Flexibility and easy programming

Bob Pitchford,Cookson Specialty Coating Systems

There are two important categories of businesses that are adapting selective conformal coating. The first is companies that have experienced rapid growth leading to the need for automation. Such companies may have masked and sprayed boards by hand in the past. They are now realizing that an automated coating system can replace six to eight workers, and accomplish in one hour what formerly required a full shift. A second category of users has already automated coating capability, but because of a need to dramatically increase throughput or have an expanded or new production line. This expansion can be accomplished using the latest coating system designs, with a minimum increase in floor space and cost.

Many categories of electronic manufacturing depend on effective conformal coating to help ensure dependable circuit performance under challenging operating conditions. Conformal coatings provide dielectric insulation, protection against moisture, hydrocarbons, dust and other contaminants and improved resistance to abrasion, thermal shock and vibration.

To be effective, a conformal coating must be free of pinholes, have sufficient film buildup on all surfaces and be applied evenly to provide the necessary barrier against the elements. It must also be capable of adhering to a diverse range of substrates, including fiberglass, plastics, ceramics, metals, legend inks, photoresists and flux residues. Coatings must accommodate differences in substrate physical make-up, chemical resistance, thermal coefficient of expansion and other important properties. Key cost issues are operator training, production labor, PCB handling (including pre-cleaning, masking, demasking), equipment, material consumption and curing. Manufacturers find that selective coating equipment designed to minimize PCB handling can substantially reduce costs.

Selective coating systems are capable of placing coating accurately on circuit surfaces so as to eliminate the need to mask test points, connectors, switches and other elements. With conventional spray or dip coating, such objects must be masked with tape or elastomer sealants before coating, and then unmasked after the coating step. Selective coating systems are computer controlled and capable of motion system resolution in the range of ±50µm, repeatable within 25µm. This level of accuracy makes it possible to coat between closely spaced components and around switches and contact points without unwanted coverage.

Selective coating systems typically use spray and needle dispenser tools in various combinations, along with multi-axis control options. Programmable Z-axis control can extend above the conveyor surface to allow for coating of assemblies with large dimensional components. New concepts allow coating dispensers to tilt under program control to reach areas that would otherwise be inaccessible. While the bulk of selective coating is done with a conventional atomized spray head, using either a flat or round spray pattern, the process can be switched on-the-fly to a needle dispenser in order to apply coating in narrow areas and tightly around selected components. Systems can apply virtually any conventional material, including solvent-based and 100% solid materials across a wide viscosity range. In these PC-controlled systems, the harddisk can store all programs separately from the spray unit’s motion controller. Computer control also allows the system to connect to a company network for file backup and other operations. Ideally, a selective coating system should integrate seamlessly into the assembly line. This integration requires mechanical compatibility (conveyor height above the floor, line speeds, etc.) as well as compatibility with SMEMA signal and timing conventions.

The primary objective in coating software design is to provide a simple, intuitive and logical interface for all aspects of operation, the input of programs for coverage of individual boards, system setup, calibration and diagnostic activities. An intuitive software interface should require a minimum of training for each level of system operation. Most people are familiar with the graphic-based Windows environment. This interface allows the user to point and click, and converting mouse commands directly to program elements. This approach allows inexperienced users to ac-curately program and operate a coatingsystem. On-screen error messages take guesswork out of diagnosis and minimize downtime.

System programming or teaching for a particular board configuration is typically done by moving the coating head from point to point (in the X, Y and Z-axes) on a path over a board and then specifying on and off points for coating applicators. The accuracy of path selection can be improved by a laser-pointing accessory, which gives the programmer a clearer indication of precisely where the coating will impinge on the board. These head layout sequences for PCB assemblies are memorized, given a designation and then executed sequentially from start to finish as each board enters the coating zone. Software control allows coating PCBs of various sizes and configurations interchangeably.

