Common SMT Design Questions

Although SMT has been used in our country for more than 20 years, due to various reasons, in some enterprises that design and produce multi-variety and small-batch products, PCB designers are still unfamiliar with SMT production equipment and processes, and cannot Good application of PCB design specifications and situations where the concepts of manufacturability are vague. As a result, there is a lack of practical experience in the selection of manufacturing process, selection of components and PCB materials, pad design, PCB layout design, thermal design, stress design, and testability design when actually designing products, and repeated revisions are required. or redesign.

The hidden dangers of quality defects caused by bad design in SMT manufacturing are very large. If the PCB layout design does not meet the specification requirements, it will cause poor manufacturability, increase the process flow and process difficulty, affect equipment utilization, reduce production efficiency, and waste man-hours. Delaying the construction period, the most serious thing is that it will cause a large number of welding defects, which will inevitably lead to PCBA repairs. We know that rework will bring hidden dangers to quality, and may damage components (some components are irreversible) and printed circuit boards, directly affecting product reliability. In the worst case, it will lead to revision or redesign, prolonging the actual product development cycle.

Common problems in SMT printed circuit board design are: no design reference mark, PCB process edge, PCB shape and size; unreasonable component layout; incorrect pad structure size; incorrect via hole design; solder mask and The silk screen is not standardized; the PCB material, thickness and width dimension ratio are not suitable; the PCB shape is irregular and not made into a jigsaw board, etc. The following will explain one by one and put forward the correct design requirements while analyzing the problem.

1. Bench mark

Fiducial marks are used to correct errors caused by PCB processing and deformation, as well as for PCB positioning and component positioning. In the whole SMT process flow, the relevant automation equipment needs to use the reference mark set on the PCB light board for accuracy calibration, such as: placement machine, silk screen printing machine, AOI, flying probe tester and automatic rework station, etc. Some PCBAs in production are assembled online without setting the benchmark mark. We know that in addition to standard shape fiducial marks, screen printing machines and placement machines can also use through-hole device soldering rings, soldering holes, and mounting device pads as fiducial marks. Many problems will cause batch repairs. It is recommended that as long as the PCB with SMD devices is wiring, the upper reference mark should be set.

2. PCB process side

The PCB should leave a certain edge to facilitate the clamping of the equipment, that is, the process edge. Generally, along the direction of PCB welding transmission, two sides should leave at least 5 mm of process side, and components and pads are not allowed to be placed within this range. Generally, the process side can be arranged on a pair of long sides.

3. PCB size is too small

In order to improve the placement efficiency, when the PCB size is less than 70 mm × 70 mm, it should be designed as a jigsaw board, and some special-shaped boards also need to be jigsawed. As shown in Figure 1, after the board passed the review of the design specifications, a panel was made, but the process side was added to the short side, and the process side should be added to the long side direction, that is, the direction of the SMT production line clamping transmission. Suggestion: When the material level of the placement machine and the feeder are sufficient, put the top surface and bottom surface of the PCB on the same side, which can save the production cost of a steel mesh and reduce the time for replacing the placement program once; Combine multiple PCBs in one project into the same large board. This can greatly improve placement efficiency and finished product efficiency.

4. Pad width size

If the pad width is too small, the soldering surface of the pins will be small, the amount of solder will be small, the strength of the solder joints will be insufficient, and the resistance to vibration and electrical conductivity will be poor. Aerospace and military products require that the width of the pad should be the same as or 1.1 times the width of the component pins, and civilian products can be reduced by 0.9 times under the limitation of high-density wiring.

5. Placement of components

When the components are close to the edge of the printed board, there will be the following problems: (a) It is not conducive to automatic assembly; (b) Mechanical stress concentration; (c) Easy to damage during turnover; (d) Metallized holes and pads are easy to be pulled hurt. It is generally required that components are not allowed to be placed within 3 mm from the process edge, clamping edge or printed board edge. When placing components near the edge of the printed board, the long side of the component should be parallel to the edge of the printed board. For chip ceramic capacitors, after high temperature aging or use for a period of time, it will be found that their failures have a common feature, mostly located near the edge of the panel, screws and sockets. close to the edge of the PCB, as shown in Figure 4.

