Brief analysis of the design process of the hottes

2022-08-14
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Brief analysis of surface mount PCB design process

Abstract: surface mount technology is widely used in the production and manufacturing of many electronic products. This paper expounds some manufacturing process problems that need to be considered in the design of surface mount PCB, and provides a reference for SMT designers

key words: PCB benchmark mark through hole wave soldering and reflow soldering testability design

previous electronic products, "plug-in + hand soldering" is the basic process of PCB, so the design requirements for PCB are also very simple. With the introduction of surface mounting technology, the manufacturing process is gradually integrated into the design technology, and the design requirements for PCB are becoming more and more stringent, which needs to be unified and standardized. At the beginning of design, product developers should not only consider the feasibility of circuit principle design, but also consider the PCB design and layout on the board, and the sequence and reasonable arrangement of process flow. Combined with the author's years of production experience, this paper summarizes the manufacturing process problems in the design of surface mount PCB, and puts forward them for the reference of designers

I. welding method and overall PCB design

reflow welding is applicable to the welding of almost all mounted components, while wave soldering is only applicable to the welding of rectangular sheet components, cylindrical components, sots, etc. and small SOPs (the number of pins is less than 28, and the spacing between pins is more than 1mm)

in view of the operability of production, the overall design of PCB should be optimized in the following order as far as possible:

(1) mixed packaging on one side, that is, placement of patch components or plug-in components on one side of PCB

(2) mount on both sides, and place patch components on one or both sides of PCB

(3) mixed installation on both sides, placement of mounting components and plug-in components on the surface of PCBA, and placement of patch components suitable for wave soldering on the surface of B

according to the above recommended PCB design, taking double-sided mixed loading (such as camera) as an example, we can design the following production process flow:

Figure 1 production process flow of double-sided mixed loading PCB

II. Selection principle of PCB substrate

substrate loaded with SMD. According to the loading form of SMD, the performance requirements of the substrate are as follows:

appearance requirements: the appearance of the substrate should be smooth and flat, without warpage or uneven, There shall be no cracks, scars, rust spots and other defects on the surface of the substrate

relationship between thermal expansion coefficient: the assembly form of surface mount components will affect the components due to the expansion and contraction stress after the substrate is heated, if the thermal expansion coefficient is different. This stress will be very large, causing the stripping of the electrode at the joint of the element and reducing the reliability of the product. Generally, the size of the element is less than 3.2 × At 1.6mm, it only suffers partial stress, and the size is greater than 3.2 × At 1.6mm, this problem must be paid attention to

relationship between thermal conductivity: during mounting and integrated circuits on the substrate, the heat during operation is mainly diffused through the substrate. When mounting circuits are dense and the heat is high, the substrate must have a high thermal conductivity

relationship between heat resistance: due to the requirements of surface mounting process, a substrate may undergo several welding processes from the end of assembly. Generally, the resistance to welding heat should reach 260 ℃ for 10 seconds

bonding strength of copper foil: the welding area of surface mounted components is smaller than that of the original components with leads. Therefore, it is required that the substrate and copper foil have good bonding strength, generally more than 1.5kg/cm2

bending strength: after the base plate is installed, due to the mass and external force of its components, it will produce disturbing bending, which will increase the stress on the components and joint points, or cause micro cracks in the components. Therefore, the bending strength of the base plate is required to reach more than 25kg/cm2

electrical performance requirements: due to the high speed of circuit transmission and the requirement of the dielectric constant of the substrate, the dielectric tangent should be small. At the same time, with the improvement of wiring density, the insulation performance of the substrate should meet the specified requirements

the reaction of the substrate to the cleaning agent, soaked in the solution for 5 minutes, its surface does not produce any adverse effects, and has good blanking performance. The storage condition of the substrate is the same as that of SMD

III. design requirements for PCB shape and processing technology

PCB process clamping edge: in the process of SMT production and plug-in wave soldering, PCB should leave a certain edge for equipment clamping. The range of this clamping edge should be 5mm, and components and pads are not allowed to be placed within this range

positioning hole design: in order to ensure that the printed board can be accurately and firmly placed on the fixture of surface mounted equipment, a pair of positioning holes need to be set, and the size of the positioning holes is 5 + 0.1mm. In order to locate quickly, one of the holes can be designed into an elliptical shape. There shall be no element within 1mm around the positioning hole

pcb thickness: from 0.5mm to 4mm, 1.6mm to 2mm is recommended

pcb slot missing: there should be no slot missing in some edge areas of the printed board, so as to avoid errors in the positioning of the printed board or sensor detection. The specific position will change due to different equipment

panel design requirements: there are the following requirements for the panel format of PCB:

(1) the size of the panel should not be too large or too small, and it should be convenient for processing in the process of manufacturing, assembly and testing without large deformation

(2) the process clamping edge and installation process hole of the panel should be determined by the manufacturing and installation process of the printed board

(3) each panel should be designed with a benchmark mark, so that the machine can treat each panel as a single board

(4) the panel can adopt the separation technology of stamp plate or double-sided V-groove. When using stamp plates, it should be noted that the lap edges should be evenly distributed around each panel to avoid deformation due to uneven stress on the printed board during welding. When the double-sided V-shaped groove is used, the depth of the V-shaped groove should be controlled at about 1/6 - 1/8 of the plate thickness