Coating software should be downward compatible so that advancements made by the manufacturer or system users can be easily incorporated in the field. Users may need to re-access a teaching sequence to make command changes or refinements without having to recreate a coating path, as in case when minor design changes are made, or when there are multiple boards in production that have only minor layout differences. If the coating system includes a computer that can display the program path as an image, this can be called from memory and used to identify and alter any points in the path on-screen. By pointing to any location and clicking, the system will move to that point and it can be altered or deleted, or additional points can be added. Selective coating efficiency requires careful, efficient programming to minimize machine motion and downtime, and thus reduce manufacturing costs. Control software commonly allows for a purge function when the system has been idle for a period or at the end of a shift. This function moves the head to a specified location and purges valves with air and/or coating to ensure consistent results. Maintenance and calibration functions control such aspects as coating assembly acceleration/deceleration rates and dispensing needle calibration. Calibration functions are also useful when other dispensers are mounted on the coating system – particularly needle dispensers. In this case, the head moves to a specific point, and measurements can be taken to accept or re-establish accuracy. The same may be required when a motor, encoder or ball screw has been de-coupled or replaced. A simple command recalibrates the system under software management. Diagnostic functions should allow the operator to look at pressure values, the accuracy of sensors used to locate boards before coating, valve positions and the performance of X, Y and Z motor functions. Positioning accuracy can be verified by moving mechanism components a specified number of encoder counts andverifying travel by optical or mechanical sensors.

Flexibility is a key in the design of the optimum coating system. Users require flexibility to meet their needs, and in terms of material – the ability to apply any conventional solvent or non-solvent coating without major system changes. Another important characteristic is a design that minimizes reliance on operator attention. For example, a system might have the capability to monitor the coating supply and provide low-level alarms and automated shutdown to avoid producing poorly coated boards that subsequently have to be scrapped.

A customer recently found that new employees having limited English language skills and no prior manufacturing or computer experience were able to learn to program and operate the SCS 4398 system in just one hour. This process demonstrates the importance of the graphic programming interface, and other well-conceived selective coating system design elements to efficient operation. One useful approach is to provide a configuration page in the system software that can be used to toggle features on and off for easier setup and troubleshooting. A field-installed upgrade can then be activated on screen and functional changes can be implemented easily, to tailor a system to user’s needs. Some manufacturers need to maintain an accumulated count of production – by board type – and these values can be recorded and reset using the configuration page.

The initial investment in automated selective coating can be leveraged to double or even quadruple production throughput in some cases. A coating assembly could be fitted with one or more slave units that are coordinated with the primary coating unit. Such production leveraging is feasible when the total surface area of PCBs can be accommodated within the maximum coating area of the system. With careful head design, the effective coating area with multiple heads can be increased beyond that of a single assembly system. Such an arrangement should be able to replicate the indexing and alignment precision of single board coating – at all four or more coating positions. Separate fluid metering and delivery for each valve may be important to provide consistent coverage from board-to-board.

Yet another method to improve productivity is to place two selective coating systems in series on the production line, with a board inverter between them. This allows both sides of each board to be coated before curing, thus eliminating the time delay of an additional cycle. With proper programming and design, a system can be equipped to switch between coating materials with minimum downtime. With multiple valves mounted to the transport mechanism, each connected to separate coating supplies, production can change from one board type and coating material to a second in a matter of seconds. Important progress is currently being made in the design of valves, in order to allow a higher flow rate without loss of accuracy. With increased fluid flow it is possible to raise line speed substantially for greater production throughput without loss of registration accuracy or consistent coating thickness.

EPP 154

Geht es um selektive Beschichtung von Baugruppen, kann man heute in der hochvolumigen Fertigung diesen Arbeitsschritt mit höchster und gleichbleibender Präzision am besten mit Automaten vornehmen. Wichtig ist, daß sich solche Systeme sehr flexibel anwenden lassen und möglichst einfach programmierbar sind.

Les machines automatiques constituent actuellement la meilleure solution pour réaliser, avec une précision maximale et constante, l’opération de revêtement sélectif des cartes dans la fabrication en masse. Il est important que ces systèmes soient très flexibles et aussi faciles à programmer que possible.

Se si tratta di rivestimenti selettivi di gruppi costruttivi, la soluzione migliore per tale fase di lavorazione consiste nell’uso di apposite macchine, che permettono di ottenere una alta e costante precisione. Di particolare importanza è il fatto che simili sistemi possono essere usati in maniera molto flessibile e che sono programmabili molto facilmente.