I have encountered numerous challenges related to PCB design. The assembly density is often excessively high, necessitating the placement of tall components like tantalum capacitors, chip inductors, and fine-pitch SOIC and TSOP devices on the PCB’s soldering surface. In such cases, the only feasible option is to employ double-sided solder paste printing for reflow soldering. Additionally, through-hole components should be arranged as compactly as possible to facilitate manual soldering. Another approach is to position through-hole components in several primary straight lines on the component surface. This arrangement accommodates the latest selective wave soldering processes, reducing the need for manual soldering, which in turn enhances efficiency and ensures solder quality. It’s important to note that dispersing solder joint distributions is detrimental to selective wave soldering, as it can exponentially increase processing time.
When adjusting component positions in the PCB layout file, it is crucial to ensure a one-to-one correspondence between the components and their silk-screen symbols. Moving components without relocating the adjacent silk-screen symbols can lead to significant quality hazards in manufacturing. In actual production, silk-screen symbols serve as an industry language that guides the assembly process.
1. The PCB must include clamping edges, positioning marks, and process positioning holes essential for automated production. Currently, electronic assembly ranks among the most automated industries. The automation equipment requires the PCB to be automatically transmitted, necessitating a clamping edge of no less than 3 to 5 mm in width along the transmission direction (typically the longer side). This ensures that components near the edge of the board can be assembled automatically.
Positioning marks are vital for the optical positioning assembly equipment widely used today. The PCB should provide at least two to three positioning marks for the optical identification system, allowing for accurate positioning and correction of PCB processing errors. Commonly used positioning marks should include two marks placed diagonally on the PCB. Standard graphics, such as solid round pads, are typically selected for these marks. For ease of identification, a clear area devoid of other circuit features or markings should surround each mark, with the size preferably not less than the mark’s diameter. Each mark should be at least 5 mm away from the edge of the board.
In PCB manufacturing, processes such as semi-automatic insertion and ICT testing require the PCB to have two to three positioning holes located in the corners.
2. Employing puzzles effectively can enhance production efficiency and flexibility.
When assembling PCBs with small or irregular shapes, various limitations arise. As a result, the common practice is to splice several smaller PCBs together to form a PCB of suitable size for assembly. Typically, for PCBs with a single side measuring less than 150mm, the boarding method is recommended. By combining two, three, four, or more small PCBs, you can achieve a larger PCB within the appropriate processing range, usually 150mm to 250mm in width and 250mm to 350mm in length. This size is more compatible with automated assembly processes.
Another splicing method involves assembling a PCB with SMD on both sides into a larger board, a process often referred to as Yin-Yang splicing. This technique is primarily aimed at reducing the costs associated with network boards; originally requiring two screens, it now only needs one. Additionally, when technicians create the programming for the placement machine, the efficiency of PCB programming using Yin and Yang splicing is significantly enhanced.
The connection between the sub-boards can be achieved through double-sided engraved V-slots, long slots, or round holes. However, the design must ensure that the separation lines are as straight as possible to facilitate final splitting. It is also crucial to position the separation edge away from PCB traces to prevent damage during the board splitting process.
There is also a cost-effective jigsaw approach that does not refer to PCB jigsawing but rather to the mesh pattern of the stencil. With the advent of fully automatic solder paste printers, modern printers (such as the DEK265) now allow the creation of multi-sided PCB mesh patterns on a steel mesh measuring 790x790mm, accommodating multiple stencils. This method of printing individual products is particularly economical, especially for manufacturers dealing with small batches and diverse varieties.
**3. Testability Design Considerations**
The testability design of SMT primarily addresses current ICT equipment conditions, anticipating testing issues that may arise during later stages of product manufacturing. To enhance testability, both process design and electrical design aspects should be taken into account.
**4. Process Design Requirements**
Factors such as positioning accuracy, substrate manufacturing procedures, substrate size, and probe type all influence detection reliability.
(1) **Precise Positioning Holes**: It is essential to incorporate precise positioning holes on the substrate. The tolerance for these holes should be within ±0.05mm. A minimum of two positioning holes is recommended, with an ideal distance between them. Non-metallized holes should be used to minimize the thickening of the solder plating layer that could compromise tolerance. If the substrate is fabricated as a single unit before individual testing, positioning holes must be included on both the main board and each sub-board.
(2) **Test Point Specifications**: The diameter of each test point should be no less than 0.4mm, with adjacent test points ideally spaced above 2.54mm and no closer than 1.27mm.
(3) **Component Height Restrictions**: Components exceeding a certain height should not be placed on the test surface, as they may hinder proper contact between the probe and the test point.
(4) **Test Point Clearance**: It is advisable to position test points at least 1.0mm away from any components to prevent damage to both probes and components. No components or test points should be within 3.2mm around the positioning hole’s ring.
(5) **Edge Clearance**: Test points should not be placed within 5mm of the PCB’s edge. This 5mm buffer ensures proper clamping of the fixture, which is typically required on the same side during conveyor belt production and SMT processes.
(6) **Plating of Test Points**: Ideally, all test points should be plated with tin or utilize a soft, easily penetrable, non-oxidizing metal to ensure reliable contact and extend the probe’s service life.
(7) **Solder Resist and Ink Limitations**: Test points should not be obscured by solder resist or text ink, as this would diminish the contact area and reliability of the tests.
**5. Electrical Design Requirements**
(1) **Via Hole Connections**: SMC/SMD test points should be routed to the soldering surface via via holes wherever possible, with a diameter exceeding 1mm. This facilitates online testing using a single-sided needle bed, thereby lowering testing costs.
(2) **Test Point for Each Node**: Each electrical node must have a corresponding test point, with POWER and GROUND test points for each IC positioned as close as possible, preferably within 2.54mm of the component.
(3) **Trace Width for Test Points**: When establishing test points on circuit traces, the width can be expanded to 40 mils.
(4) **Distribution of Test Points**: Test points should be evenly distributed across the printed board. Concentrating probes in one area can lead to excessive pressure, potentially deforming the board or the needle bed, and may prevent some probes from contacting their respective test points.
(5) **Power Supply Line Break Points**: Test break points should be established along the power supply line in various areas of the PCB, allowing for quicker and more accurate fault location when a power supply decoupling capacitor or other components short-circuit. When designing breakpoints, consideration should be given to restoring test functionality.
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