6. Component placement direction

A certain maintenance space should be reserved around large devices to facilitate the operation of the heating head of the repair equipment. When placing components near the edge of a large device, the long side of the component should be parallel to the edge of the device.

7. Location of key and valuable components

The proximity of critical and valuable components to areas of high stress concentration can easily lead to solder joint fatigue or solder joint fracture. Do not place it on the corners and edges of the PCB, nor near connectors, mounting holes, slots, cuts, gaps, and corners of the panel. As shown in Figures 9 and 10. BGA device inspection generally adopts non-destructive X-ray inspection method.

8. Via

Vias are not allowed on pads. Vias on the pads will cause the solder to melt and leak into the metallized hole or the bottom layer through the via holes during soldering, causing defects such as too little solder in the solder joints, virtual soldering, tombstones, and thermal stress. Vias are generally not set under the device. When the device package is smaller than 0805, it is easy to cause a short circuit; when the via is solder masked, the thicker solder mask will hold up the device, causing desoldering, tombstone and virtual soldering, etc. .

9. Wing pin design requirements

The soles of the wing pins should all sit on the pads. Ensure that a solder joint is formed at the heel of the pin, that is, the main solder joint. Whether the solder joint is formed here is the key point to ensure the welding strength of this type of pin. Ensure that the solder pad extends more than 0.3 mm from the toe of the pin.

10. BGA layout requirements

Early design specifications do not recommend placing BGA devices on both sides, but the demand for thin and light electronic products is becoming more and more urgent in practical applications. For example, in the case product, 10 BGA devices are installed on the top surface of the PCB, and 6 BGA devices are installed on the bottom surface of the PCB. It has become the norm to place BGA devices on both sides. The temperature cycle life of the solder joints in the PCBA assembly state has a lot to do with the assembly density, especially when the array terminals are assembled at the same position on both sides of the PCB, the life of the joints will be reduced by about 50%.

11. Electrical connector aperture ratio

The aperture ratio of the electrical connector should be set reasonably. The large aperture ratio leads to a large gap between the hole wall and the pin, and it is easy to cause a short circuit through tin when soldering. For special cases, such as the J30J micro-rectangular electrical connector, the minimum welding gap is 0.136 mm, which is smaller than the minimum safety gap of 0.2 mm for through-hole device welding. It is easy to cause tin connection during soldering. An oval hole can be set to increase the gap between the two pads. The gap can reach 0.25 mm, which can effectively prevent short circuit during welding.

Process summary

According to the actual application experience, the following 8 measures are adopted, and the standardization of PCB design can be quickly, comprehensively and effectively applied within the enterprise, eliminating bad design and realizing manufacturability design.

(1) Formulate internal PCB design specification standards (or guidelines) and review systems;

(2) Set up the post of PCB design specification process designer in the enterprise;

(3) Purchase professional PCB design specification software tools;

(4) Establish training plans and files for PCB designers, and confirm that all PCB designers have undergone manufacturability design training;

(5) Make full use of the internal network of the enterprise, establish a fast and effective way to obtain PCB design standard information, and establish a fast and effective implementation process for feedback, approval and approval of PCB design issues;

(6) Establish a PCB design defect case library, which is a green channel for different designers and craftsmen to obtain and update in daily work;

(7) Effective communication and collaboration between PCB designers and SMT processing plants; (8) SMT processing plants provide timely feedback to customers, and continuously improve and perfect product PCB design.

The above are the common problems and solutions of SMT printed circuit board design introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. for you. If you have software and hardware function development needs for smart electronic products, you can rest assured to entrust them to us. We have rich experience in customized development of electronic products, and can evaluate the development cycle and IC price as soon as possible, and can also calculate PCBA quotations. We are a number of chip agents at home and abroad, including MCU, voice IC, Bluetooth IC and modules, wifi modules. We have hardware design and software development capabilities. Covering circuit design, PCB design, single-chip microcomputer development, software custom development, APP custom development, WeChat official account development, voice recognition technology, Bluetooth wifi development, etc. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS solutions, the development of Bluetooth audio, the development of children's toys, and the development of electronic education products.