(5) when designing printed boards that are pasted on both sides without wave soldering, double number panels can be used with half of the front and half of the back, and the graphics on both sides are arranged in the same way, which can improve the equipment utilization rate (the equipment investment can be halved under the conditions of medium and small batch production), and save production preparation costs and time

warpage of PCB. For printed boards used for surface mounting, the warpage is required to be less than 0.0075mm/mm. The details are as follows: Table 1 allowable warpage of PCB

IV. PCB pad design process requirements

pad design is an extremely critical part of PCB circuit design, because it determines the welding position of components on the printed board, as well as the reliability of solder joints, possible welding defects in the welding process, cleanability Testability and maintenance volume play a significant role

factors to be considered in the design of solder mask

(1) the opening size of solder mask corresponding to each pad on the printed board should be 0.05 ~ 0.25mm wider and longer than the pad size respectively. The specific situation depends on the pad spacing. The purpose is to prevent solder resist from polluting the pad, and to avoid continuous printing and welding during solder paste printing and welding

(2) the thickness of the solder mask shall not be greater than the thickness of the pad

the pad and the printed wire

(1) reduce the width of the connecting pad of the printed wire, unless limited by factors such as hand charge capacity and the processing limit of the printed board, the maximum width shall be 0.4mm, or half of the pad width (whichever is smaller)

(2) when the pad is connected with a large area of conductive area such as ground, power and other planes, it should be thermally isolated through a thin conductive line

(3) the printed wire should avoid being connected with the pad at a certain angle. As long as possible, the printed wire should be connected with it from the center of the long side of the pad

through hole layout

(1) avoid setting through holes within the surface mounting pad or within 0.635mm from the surface mounting pad. If it cannot be avoided, the solder loss channel must be blocked with solder resist

(2) as a test support through hole, when designing the layout, it is necessary to fully consider the minimum spacing of probes with different diameters during automatic testing

for the same component, all pads used symmetrically (such as sheet resistance, capacitance, SOIC, QFP, etc.) should be designed to strictly maintain their overall symmetry, that is, the shape and size of the pad pattern should be completely consistent. 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 form an ideal solder joint

for components with multiple pins (such as SOIC, QFP, etc.), the short connection between the pin pads is not allowed to be connected directly, and the short connection should be made after the pad is added with the outgoing interconnect, so as to avoid bridging. In addition, try to avoid crossing the interconnection line between its pads (especially the pin devices with fine spacing). The interconnection line between adjacent pads must be shielded with solder mask

characters and graphic marks are not allowed to be printed in the pad, and the distance between the mark symbol and the edge of the pad should be greater than 0.5mm. No through holes are allowed between the pads of devices without external pins to ensure the cleaning quality

when the wave soldering process is adopted, the through hole of the inserted pin is generally 0.05 - 0.3mm larger than the wire diameter of the pin, and the diameter of the pad should be greater than 3 times the aperture

pad graphic design (see Table 2) table 2 pad area size of chip components

(1) pad graphic design of chip components

(2) SOP, QFP pad graphic design: there is no standard calculation formula for SOP, QFP pad size, so the design of pad graphic is relatively difficult

reference Panasonic SOP and QFP pad graphic design standards, as shown in Table 3. Table 3sop, QFP pad graphic design size

v. requirements for component layout

component layout should meet the requirements of SMT production process. Product quality problems caused by design are difficult to overcome in production; Therefore, PCB design engineers should understand the basic SMT process characteristics, and carry out component layout design according to different process requirements. The correct design can minimize welding defects. The following points should be considered in the layout of components:

the distribution of components on PCB should be as uniform as possible; The heat capacity of reflow soldering of high-quality devices is large. Therefore, too concentrated layout is easy to cause local low temperature and false soldering

a certain maintenance gap should be left around large devices (a size that the heating head of SMD repair equipment can operate)

power devices should be placed evenly on the edge of PCB or on the ventilation position in the chassis

when one side is mixed, the cloth for mounting and inserting components should be placed on side a; When using double-sided reflow soldering for mixed assembly, the large mounted and inserted components should be placed on side a, and the large components on both sides of PCB A and B should be staggered as far as possible; When using reflow soldering on side a and wave soldering on side B, large mounting and inserting components should be placed on side a (reflow soldering), rectangular and cylindrical chip components suitable for wave soldering, sots and smaller SOPs (the number of pins is less than 28, and the pin spacing is more than 1mm) should be placed on side B (wave soldering surface). Devices with pins on four sides, such as QEP, PLCC, etc., cannot be placed on the wave soldering surface

the component package on the wave crest welding surface must be able to withstand the temperature of more than 260 degrees and be fully sealed

valuable components should not be placed at the corners and edges of PCB, or close to connectors, mounting holes, slots, cutting, notches and corners of panels. These positions are high stress areas of printed boards, which are easy to cause cracks or cracks in solder joints and components, such as 3D printing and mobile phone heat dissipation

direction of wave soldering components

Figure 2 component direction in wave soldering applications

all polar surface mount components are possible

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