PCB Optimization Design (reprint)

PCB Optimization Design (i.)

2011-04-25 11:55:36| Category: PCB design

SMT technology is now very mature and widely used in electronic products, therefore, electronic product designers need to understand the SMT technology common Sense and manufacturability design (DFM) requirements. SMT process products, at the beginning of the design should be integrated into consideration of the production process, raw material selection, equipment requirements, device layout, test conditions and other elements, as far as possible to shorten the design time, to ensure the design to the manufacture of one-off success.

SMT (Surface mount Technology) is a kind of electronic assembly technology which can be welded and assembled by means of reflow soldering or wave soldering, by placing the leadless or short lead surface assembled component (SMC/SMD) on the surface of the printed circuit board.

With the miniaturization and complication of electronic products, SMT technology has been developed rapidly, and is becoming mature and perfect. Circuit design, structural design and process technology are the three major technical elements of electronic products, SMT process is an important link in the process technology. In SMT process, the rationality of PCB design is more concerned about the SMT manufacturers, but also the key factors affecting product quality. Some designers do not pay much attention to product manufacturability, that as long as the circuit design no problem can be, so the design of the product can not only poor manufacturing, but also need to continue to change the process, resulting in product development progress, design costs increased, reducing the market competitiveness of products. Therefore, designers must pay attention to the design of PCB manufacturability.

I. Common undesirable phenomena in PCB design

1.1 PCB lack of process edge or process edge design unreasonable, resulting in equipment can not be mounted. As shown in 1.1.

1.2 PCB missing locating hole, positioning hole position is not correct, equipment can not be accurate, firm positioning. As shown in 1.2.

1.3 Missing Mark Point, Mark Point design is not standardized, resulting in machine identification difficulties. 1.3a, 1.3b and 1.3c are shown.

1.4 Screw hole metallization, the design of the pad is unreasonable.

Screw holes are used to secure the PCB board with screws. In order to prevent excessive wave soldering after plugging, screw hole inside the wall does not allow copper foil, over the crest surface of the screw hole pads need to be designed to "m" or plum-shaped (if the wave soldering using the carrier, may not exist above problems). 1.4A and 1.4b are shown.

1.5 PCB solder Pad size design error.

Common problems with the size of the pad plate size error, too large or too small pad spacing, welding plate asymmetry, compatible with the design of the pad is not reasonable, such as welding prone to false welding, displacement, vertical monument and other undesirable phenomena. 1.5a, 1.5b and 1.5c are shown.

The 1.6 pads have vias or pads that are too close to the vias.

Solder melting and then flow to the bottom surface of the PCB, resulting in solder joints less tin defects. 1.6A and 1.6b are shown.

1.7 The test point is too small, the test point is placed under the component or too close to the component. As shown in 1.7.

1.8 Screen printing or resistance welding on the pad, test point, the position number or polarity mark missing, the position number is reversed, the character is too large or too small. 1.8a, 1.8b and 1.8c are shown.

1.9 The distance between the components is not standardized, maintainability is poor.

There must be sufficient distance between the SMD, the general requirements for reflow soldering between the patch parts of the minimum distance of 0.5mm, wave soldering patch pieces from the minimum distance of 0.8mm, tall devices and the back of the patch should be greater distance between. SMD is not allowed in 3mm around BGA devices. As shown in 1.9.

1.10 IC Solder pad design is not standard.

QFP pad shape and the distance between the pad is inconsistent, the interconnection between the pads short circuit design, BGA pad shape irregular. 1.10A and 1.10b are shown.

1.11 PCB Board design is unreasonable.

PCB after the component interference, v-cut increase lead to deformation, yin and yang puzzle caused by heavy component welding bad. As shown in 1.11.

1.12 Using the wave soldering process IC and connector lack of guide solder pads, resulting in short-circuit after welding. As shown in 1.12.

