SMT assembly manufacturability design
Design for Manufacture (DFM: Design for Manufacture) is a design method. Its main idea is to consider the possibility, efficiency and economy of manufacturing at the same time when designing a product, that is, the manufacturability (or processability) of the product. . In this case of simultaneous consideration of design and process, not only many hidden process problems can be exposed early, avoiding design rework; at the same time, the manufacturability of different design schemes is evaluated, and the processing cost is optimized, which can significantly reduce costs. , Shorten the manufacturing cycle and enhance the competitiveness of products. At present, DFM technology has developed into DFx series technology, which refers to the design for manufacturing process, assembly, rework, testing, reliability and environmental protection, that is, the full life cycle process of product manufacturing and use is fully considered in the design stage.
1. Technical content of manufacturability design in SMT assembly
Just like in other industries, DFM is equally applicable and very important in SMT assembly and SMT product design. Taking the PCB-level circuit module assembly design as an example, the DFM technology of PCB circuit modules aims to carry out the assembly design of high-density, high-precision surface-mounted circuit modules, and the constraint analysis of manufacturing system resource capabilities and status, and finally form a support for developers to circuit Module manufacturability design standards and guiding specifications. The main content involved are:
① Establish the interface module between EDA design software and manufacturability analysis system;
② New packaging components, such as BGA, CSP, FLIP-CHIP and other land pattern design technology;
③ Design of conveying edges, positioning holes, and positioning symbols required for automated production;
④ Selection technology of assembly process accessories;
⑤ Solder paste and patch coating technology for surface assembly circuit modules;
⑥ Surface mounting circuit module mounting technology;
⑦ Surface assembly circuit module welding technology;
⑧ Manufacturing equipment resource capacity analysis technology;
⑨ Establish design specifications for manufacturability of surface mount circuit modules.
The manufacturability design analysis software system involves two major parts: one is PCB assembly analysis; the other is PCB
Machinability analysis. The above two categories of content correspond to some specific classifications, and the specific classifications are as follows.
(1) PCB assemblyability analysis
Reference point analysis: presence or absence of reference point, size of reference point, distance between reference point (distance between reference point and board edge, distance between reference point and component, distance between reference point and wiring).
Component analysis: component layout analysis, component spacing analysis (component and component spacing, component and board edge, component and wiring, component and silk screen characters), component height, spacing.
Pad analysis: pad size design, pad spacing, via analysis (via under SMD, spacing between vias, spacing between vias and components).
Test point analysis: test point size, test point spacing, test point inspection on wiring; solder mask analysis. Silkscreen analysis: silkscreen position check, polarity labeling, and tag repeatability check.
PCB analysis: the presence or absence of the transmission side, the size of the transmission side, and the direction of the transmission side.
Package and pad matching analysis: the ratio of the length of the pin to the pad, the ratio of the width of the pin to the pad, and the area ratio of the pin to the pad.
(2) PCB processability analysis content
PCB wiring inspection, via inspection, process hole inspection, signal layer analysis.
2. Manufacturability design analysis software system
The architecture model of the manufacturability design analysis software system is shown in Figure 5.1.
3. Key technical points of manufacturability design
(1) Requirements for PCB substrate
In addition to providing the basic structure required for assembly, the function of the substrate also provides the wiring and heat dissipation functions required for power and electrical signals. It is required to have sufficient mechanical strength (resistance to distortion, vibration and impact, etc.), be able to withstand heat treatment and impact in the assembly process, have sufficient flatness, and be suitable for PCB manufacturing processes, and have good electrical properties (such as impedance, dielectric constant) Wait).
① PCB size range
From the perspective of production, the smallest veneer size should not be less than 250mm X 200mm; the general ideal size range is width (200mm ~ 250mm) X length (250mm-350mm), for the long side size is less than 125mm, or the short side is less than 100mm, Or the size range is less than 125mm X 100mm
The PCB should be assembled in a way that it can be converted to an ideal size that meets the production requirements for easy assembly.
② PCB thickness
PCB thickness refers to its nominal thickness (that is, the thickness of the insulating layer and copper foil). The PCB thickness should be selected according to the board size and the quality of the installed components. Recommended thickness is 0.5mm, 0.7mm, 0.8mm, lmm, 1.5mm, 1.6mm, (1.8mm), 2mm, 2.4mm, (3.0mm), 3.2mm, 4.0mm, 6.4mm, 0.7mm and] .5mm PCB is used for double-sided design with gold fingers. 1.8mm and 3.0mm are non-standard sizes, so use as little as possible.
③ PCB appearance requirements
For wave soldering, it is required that the shape should be rectangular. If there is a missing groove, consider using a craft to fill up the missing groove; for pure SMT boards, missing grooves are allowed, but the size of the missing groove should be smaller than where it is. 1/3 of the side length to ensure that the PCB is transported smoothly on the chain.
④ PCB transmission requirements
In terms of the transmission direction, it should be considered from the perspective of reducing the deformation of the PCB during welding. For PCBs that are not to be joined, the long side direction is generally used as the transmission direction; for small boards to be joined, the long side direction of the joint is used as the transmission direction. In terms of the transmission side, the PCB that is not used as a panel should be left with a transmission side not less than 3.5mm in width. Usually, the long side of the PCB or the panel is used as the transmission side for transmission on the assembly line. If it is a double-sided assembly PCB, the front side cannot have any components and pads within 3.5mm from the side, and the reverse side cannot have any components and pads within 6mm from the side.
⑤ Jigsaw design
When designing the jigsaw, consider how to place the small boards, how to make several pieces, how to save materials and ensure sufficient rigidity after joining. When joining the boards, it is best to use the same direction for all the small boards, and the special-shaped boards can be joined in a relative way. The size of the assembled board should meet the requirements of the PCB board size range, and should not be too large, otherwise it will be easily deformed during soldering.
