Electronic manufacturing centers face multiple challenges as they implement new processes and procedures to support leadfree manufacturing requirements. Continuous miniaturization of components coupled with inconsistent wetting behavior of lead-free solder paste has created increased concerns for stencil fabricators and manufacturers of stencil printers. Today, more than ever, stencil printers must provide accurate, repeatable deposition of solder paste, meet production thru-put requirements, and provide real-time printing and inspection solutions. Recent stencil printer technology advancements have achieved a reliable, automatic inspection process of the substrate, with the capability to auto-correct substrate misalignment, prior to printing. By coupling high-resolution optics to precision motors utilizing digital servo controllers with optical feedback, fiducial recognition and real-time correction to mis-aligned substrates is achieved. The high resolution optic recorder is mounted to a servo-controlled, XY gantry located above the stencil. Levels of direct and diffused intensity-controlled illumination are obtained for evaluation. This unique process acquires any shape on the substrate or stencil which can be used as a fiducial; multiple fiducial locations can be selected to further increase alignment accuracy.

Additional parameters must be recognized to achieve proper PCB solder paste printing. Excessive squeegee pressure is a known contributor to numerous print errors including solder bridges. To address this concern, some stencil printer manufacturers integrate two independent servo-controlled motor-driven print heads to provide real-time pressure measurement. Based on product process requirements, the squeegee pressure is pre-programmed via a user interface. Squeegee pressure is then monitored up to 2000 times per second and when deviations are identified, corrections to pressure are automatic thus ensuring optimal squeegee pressure. In addition to monitoring and managing squeegee pressure, each print head is equipped with an optical encoder that constantly monitors print head height, automatically implementing adjustments when required. Manufacturers of stencil printers are implementing technology-based process controls to ensure maximum yield and achieve the requirements of quality standards.

Exerra Corporation has recently announced its 2D&DTM print quality inspection system. The 2D&DTM is based on a proprietary image processing generator that enables 100% inspection of 100% of assemblies - without compromising on throughput. During each inspection cycle, the color camera rapidly scans the PCB and acquires a set of color images at a resolution of 8 microns. The images are then compared to reference images and adjusted to meet product acceptability standards. The application of a color camera enables the inspection system to analyze solder color differentiation, which if often not possible with some existing monochrome camera-based processes. The intuitive interface enables the user to define the entire PCB or select “areas of interest” as required. Each “area of interest” is presented as an icon on a graphical representation of the PCB, and can be magnified for close-up inspection. Each type of defect is displayed on the screen in a unique color, making it easy to identify various types of problems, when identified. The frequency and level of inspection is userdefined and can be executed after every printing and stencil cleaning cycle. The inspection system generates both statistical and graphical reports, featuring automated alerts when measurements exceed defined tolerances.

The Exerra ep-33 features an automated stencil aperture inspection process to ensure that apertures are free of clogged paste. As in the printing inspection, the user can define whether to inspect the entire stencil or only selected areas of interest (for example, fine-pitch components). The results of the stencil inspection can be set to trigger automatic or manual stencil cleaning. The In Process Stencil Cleaner (IPCTM) operates simultaneously as the stencil shuttle retracts, ensuring maximum throughput. The stencil cleaner offers both dry and wet wiping, or can be programmed for a combination of both. For wet cleaning, the solvent is evenly distributed on the surface of the cleaning paper with a unique spraying mechanism. The amount of solvent used and the rolling speed can be pre-programmed by the operator. The paper used in each cleaning cycle is also constantly monitored. The Exerra ep-33 utilizes a flexible and easy to set-up print nest with magnetic support pins and bars for underside support. Boards are held firmly in place during the print cycle by pressure-adjustable side clamps, which guarantee reproducible printing; optional vacuum system and dedicated work-holders are also available. The Exerra ep-33 features a clearly structured and intuitive, mouse-driven user interface that simplifies operation and drastically reduces training times. Utilities and parameters are logically displayed on the screen, enabling quick access and extremely fast programming. Unused functions can be switched off to prevent any unauthorized process changes, and all levels can be locked, ensuring that stable processes cannot be changed.