News
How Accurate Should the Printer Be?
As accurate as is needed for the products being built today - and tomorrow.
I just returned from a four-week trip to China to conduct process seminars for customers there. Among the seminar topics presented, the most popular by far was the 01005/0201-component assembly process. None of these customers is currently using 01005s. Some had just starting using 0201s, but all have heard that products they will soon begin building may contain 01005s.
Assembly of small 01005 components creates a number of challenges for standard SMT processes (printing, placement, reflow). Those who have not seen an 01005 (Figure 1) would be amazed at how small they are. I have "looked" at many but have not necessarily seen them since they are barely visible to the naked eye.
Imagine lining up the very tiny stencil apertures used for 01005s with the very small board pads (one customer in Asia uses a pad of 0.006 x 0.006" with a 0.004" space between the pads for 01005s, although most customers are using slightly larger pads). Now imagine aligning the 10, 50, 100 or perhaps 1000 01005 component apertures with their board pads. Printing equipment accuracy, repeatability and stability (the capability of the equipment to maintain its original accuracy and repeatability for its entire service life) are critical in aperture-to-PCB pad alignment. Several other factors such as stencil accuracy and PCB accuracy also influence alignment precision.
The combination of Pb-free materials that do not solder as well as SnPb materials with extremely small components requires that each process be fully understood, characterized and optimized. Achieving acceptable yields and quality levels will require sophisticated engineering. Process optimization via trial and error - never a good idea - is now a near-certain path to defects, scrap and rework.
One large customer understood the concept that "a machine is not a process" (as discussed in Circuits Assembly, June 2006), but wanted to know how an equipment manufacturer determines how accurate a printing machine should be and how accurate the printer he purchases should be. Great questions.
The answer to the first is simple: we do not. When we design a new printer we survey customers to determine what performance specs and features the machine must have to provide the capabilities they require. We use other sources such as the iNEMI Roadmap, the IPC technology roadmap and industry forecast reports. This is basic marketing research that any company should do before developing any product. There is also the practical consideration of developing a machine with the identified capabilities and features for a competitive cost. The combination of these inputs determines the machine's performance specifications.
From customer roadmaps we derived a need for more accurate, repeatable and stable printing equipment. We presented proposed equipment specs for improved accuracy and repeatability, and a stable platform that maintains the initial accuracy and repeatability over the life of the printing equipment - requirements our customers verified. We along with our customers know the accuracy and repeatability of the printing equipment that has been available for a number of years. Identifying how much more accurate the new equipment must be is an exercise in understanding the component, PCB size and other process trends. A number of other factors must be considered including process cycle time, post print inspection, maximum and minimum board size, board thickness, etc.
The answer the second question is also simple: As accurate as is needed for the products being built.
Having said that, the customer must also think about what new products it might build over the next several years. Companies that make mobile products, for which 01005s are being considered, should consider more accurate, repeatable printers. EMS companies, which often move process equipment from site to site, should consider the stringent applications they will be required to satisfy.
Many companies' products will not require the smallest components. That is why major printer OEMs make several equipment models with different levels of accuracy and repeatability.
The approach to selecting printers that provide the required accuracy, repeatability and stability is a combination of engineering analysis, technology tends, your particular business and the equipment supplier's understanding of your requirements.
Joe Belmonte is project manager, advanced process development, at Speedline Technologies (speedlinetech.com); jbelmonte@speedlinetech.com. His column appears semimonthly.
