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Low-defect Printing for Large PCBs
December 31, 1969 |Estimated reading time: 6 minutes
New process methods that can provide high yields on today's largest ever PCBs are discussed.
By Jim Bernhard
Over the past year and a half, the electronics market has diverged into two distinct directions. One direction continues decreasing in size of electronics products, and thus the size of printed circuit boards (PCB). The other direction drastically drives PCBs to larger sizes. This direction has been created mainly by Internet infrastructure and storage markets that require advanced technology backplanes as an integral part of the products in those markets. The increase in processing speed requirements has caused an evolution from systems that previously used several backplanes cabled together to systems with one large panel encompassing more direct signal paths. These performance requirements have pushed the size of PCBs that need to be processed past the 20 x 20" panel size to panels of 36 x 24" and even up to 60 x 36" (Figure 1). This increase has caused some monumental challenges for the OEM and contract manufacturers (CM) who are processing these assemblies. In turn, these same challenges fall to equipment manufacturers. These advanced technology backplanes incorporate various fine-pitch and array components, requiring equipment to provide the same results on large PCBs as on small.
Figure 1. Today's largest PCBs require entirely new equipment designs to deal with their increased size and weight. Stencil and process design mandate thinking outside the box of traditional SMT.
Basic Machine Design PrinciplesThe foundation of any piece of equipment designed for large-scale processing is the frame itself. The inherent stresses involved in printing such large and heavy substrates require robust mechanisms to maintain reliability and quality control. To ensure reliable positioning in the X-, Θ- and Z-axes, the drive systems should be mounted on a frame of sufficient mass to prevent any machine flexing. Also, print areas of such substrates require high print pressures to achieve a clean wipe of the stencil surface as well as maintain the board and stencil gasketing. A rigid frame is required to house the stencil clamping and squeegee drive mechanisms without suffering from deflection. Some systems are designed with castings of sufficient mass to be able to handle the forces in the machine. These castings generally are developed using finite element analysis to guarantee structural stability.
Figure 2. Robust print head mechanisms are required to effectively apply the needed higher print forces reliably without frame deflection. Closed-loop pressure feedback is essential in overcoming substrate warpage and achieving acceptable print definition.
Print Head. Squeegee print head control can be the most important contributor to a consistent print across such a wide area (Figure 2). Closed-loop pressure control systems are ideal for maintaining the consistency required. The squeegees must work to not only print the paste with a clean wipe across the stencil, but also to conform to any board/stencil planarity differences and maintain the all-important print gasket along the entire print length. Pneumatic print head systems work best because of the fast response time while being least susceptible to Z-axis changes throughout the print stroke.
Substrate Support. Substrate support is more critical in thinner, processor board-type substrates vs. thicker backplanes. In either case, however, the table must be able to accommodate support across the substrate's entire length. Generally, support must be repeatable and effective so that the print process achieves the same results week after week, set up after set up. A critical element to success is shear access to the print nest for ease of set up and cleaning. Finally, the best printers have systems to guarantee the support consistency. Laser guidance of support pins followed by vision verification of pin location has been found to be a simple, repeatable and virtually maintenance-free solution.
Board Hold Down and Clamping. Sizable substrates must be held securely for the most accurate alignment and to prevent shifting during the high-pressure print stroke. Thicker backplane style substrates typically have more than sufficient component edge clearance, which accommodates top clamping mechanisms to flatten the substrate. Other PCB types offer very little edge clearance, which then requires side clamping to achieve the best print definition in the areas close to the substrate's edge. The latest systems have both top clamping and side clamping to accommodate a variety of board styles. These clamping systems can provide a more consistent print than vacuum hold-down systems. Given the large amount of through holes, mounting holes and vias on these substrates, vacuum leakage can both pull paste between the board/stencil gasket, and hinder the board's slow release from the stencil after print. Still, when combined with dedicated tooling to avoid these holes, vacuum can be helpful in some applications. For these cases, the ability to accommodate vacuum hold down is a desirable printer capability.
Figure 3. An automatic look up/look down vision system can be an indispensable tool in achieving the best alignment for a board and stencil combination with shifted/stretched artwork.
Alignment. A key challenge in achieving a high-yield print is the shifting geometries of such large entities. The substrates themselves can have significant artwork stretch. Stencils with artworks of this size also are a challenge for most stencil houses. Aside from requiring larger and thicker stencil frames, the majority of laser and chemical etching equipment is not capable of cutting the stencil apertures into the larger foil in one pass, if at all. Because of this difficulty, the stencils are subject to stretching and inaccuracies from step and repeat laser-cutting procedures. The best way to handle such mismatches is to use a vision system with automatic best fit algorithms to minimize the error across as many as four fiducials (Figure 3). Furthermore, enhanced software features, such as the ability to designate a critical fiducial nearest a cluster of the finest-pitch devices can be beneficial. This allows the best alignment, specifically in the most critical area. Dual light source systems also can help in achieving the best fiducial contrasts, even when plating quality varies significantly.
Figure 4. Thicker and heaver backplanes require specialized handling to reliably convey them in a fully in-line process. Minimizing operator lifting and intervention is key to a safe and controlled process.
Handling/Ergonomics. As boards become larger and heavier, weighing as much as 70 lb, using semiautomatic clamshell style printers violates ergonomic standards for lifting. Substrate loading and unloading would require two people to be performed properly and without risk of injury. Prudence dictates that these style boards be conveyed and handled automatically. The latest printers use heavy-duty conveyors for complete in-line processing with no manual intervention (Figure 4). The in-line process also can facilitate a true vertical release of the board from the stencil at a controlled speed to achieve optimum print definition. Finally, the specific in-line design can offer floor space advantages.
Wiping. Considering how fine-pitch SMT and area-array packages are becoming much more prevalent on these assemblies, they require all the process controls currently used on today's smaller high-density assemblies. Automatic wet/dry/vacuum stencil wiping is critical to a successful print yield. Due to the shear inability to perform this process effectively over such a wide area by manual means, a large format printer must be able to perform this task automatically.
Post-print Inspection. Some rather large gaps in the production line still exist as the equipment industry works to catch up with the emerging need for large-format processing. One of these critical areas is automatic optical inspection, in which no large-format solution currently exists. In this circumstance, a printer capable of inspecting paste brick deposits, as well as bridging between pads becomes an invaluable process control and verification tool. Furthermore, printers capable of inspecting stencil apertures prior to print can be effective defect prevention tools, especially if clogged apertures can be cleaned automatically with a vacuum wiper.
ConclusionAs large-format processing becomes more and more prevalent, both equipment suppliers and users will have to learn new process methods to achieve high yields. The learning curve will need to be fast for any company that wishes to be successful. Partnering with the suppliers that are on the forefront of this technology can be a key factor in overcoming the large-format challenges ahead.
Jim Bernhard, vice president, sales and customer support, may be contacted at EKRA America, 34 Saint Martin Dr., Marlborough, MA 01752; (508) 486-9566; Fax: (508) 486-9567; E-mail: jbernhard@ekra-america.com; Web site: www.ekra.com.