1.13 The arrangement of the components does not meet the corresponding process requirements.

When using reflow soldering process, the layout direction of the components should be consistent with the direction of the PCB into the reflow oven, wave soldering effect should be considered when using wave soldering technology. As shown in 1.13.

PCB design causes the main reasons are as follows: (1) because the designers are not familiar with SMT process, equipment and manufacturability design, (2) the company lacks the corresponding design norms, (3) in the product design process without the participation of the process personnel, the lack of DFM review, (4) management and institutional issues.
In order to solve this problem effectively, PCB optimization design is necessary.

Second, the basic concept of optimal design

Optimized design, commonly known abroad as DFX, was put forward by the United States in the 90 's mid-century. DFX is based on the idea of concurrent design, in the first design of the product to consider the manufacturing and testing process requirements, and in the design phase to solve, so as to achieve the goal of design to manufacturing one-time success. Optimized design, including manufacturability (Design for manufacturing, DFM), testable/analytical designs (design for test/design for Diagnosibility, DFT/DFD), Assembly design ( Design for Assembly, DFA); eco-friendly designs (designed for environment, DFE); Designed for PCB manufacturability (design for fabrication of the PCB, DFF), and a design for Sourcing, DFS; Reliability Designs (Design for reliability, DFR); Designed for costs (design for cost, DFC), the domestic general Dutong is called Manufacturability Design (DFM). This design concept and design approach can shorten the time to market, reduce manufacturing costs, improve production and product quality. The optimization design should be run through the entire design process, not the final drawing. As shown in 2.

Third, the basic principles of PCB optimization design

3.1, SMT process of determination.

In the concept of product design start-up phase (Concept Start,cs) should be based on product planning requirements, determine product production process. There are six ways to assemble the surface of the PCB (see table 3-1 below):

Several typical assembly processes are as follows:

(1) Single side mixing (SMD and THC both on a side)
A-face printing solder paste → component placement → reflow soldering →a surface thc→b surface over wave soldering → Inspection
(2) Double-sided mixing (THC on a side, a, b both sides have SMD)
A-face printing solder paste → component placement → reflow soldering → flip plate →b surface printing or dispensing → component placement → gel-cured →a face mount Thc→b surface over wave soldering → Inspection
or use the following process:
B-face printing solder paste → component placement → reflow soldering → flip plate →a surface printing solder paste → component mount → reflow soldering →a surface thc→b surface over wave soldering → Inspection
(3) Double-sided surface assembly
A-face printing solder paste → component placement → reflow soldering → flip plate →b surface printing solder paste → component placement → reflow soldering → inspection
The main factors to be considered in selecting the Surface assembly process:
(1) The assembly density of PCB board;
(2) SMT equipment conditions;
(3) Cost and efficiency.
General principle: In the reflow soldering equipment and wave soldering equipment are equipped with the conditions, preferential selection of reflow soldering; Select the product assembly method is generally preferred single-sided mixed and single-sided full surface assembly; Try to avoid the PCB on both sides have a large proportion of IC and other devices.

3.2, PCB base material selection.

The substrate for the PCB variety, mainly divided into two major categories: organic substrate materials and inorganic substrate materials. Organic substrates, also known as copper clad laminates (Copper clad laminates, CCL) are the main materials for PCB manufacturing. The inorganic substrates are mainly ceramic plates and enamel clad steel substrates. For example, we use a large number of double-panel FR-4 is glass cloth CCL.

The choice of substrate should consider the following factors: the choice of substrate should be based on the use of the PCB and mechanical, electrical performance requirements; According to the PCB structure to determine the substrate copper clad layer (single, double or multilayer board); According to the PCB size and component weight to determine the thickness of the substrate; substrate parameters TG, CTE and flatness, etc. meet the requirements , price factors and so on. PCB substrates for SMT processes have the following requirements:

(1) The glass transition temperature (Tg) is higher. The glass transition temperature (glass transition temperature, Tg) is a unique property of the polymer, which determines the critical temperature of the material properties and is a key parameter to select the substrate. Epoxy resin TG in 125~140℃, reflow soldering temperature around 245 ℃, far higher than the PCB substrate TG, high temperature prone to thermal deformation of the PCB, serious damage to components. Therefore, the choice of the substrate when the high TG base material, recommended TG in 140 ℃ or more.

