PCB Assembly Tooling -Pallet (/Carrier)
In electronic manufacturing industry, when mention Printed Circuit Board Assembly (PCBA) Tooling, people always think of Stencil, Pallet (or Carrier) and Test Jig (or Fixture). Today, we talk all about the Pallet (/Carrier) in SMT process and wave soldering process.
Surface mounting pallets (or carriers) are often an essential part of achieving consistent, highest quality success with new product introduction (NPI) or high-volume PCB assembly runs. These tools are not new to the electronic manufacturing industry and are a proven part of designing a robust surface mounting (SMT) process. Oftentimes only mechanical support is considered when deciding upon the use of SMT pallets, but there are many purposes and advantages of using SMT pallets.
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.
Preventing Rigid & Flex PCB Warpage
Providing mechanical support throughout the entire SMT line (top and bottom assembly) can reduce warpage of rigid PCBs, Rigid-flex boards and flex circuits that can significantly reduce yields. SMT pallets prove to be really essential when processing thinner substrates, such as flexible 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 PCB board to the pallet withing the reflow process. Utilizing tapered tooling pins located in the tolling holes of the circuit board (plated or non-plated), a wedge effect is created t secure the printed board to the pallet PCB pocket during screen print, pick and place, and reflow processes. Tension or pressure against these tapered pins is generated by utilizing flush-mounted “pushers” with varying spring K-factors (dependent upon PCB board thickness and stack-up characteristics) helping to maintain printed board flatness throughout the entire process.
Improved Stencil Alignment after De-paneling
Printing smaller images after depaneling operations can improve yields when fine pitch devices are present. PCB boards are oftentimes received in a de-paneled condition and the SMT pallet is the only solution to conveying and processing the SMT 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-PCB 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 PCB 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 printed circuit boards.
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, engineer support, or equipment maintenance support.
Protection /Heat Shielding
Heat-sensitive components can often be shielded through the secondary reflow process. A robust design will also distribute temperatures evenly across the PCB 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 MADPCB that is experienced in the PCB assembly 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 (/carrier) for each unique circuit board assembly is critical to the success of the fixture. A through 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, PCB board /Fab drawing, and a complete BOM (with manufacturer P/Ns). Also needed will be “smart data” files (ODB++) containing part specification information along with a BOM. What’s more, material proper thickness, composite formulation, ESD properties, etc. are selected for your application.
Pallet Material: Durostone
When visit PCB assembly house, you may see dark grey pallets on each machine and conveyor with the SMT line. What’s the pallet material? Many people concern this question. In MADPCB, pallet material is always Nanometer Composite Stone, also called Durostone, but sometimes is Aluminum. Durostone is the most commonly used when making pallets. Durostone is a heavy-duty glass fiber reinforced plastic, which offers extreme strength and excellent mechanical and thermal properties at elevated temperatures and are dimensionally stable and retain pallets flatness through repeated reflowing cycles in SMT process. Durostone is manufactured by using Polyester (PET), Vinylester, Epoxid- and modified Epoxy resins, combined with carbon, glass, or basalt fiber reinforcements. Durostone solder pallets can be continuous used again over 20,000 time under high temperatures. SMT pallets manufactured with durostone bring a lot of benefits to all PCB and assembly manufacturers. Pallets cost is NRE charge, and you can consider it as a valuable tooling option that will have a short return on investment period.