1. Preface Of PCB Design
With the increasing market competition of communication and electronic products, the life cycle of products is shortening. The upgrading of original products and the release speed of new products play an increasingly critical role in the survival and development of the enterprise. In the manufacturing link, how to obtain new products with higher manufacturability and manufacturing quality with less lead-in time in production has become more and more the competitiveness pursued by people of vision.
In the manufacture of electronic products, with the miniaturization and complexity of products, the assembly density of circuit boards is becoming higher and higher. Accordingly, the new generation of SMT assembling process which has been widely used requires designers to consider the manufacturability at the very beginning. Once the poor manufacturability is caused by poor consideration in the design, it is bound to modify the design, which will inevitably prolong the time of product introduction and increase the cost of introduction. Even if the PCB layout is slightly changed, the cost of re-making the printed board and SMT solder paste printing screen board is up to thousands or even tens of thousands of yuan, and the analog circuit even needs to be re-debugging.
The delay of the import time may cause the enterprise to miss the opportunity in the market and be in a very disadvantageous position strategically. However, if the product is manufactured without modification, it will inevitably have manufacturing defects or increase manufacturing costs, which will be more costly. Therefore, when enterprises design new products, the earlier the manufacturability of the design is considered, the more conducive to the effective introduction of new products.
2. Contents to be considered in PCB design
The manufacturability of PCB design is divided into two categories, one is the processing technology of producing printed circuit boards; The second refers to the circuit and structure of the components and printed circuit boards of the mounting process. For the processing technology of producing printed circuit boards, the general PCB manufacturers, due to the influence of their manufacturing capacity, will provide designers with very detailed requirements, which is relatively good in practice. But according to the author's understanding, the real in practice that has not received enough attention, is the second type, namely manufacturability design for electronic assembly. The focus of this paper is also to describe the manufacturability issues that designers must consider in the stage of PCB design.
Manufacturability design for electronic assembly requires PCB designers to consider the following at the beginning of PCB design:
2.1 Appropriate selection of assembly mode and component layout in PCB design
The selection of assembly mode and the component layout is a very important aspect of PCB manufacturability, which has a great impact on assembly efficiency, cost and product quality. In fact, the author has come into contact with quite a lot of PCB, and there is still a lack of consideration in some very basic principles.
(1) Select the appropriate assembly method
Generally, according to different assembly densities of PCB, the following assembly methods are recommended:
As a circuit design engineer, I should have a correct understanding of the PCB assembling process, so that I can avoid making some mistakes in principle. When selecting the assembly mode, in addition to considering the assembly density of PCB and the difficulty of wiring, it is necessary to consider the typical process flow of this assembly mode and the level of process equipment of the enterprise itself. If the enterprise does not have a good wave welding process, then choose the fifth assembly method in the table above may bring you a lot of trouble. It is also worth noting that if the wave soldering process is planned for the welding surface, it should be avoided to complicate the process by placing a few SMDS on the welding surface.
(2) Component layout
The layout of PCB components has a very important impact on production efficiency and cost and is an important index to measure the PCB design of the connectability. Generally speaking, the components are arranged as evenly, regularly, and neatly as possible, and arranged in the same direction and polarity distribution. The regular arrangement is convenient for inspection and conducive to improving the patch/plug-in speed, uniform distribution is conducive to heat dissipation and optimization of the welding process.
On the other hand, in order to simplify the process, PCB designers should always be aware that only one group welding process of reflow welding and wave welding can be used on either side of the PCB. This is especially noteworthy in the assembly density, PCB welding surface must be distributed with more patch components. The designer should consider which group welding process to use for the mounted components on the weld surface. Preferably, a wave soldering process after patch curing can be used to weld the pins of the perforated devices on the component surface at the same time.