(2) Requirements for pad design
The design of the pad is a key content in the manufacturability design technology. The size of the pad has a great influence on the manufacturability and life of SMT products, so it is a key task that must be done in SMT applications. Commonly used component pads can be designed with reference to some standards, such as IPC-SM-782o. For the same component, all pads used symmetrically (such as chip resistors, capacitors, SOIC, QFP, etc.) should be designed strictly to maintain their comprehensiveness The symmetry, that is, the shape and size of the land pattern should be exactly the same. In order to ensure that when the solder melts, the surface tension acting on all solder joints on the components can be balanced (that is, the resultant force is zero), so as to facilitate the formation of ideal solder joints.
In the SMC/SMD pad design, the pad patterns of standard size components can be directly called from the component library of the CAD software, but the actual design must also be based on the assembly density of the specific product, different processes, different equipment and special The requirements of the components are designed. The following mainly introduces the pad design of several types of commonly used components.
① Rectangular chip component pad design
Land width: A =-K
Pad length (resistance): B = H^+ Ding Si + K
Pad length (capacitance): B = + T^- K
Pad pitch: G =-2-K
In the formula: L is the component length (mm); W is the component width (mm); T is the component welding end width (mm); H is the welding end height (mm); K is a constant, generally 0.25mm.
② Transistor (SOT) pad design
The design requirements of the pad for a single pin: pad width 2 pin width; pad length = component pin length + b, +e.g. Among them: hunger=milk, the value range is 0.3mm~0.5mm.
For small-outline transistors, on the basis of keeping the center distance between pads equal to the center distance between leads, the four sides of each pad should be extended at least 0.35mm.
③ Land design for wing-shaped small outline package device (SOP) and quad flat package device (QFP)
In general, the width of the pad is equal to the pin width Wl, and the pad length is 2.0mm±0.5mm. The center distance of the pad is equal to the center distance of the pin.
The general principle of the design of the width of a single pin pad: when the device pin spacing is Wl.0mm, Wl; when the device pin spacing is 1.27mm, Wp VI. 2 tons; pad length fart = pin length + our + b2o bx = If the value range is 0.3mm~0.5mm. .
The relative distance between the two rows of pads (the inner outline of the pad pattern) is calculated by the following formula.
G = F-K
In the formula: G is the distance between the two pads (mm); F is the package size of the component shell (mm); K is a constant, generally 0.25mm.
④ Land design for J-shaped small-outline packaged device (SOJ) and plastic-encapsulated leaded packaged device (PLCC)
The pins of SOJ and PLCC are all J-shaped, and the typical pin center spacing is 1.27mm. The general principle of single pin pad design: (0.5mm~0.8mm) X (1.85mm~2.15mm); the center of the pin should be between the inner 1/3 of the pad pattern and the center of the pad; the SOJ is relative to two rows of pads The distance between (the inner profile of the land pattern) is generally 4.9mm.
⑤ Land design of ball grid array device (BGA)
The shape of the pad of the BGA device is circular, and the diameter of the pad should be 20% smaller than the diameter of the solder ball. The through hole next to the pad must be soldered to prevent the loss of solder and cause short circuit or false soldering when making the board. The BGA pad spacing should be designed according to the metric system. Since the component manual will give both metric and inch sizes, the components are actually produced according to the metric system. Designing the pads according to the inch system will cause assembly deviation.
(3) Requirements for wiring design
The basic principles of printed conductor design: the shortest wiring principle, as little as possible through the pads, avoid sharp corners, uniform and symmetrical design, and make full and reasonable use of space.
For components mounted on two pads, such as resistors and capacitors, the printed lines connected to their pads should preferably be drawn symmetrically from the center of the pad, and the printed lines connected to the pad must have the same width. For the pads connected to the wider printed lines, it is better to pass through a narrow printed line in the middle. This narrow printed line is usually called the “insulation path”. Otherwise, for chip components of 0805 and below, The phenomenon of “tombstone erection” is prone to occur during welding. When the circuit is connected to the pads of QFP, PLCC, SOT and other devices, it is generally recommended to lead from the two ends of the pad instead of directly connecting in the middle. When the pad is connected to a large area ground, the cross paving method and the 45° paving method should be preferred. The length of the wire drawn from the large-area ground or power line is greater than 0.5mm and the width is less than 0.4mm. Avoid crossing the wires between the pads of fine-pitch components as much as possible. If it is necessary to pass the wires between the pads, the solder mask should be used to reliably shield them. When the assembly density permits, try to choose a lower density wiring design to improve reliability.
PCB design is divided into 3 levels according to the assembly industry’s agreement by wiring density:
① Low density. A typical circuit is a plug-in circuit board, and its technical requirement is to pass through 2 wires with a wire diameter/between diameter of 0.23mm within a 2.54mm component pin center spacing.
② Medium density. Usually surface mount and plug-in hybrid circuit boards, pass a wire diameter of 0,15mm within the 1.27mm component pin center spacing.
③ High density. Usually it is a surface-mount PCB design, and passes 2 to 3 wires of finer wire diameter/pitch diameter within 1.27mm component pin center spacing.
(4) Requirements for assembly quality
① Typical assembly defects
Typical assembly defects are: solder balls, solder balls, bridges, insufficient solder joints, too full solder joints, voids, tombstones, cold soldering, poor wetting, etc. The design should take targeted measures to prevent assembly defects.
② Assemble quality judgment
It can be divided into three types: general-purpose electronic products, special-purpose electronic products, and high-reliability electronic products. The corresponding standards should be used as the basis for quality design. From the perspective of acceptance, it is divided into three levels: best, acceptable and unqualified, and the design should aim at the best.
