.

Optimize the Selective Soldering Performance by Improving Nitrogen Environment at dip solder Process

In PCB circuit assemblies the trend is moving to more SMD components with finer pitch connections. The majority of the assemblies still have a small amount of through hole components. Some of them can’t withstand high reflow temperatures, while others are there because of their mechanical robustness. In order to meet requirements for mass solder production a selective dip soldering process using nozzle plates is a preferred soldering process because of its short cycle time.

The challenge in a dip soldering process is to get a sufficient hole fill, without bridging and minimize the number of solder balls. A new cover was designed to improve the nitrogen environment. Reducing oxygen levels benefits the wetting, but increases the risk for solder balling. Previous investigations showed that solder balling can be minimized by selecting proper materials for solder resist and flux.