However, wave welding patch components have relatively strict constraints, only 0603 and above size chip resistance, SOT, SOIC (pin spacing ≥1mm and height less than 2.0mm) welding. For components distributed on the welding surface, the direction of the pins should be perpendicular to the transmission direction of PCB during wave crest welding, so as to ensure that the welding ends or leads on both sides of the components are immersed in welding at the same time.
The arrangement order and spacing between adjacent components should also meet the requirements of wave crest welding to avoid the "shielding effect", as shown in FIG. 1. When using wave soldering SOIC and other multi-pin components, should be set in the direction of tin flow at two (each side 1) solder feet, to prevent continuous welding.
Components of similar type should be arranged in the same direction on the board, making it easier to mount, inspect, and weld the components. For example, having the negative terminals of all radial capacitors facing the right side of the plate, having all the DIP notches facing the same direction, etc., can speed up instrumentation and make it easier to find errors. As shown in Figure 2, since board A adopts this method, it is easy to find the reverse capacitor, while Board B takes more time to find it. In fact, a company can standardize the orientation of all the circuit board components it makes. Some board layouts may not necessarily allow this, but it should be an effort.
What manufacturability issues should be considered in PCB design
Also, similar component types should be grounded together as much as possible, with all component feet in the same direction, as shown in Figure 3.
However, the author has indeed encountered quite a number of PCBS, where the assembly density is too high, and the welding surface of the PCB must also be distributed with high components such as tantalum capacitor and patch inductance, as well as thin-spaced SOIC and TSOP. In this case, it is only possible to use double-sided printed solder paste patch for backflow welding, and plug-in components should be concentrated as far as possible in the distribution of components to adapt to manual welding. Another possibility is that the perforated elements on the component face should be distributed as far as possible in a few main straight lines to accommodate the selective wave soldering process, which can avoid manual welding and improve efficiency, and ensure the quality of welding. Discrete solder joint distribution is a major taboo in selective wave soldering, which will multiply the processing time.
When adjusting the position of components in the printed board file, it is necessary to pay attention to the one-to-one correspondence between components and silkscreen symbols. If the components are moved without corresponding moving the silkscreen symbols next to the components, it will become a major quality hazard in manufacturing, because in actual production, silkscreen symbols are the industry language that can guide production.
2.2 The PCB must be equipped with clamping edges, positioning marks and process positioning holes necessary for automatic production.
At present, electronic mounting is one of the industries with a degree of automation, the automation equipment used in the production requires automatic transmission of PCB, so that the transmission direction of PCB (generally for the long side direction), the upper and lower each have a not less than 3-5mm wide clamping edge, in order to facilitate automatic transmission, avoid near the edge of the board due to the clamping can not automatically mounting.
The role of positioning markers is that PCB needs to provide at least two or three positioning markers for the optical identification system to accurately locate PCB and correct PCB machining errors for the assembly equipment which is widely used in optical positioning. Of the positioning markers commonly used, two must be distributed on the diagonal of the PCB. The selection of positioning marks generally uses standard graphics such as a solid round pad. In order to facilitate identification, there should be an empty area around the marks without other circuit features or marks, the size of which should not be less than the diameter of the marks (as shown in Figure 4), and the distance between the marks and the edge of the board should be more than 5mm.
In the manufacturing of PCB itself, as well as in the assembling process of semi-automatic plug-in, ICT testing and other processes, PCB needs to provide two to three positioning holes in the corners.
2.3 Rational use of panels to improve production efficiency and flexibility
When assembling PCB with small sizes or irregular shapes, it will be subject to many restrictions, so it is generally adopted to assemble several small PCB into PCB of appropriate size, as shown in Figure 5. Generally, PCB with a single side size of less than 150mm can be considered to adopt the splicing method. By two, three, four, etc., the size of large PCB can be spliced to the appropriate processing range. Generally, PCB with a width of 150mm~250mm and length of 250mm~350mm is the more appropriate size in automatic assembling.