(2) Low coefficient of thermal expansion (CTE). For multilayer board structure of the PCB, because the x, y direction (that is, the length, width direction) and z direction (that is, thickness direction) of the thermal expansion coefficient is inconsistent, prone to PCB deformation, serious will cause metal hole rupture and damage components. As shown in 3.2.

(3) high heat resistance. Usually PCB to go through two times reflux, in order to ensure two times the reliability of SMD, must require a high temperature after the PCB deformation to small. The recommended value for T260 is more than 30 minutes and the recommended value for T288 is greater than five minutes.

(4) Good flatness. General PCB allowable warping curvature in 0.75%, for PCB thickness of 1.6mm substrate, warping ≤0.5mm, under warping ≤1.2mm.

(5) Good electrical performance. Due to the development of communication technology to high-frequency, the high-frequency characteristics of PCB requirements also increased. Increasing frequency will cause the dielectric constant (ε) of the PCB substrate to increase, resulting in a decrease in the transmission speed of the circuit signal. Other electrical performance indicators are dielectric loss angle tangent, electrical strength, insulation strength, arc strength and so on.

3.3. PCB dimensions and process design requirements.

In PCB design, the first thing to consider the PCB dimensions. PCB size too large impedance will increase, anti-noise capacity is reduced, the form size is too small, the heat dissipation is not good, and adjacent lines susceptible to interference, and PCB dimensions to meet the requirements of SMT equipment.

(1) PCB profile

The PCB is generally rectangular, the best aspect ratio is 3:2 or 4:3, the length and width ratio is easy to produce warping deformation. It is recommended to standardize the PCB size as much as possible, simplifying the processing process and reducing processing costs.

(2) PCB size

Different SMT equipment on the PCB size requirements, in the PCB design must consider the SMT equipment PCB maximum and minimum placement size, the general size in 50x50~350x250mm (the latest SMT equipment PCB size has been greatly improved, For example Universal's Genesis GX maximum PCB size reaches 813x610mm).

(3) PCB thickness

PCB thickness should consider the PCB board mechanical strength requirements and PCB unit area of the weight of components, generally in 0.3~6mm. Commonly used PCB thickness is 1.6mm, extra large plate available 2mm, RF Microstrip board and so on generally in 0.8~1mm.

(4) PCB Process Edge

PCB in the SMT production process, is through the rail transmission to complete, in order to ensure that the PCB is reliable fixed, generally in the transmission track edge (long side) to reserve the size of 5mm to facilitate equipment clamping, in this range is not allowed to mount devices. You must increase the process edge when you cannot reserve. For some of the plug-in wave soldering products, the general side (short side) need to reserve 3mm size in order to block the tin Bar.

(5) PCB Locating hole

Some SMT equipment (such as mounter) using hole positioning method, in order to ensure that the PCB can be accurately fixed on the device fixture, the PCB is required to set up a locating hole. Different equipment for locating hole requirements, generally need to set a pair of locating holes in the lower left and right corner of the PCB, the aperture is φ4mm (also have φ3mm or φ5mm), the hole wall does not allow metallization, one locating hole can also be designed as an elliptical hole, in order to locate quickly. General requirements of the main positioning hole and PCB on both sides of the distance is 5mmx5mm, adjust the hole distance from the bottom of the PCB 5mm. SMD elements are not allowed in the 5mm range around the locating hole. The image of the locating hole is shown in Figure 3.3a.

