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Tooling for the SMT Process
By Chad Gilliland and Eric Miller, Ascentec Engineering, Sherwood, OR
Surface mount pallets (or carriers) are often an essential part of achieving consistent, highest quality success with new product introduction (NPI) or production volume builds. These tools are not new to the industry and are a proven part of designing a robust SMT process. Oftentimes only mechanical support is considered when deciding upon the use of SMT pallets, but a longer list is really needed.
Whenever pallets are being considered and/or evaluated, take the time necessary to remind yourself of the benefits of using an SMT pallet and how you would want it designed. There is a short of items to consider when reviewing the benefits of SMT pallets and the design elements that are important. These items are: PC board protection, reduction of board warpage, improved stencil alignment after depanel, vision and alignment, conveyance, edge clearance, process standardization, protection/heat shielding, and critical alignment of parts.
PC board Protection.
SMT pallets reduce the amount of PC board handling by line operators and may provide for easier, lower-risk transportation and/or storage. Damage can occur in-line and off-line and pallets can help minimize that damage.
Reduction of Board Warpage.
Providing mechanical support throughout the entire SMT line (top and bottom assembly) can reduce board warpage that can significantly reduce yields. SMT pallets prove to be really essential when processing thinner substrates — such as PCMCIA, flex circuits, and rigid flex boards — but quite often prove beneficial with substrates that are considered "thick". The specific design features chosen for a given SMT pallet are critical to minimizing warpage. Threaded inserts may be utilized to secure the board to the pallet within the reflow process. Utilizing tapered tooling pins located in the tooling holes of the PC board (plated or non-plated), a wedge effect is created to secure the PC board to the pallet PC board pocket during screen print, pick and place and reflow processes. Tension or pressure against these tapered pins is generated by utilizing flush-mounted "pushers" or "pullers" with varying spring K-factors (dependent upon PC board thickness and stack-up characteristics) helping to maintain PC board flatness throughout the entire process.
Improved Stencil Alignment after Depanel.
Printing smaller images after depaneling operations (singulation) can improve yields when fine pitch devices are present. PC boards are oftentimes received in a de-paneled condition and the SMT pallet is the only solution to conveying and processing the assembly. Proceed with caution when deciding the number of images to place into one SMT pallet. Productivity is obviously improved as the quantity increases, but stencil-to-PC board alignment is reduced exponentially with each image added. A good rule of thumb is to never process more than 2 images in a single SMT pallet.
Vision and Alignment.
SMT pallets can provide the required or additional fiducials needed to achieve optimum stencil registration and placement accuracy. Ask your tooling provider to explain how they design and fabricate pallet fiducials to be robust and easily seen with your process equipment.
Many "odd-shaped" PC board layouts are not easily conveyed through SMT process equipment and pallets provide solid edge step features making smooth conveyance a non-issue. Pay attention to the edge step thickness whenever a pallet is designed.
Parts too close to the PC board edge are at risk of being damaged during first or second side assembly processing without an SMT pallet. SMT pallets can be designed with clearance for these parts (on either side) while still providing solid support for the rest of the PC board.
Machine and/or line changeover can be reduced if a standard pallet design (width, thickness, material) is used with all the assemblies in your portfolio of products. For single-sided processing, pallets can often be used in other "downstream" processes such as AOI or electrical testing. SMT tooling can also eliminate the need for high-cost vacuum or custom support plates in printing or placement equipment. Pallets also provide the added benefit of not creating additional operator training, engineering support, or equipment maintenance support.
Heat-sensitive components can often be shielded through the secondary reflow process. A robust design will also distribute temperatures evenly across the PC board minimizing thermal impact.
Critical Alignment of Parts.
Top cover alignment plates or integrated pallet features can help achieve critical alignment or spacing of parts such as edge connectors during reflow.
Partnering with a tooling vendor that is experienced in the PCBA industry is an important first step when considering SMT pallets. Be sure to review each of the previously listed considerations with your internal process development team and with your tooling provider.
Creating a Unique Pallet
Creating each SMT pallet for each unique circuit board design is critical to the success of the fixture. A thorough understanding of thermodynamics and the effects a composite carrier will have in the reflow environment is an essential part of the success or failure of any pallet. A PCBA data package should include Gerber files, PC board/Fab drawing, and a BOM (w/AVL). Also needed will be "smart data" files (Valor ODB++) containing part specification information along with a BOM and AVL. Also, material selection should be discussed with your tooling vendor to ensure that the proper thickness, composite formulation, ESD properties, etc. are selected for your application.
It is also important to note that it is often advantageous to use a composite that is optically sensitive such as VonRoll CDM 68.940 Grey. The use of a composite such as this will allow your pallet to be "seen" by the each machine and conveyor with the SMT line and minimizes (or eliminates) the need for high temp paint, Kapton tape, metal inserts or tape, etc. used by many to trip line sensors. SMT processing presents several challenges; it's important to minimize factors that keep you from a low-yield process. Doing this successfully can sometimes seem overwhelming or impossible. If designed and fabricated correctly, SMT pallets will help to minimize several of these factors and should be considered a valuable tooling option that will have a short return on investment period.
Contact: Ascentec Engineering, 13565 SW Tualatin-Sherwood Rd., Suite 800, Sherwood, OR 97140
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