-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Practical ideas on Large-board Printing
December 31, 1969 |Estimated reading time: 6 minutes
Tips for production printing of the latest large-panel PCBs are provided, including handling, tooling, processing, vision and understencil alignment, and accessibility.
Large-panel boards and by that we mean individual boards 24 x 28" and larger began appearing in the industry more than five years ago. Though this trend began as a trickle (with no more than a question such as, "Can you process this?"), it has become a virtual flood, now featuring ultra-large printed circuit boards (PCB) of many layers, sometimes 0.5" thick, and heavy. Nor are they especially all backplanes or backpanels. The vast majority are simply PCBs: large motherboards, high-density, SMT, through-hole and mixed technology. The large size of these boards is, in part, driven by electrical, packaging, thermal and cost considerations; but whatever the reason, these boards present an entirely new slate of concerns in processing, in this case, stencil printing.
Naturally, it has become necessary for equipment manufacturers from printing, to pick-and-place and onward to redesign their equipment to accommodate these larger sizes. This article will discuss issues involved in production printing of large-panel circuit boards from the perspective of the solder paste stencil printer manufacturer.
Handling and TransportWhile not necessarily involved with the actual printing, handling issues in this case, loading and unloading, and transferring the boards from one machine in the line to another are very much a part of large-board processing. Conveyorized transport systems that easily can handle a 12 x 12" PCB are not necessarily capable of properly handling a 24 x 52" board that weighs 60 lbs or more. There may be a need for significant investment in racks and other material handling equipment designed to properly support and manage such boards. Managing such awkward, large, heavy boards also brings personnel issues into play, particularly when manual loading/offloading is used. As a result, these assemblies can be worth from $10 to 30K when finished. Many of these boards/assemblies contain fine- and ultra-fine-pitch components. Clearly, the costs of mishandling, dropping or damaging such a board can be considerable.
Work Platforms and ToolingConsistent, repeatable, precise solder paste stencil printing requires a substrate to be held as flat as possible. Precision tooling with vacuum has long been the industry standard. For second-side printing applications, however, when a large panel has passed through reflow and may have experienced some additional warping, how well will vacuum tooling hold a robust 0.5" many-layer board effectively flat? In such cases, hard tooling again a board support issue often is needed, and the quality and nature of the tooling is key. Clamping, hard tooling and custom machined tooling must come into play. There can be significant cost in this. Consider that the larger the board, the more pronounced the effect of warping could be as one travels to the outer edges. Still, for most applications, vacuum tooling and under board supports are sufficient to flatten the board and hold it securely, aided by the large surface area of the board itself.
Figure 1. Large boards may exhibit warpage, particularly double-sided boards following the first reflow pass. Keeping the board flat and stabilized is essential to achieving good and repeatable printing results.
Larger Boards = Narrower Process WindowsThough it seems paradoxical, it nonetheless is true that the larger the board becomes, the more difficult it is to keep variability out of the printing process. For this reason, we need to build in automated functions to correct such things as variables in the boards themselves (artwork and registration shift, as well as bow and twist). Advanced features such as adjustable PCB support, programmable squeegee drive, pressure, stroke and speed also may be helpful. Automatic vision alignment systems, an understencil wiper module, and motorized or programmable conveyor width adjustment are essential to ensure consistency and to compensate "on the fly" for board variations. Newer printers designed to handle the largest print areas now 24 x 52" and larger must incorporate such means of physical board management as PCB flatteners to optimize the vacuum hold down that must be present to offset the warpage, bow and twist (Figure 1).
Vision Alignment and Auto Vision CorrectionAutomatic vision recognition of fiducials and motor-driven alignment of the tooling plate results in the highest accuracy and greatest possible throughput. This is especially true for large boards because it adds consistency and accuracy (widens process window) and is necessary, quite simply, because of the enormous cost of each board, as previously mentioned.
Typical vision alignment systems that are available in the current large-panel processing marketplace include the simple, but effective, closed circuit television (CCTV) vision alignment on manually loaded machines. However, for optimum alignment results, an NT-based auto correction system is recommended. These computer-based vision systems can be specified on manually loaded or automatically fed systems.
Programmable SqueegeesA key to establishing and maintaining the process is the precise control of the squeegee assembly. On these large PCBs the consistent pressure and stroke length is of paramount importance. It is extremely important that each squeegee is controlled independently. The tolerance build up in this process type is such that precision pressure control must be maintained to a minimum of 0.001".
Understencil WipingWith a perfectly flat stencil, the "gasketing effect" will reduce solder paste significantly from squeezing out under the stencil during the printing process, causing wet bridges. Life and printing are less than perfect, and unfortunately so are many large boards, in planar sense. Programmable automatic understencil wiping is necessary to remove this excess material from the bottom of the stencil, which can spread paste contamination to subsequent boards when slight warping and variations in flatness or board topography are encountered.
Accessibility and Ease of UseThis may seem, at first glance, to be a matter of lesser importance; however, this is not the case. Accessibility is key when maneuvering a large and awkward board into or out of a printer (Figure 2). In the event that there is a problem in the process area, a technician must be able to get into the process area quickly and correct the problem. There is little value in aerodynamic exterior cosmetics if they restrict access to the print area.
Smaller Dimension Print DirectionAdditionally, it is imperative, as boards become larger and larger, to try to keep squeegee clamp size from getting too unmanageable because the longer the squeegee, the more difficult it is to maintain a stable, smooth print. Consider that most printers print in the front to back direction, across the line of process flow. If we consider the challenge of printing a
Figure 2. Wide-open access to the printing area is essential for handling and properly managing large boards during the printing process. Handling and ease of handling is a major consideration in large-board printing.
24 x 60" board a probable, potential size in the near future a printer that prints front to back will have to have a squeegee 5' feet wide. Therefore, printer manufacturers should consider printing in the direction of the smallest dimension, (i.e., the same as the direction of flow through a line right to left, or the reverse). There are printers available that will print in this direction. An added benefit to this is solder paste usage and waste reduction. Also, consider that even with the technology available, such control will become even more difficult as squeegee lengths increase dramatically.
ConclusionLarge-board processing is uncharted territory. Those used to 12 or 18" boards with features that are in sync with the Gerber data will be astounded to find as much as a 0.020" (20 mil) shift of the artwork on a large-panel PCB. Still, the board simply cannot be rejected; printers must print it correctly and build a robust, working assembly. Learning the unique requirements of these boards and effecting a paradigm shift in thinking will be necessary for reliable, repeatable, and high-yield printing and assembly of large-panel boards.
JIM GLASGOW, president, may be contacted at Surface Mount Techniques, 23042 Alcalde, Unit C, Laguna Hills, CA 92653-1326; (949) 768-0178; Fax: (949) 768-0397; Web site: www.smtprinters.com.