6) PCB Reference identification point (Fiducial Mark)

The reference point, also known as Mark, provides a common, measurable point for all steps in the SMT assembly process, ensuring that each device used in the assembly can accurately position the circuit pattern. Therefore, Mark Point is critical to SMT production. Mark points are generally divided into the whole board mark, puzzle mark, local identification mark (foot spacing ≤0.5mm), the mark point is generally specified in the center of metal foil, diameter of 1.0mm, the surrounding open contrast zone 3mm diameter, metal foil and the surrounding open area of the color contrast to be obvious. In the φ3mm range is not allowed to have silk screen, pad or v-cut and so on. 3.3b and 3.3c are shown. Mark is typically located in the diagonal position of the PCB, component, and panel, the General board benchmark Mark points placed 3, respectively placed in the left and right corner and an upper corner, the "L" shape distribution. The mark Point edge is at least 5mm,3.3d from the PCB edge.

(7) PCB board Design

General principle: When the size of PCB veneer <50mmx50mm, must make the puzzle. It is recommended that when the size of the PCB is <160mmx120mm, the board design is converted into the ideal size to meet the production requirements, so that the plug-in and welding can improve production efficiency and equipment utilization. But pay attention to the size of the puzzle is not too large, and to meet the requirements of the equipment. The plate can be used in a V-groove, a stamp hole or a punching groove, etc., it is recommended that the same board only a kind of sub-board method. As shown in the 3.3e:

For part of the full-surface assembly of the double-sided SMD board, can be used in the design of yin and yang, so that the same board can be used to save programming time to change the line, improve production efficiency. But for larger, heavier-weight devices, the restrictions are as follows:

a= Device weight/pin-to-pad contact area
Chip-type device: a≤0.075g/mm2
Wing PIN Device: a≤0.300g/mm2
J-Pin Device: a≤0.200g/mm2
Surface Array Device: a≤0.100g/mm2
If an overweight device must be on a bottom surface, it should be tested to verify the feasibility.

(8) Guide Fillet

Right-angled PCB in the transmission of easy to produce card board, in order to facilitate processing, it is recommended that the PCB corner of the design of the guide fillet, and the PCB fillet is conducive to vacuum packaging, transport, to avoid packaging bag puncture, wear. The radius r of the fillet is generally 2mm~4mm. There are special requirements according to the structure diagram of the method clearly marked R size, so that manufacturers processing. The guide fillet design is shown in Figure 3.3a.

(9) PCB weldability

According to IPC-6012 requirements, conditions: 245±5℃, Time 3~5s, receive the standard: The tin area is greater than 95%.

3.4, the choice of components

The selection of components takes into account product requirements, PCB size, assembly process, equipment processing capacity and cost.

(1) The shape of the component is suitable for SMT equipment placement, choose the special-shaped device;

(2) As far as possible the use of conventional components, component size, shape should be standardized. The differences between the various standards should be considered;

(3) The selection of components should meet the SMT machine processing corresponding component size range and height;

(4) The packaging form of components suitable for placement machine automatic placement requirements;

At present, there are four kinds of packaging methods: Taping (Tape and Reel), tubular (Tube, Stick), Pallet (Tray), Bulk (Bulk). When mass production, SMD device packaging as far as possible to choose the form of taping.

(5) The solder end of the component or pin can be welded to meet the requirements;

In general, the following requirements should be met: 235℃±5℃,2±0.2s or 230℃±5℃,3±0.5s, under 20 times times the microscope to check the solder end of the tin, the component solder end requires more than 90% of tin.

(6) The high temperature welding requirements of components under lead-free condition;

IPC in the latest standard j-std-020, according to the thickness of the package body, the volume of the corresponding reflow soldering peak temperature requirements, as shown in table 3-4.

(7) IC Device Selection Guidelines: Plug-in IC can not adapt to product development, as far as possible to avoid the selection; pitch less than 0.4mm SOP/QFP process capacity may be insufficient, should be cautious choice, SOJ/PLCC inconvenience detection and repair, should be careful selection, LCC/QFN, such as Leadless IC, should try to avoid the selection 0.5mm the following BGA domestic PCB factory processing capacity is insufficient, carefully selected, COB/FC non-processing capacity, prohibit selection.

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PCB Optimization Design (reprint)