Another way of the board is to arrange the PCB with SMD on both sides of a positive and negative spelling into a large board, such a board is commonly known as Yin and Yang, generally for the consideration of saving the cost of the screen board, that is, through such a board, originally need two sides of the screen board, now only need to open a screen board. In addition, when the technicians prepare the running program of the SMT machine, the PCB programming efficiency of Yin and Yang is also higher.
When the board is divided, the connection between the sub-boards can be made of double face V-shaped grooves, long slot holes and round holes, etc., but the design must be considered as far as possible to make the separation line in a straight line, in order to facilitate the board, but also consider that the separation side can not be too close to the PCB line so that the PCB is easy to damage when the board.
There is also a very economical board and does not refer to the PCB board, but to the mesh of the grid graphic board. With the application of an automatic solder paste printing press, the current more advanced printing press (such as DEK265) has allowed the size of 790×790mm steel mesh, set up a multi-sided PCB mesh pattern, can achieve a piece of steel mesh for the printing of multiple products, is a very cost saving practice, especially suitable for the product characteristics of small batch and variety of manufacturers.
2.4 Considerations of testability design
The testability design of SMT is mainly for the current ICT equipment situation. Testing issues for post-production manufacturing are taken into account in circuit and surface-mounted PCB SMB designs. To improve testability design, two requirements of process design and electrical design should be considered.
2.4.1 Requirements of process design
The accuracy of positioning, substrate manufacturing procedure, substrate size and probe type are all factors that affect the reliability of the probe.
(1) positioning hole. The error of positioning holes on the substrate should be within ±0.05mm. Set at least two positioning holes as far apart as possible. The use of nonmetallic positioning holes to reduce the thickness of the solder coating can not meet the tolerance requirements. If the substrate is manufactured as a whole and then tested separately, the positioning holes must be located on the motherboard and each individual substrate.
(2) The diameter of the test point is not less than 0.4mm, and the spacing between adjacent test points is more than 2.54mm, not less than 1.27mm.
(3) Components whose height is higher than *mm should not be placed on the test surface, which will cause poor contact between the probe of the online test fixture and the test point.
(4) Place the test point 1.0mm away from the component to avoid impact damage between the probe and the component. There should be no components or test points within 3.2mm of the ring of the positioning hole.
(5) The test point shall not be set within 5mm of the PCB edge, which is used to ensure the clamping fixture. The same process edge is usually required in conveyor belt production equipment and SMT equipment.
(6) All detection points shall be tinned or metal conductive materials with soft texture, easy penetration, and non-oxidation shall be selected to ensure reliable contact and prolong the service life of the probe.
(7) the test point can not be covered by solder resistance or text ink, otherwise, it will reduce the contact area of the test point, and reduce the reliability of the test.
2.4.2 Requirements for electrical design
(1) The SMC/SMD test point of the component surface should be led to the welding surface through the hole as far as possible, and the hole diameter should be greater than 1mm. In this way, single-sided needle beds can be used for online testing, thus reducing the cost of online testing.
(2) Each electrical node must have a test point, and each IC must have a test point of POWER and GROUND, and as close as possible to this component, within the range of 2.54mm from the IC.
(3) The width of the test point can be enlarged to 40mil wide when it is set on the circuit routing.
(4) Evenly distribute the test points on the printed board. If the probe is concentrated in a certain area, the higher pressure will deform the plate or needle bed under test, further preventing part of the probe from reaching the test point.
(5) The power supply line on the circuit board should be divided into regions to set the test breakpoint so that when the power decoupling capacitor or other components on the circuit board appear short circuit to the power supply, find the fault point more quickly and accurately. When designing breakpoints, the power-carrying capacity after resuming the test breakpoint should be considered.
Figure 6 shows an example of a test point design. The test pad is set near the lead of the component by the extension wire or the test node is used by the perforated pad. The test node is strictly forbidden to be selected on the solder joint of the component. This test may make the virtual welding joint extrude to the ideal position under the pressure of the probe, so that the virtual welding fault is covered up and the so-called "fault-masking effect" occurs. The probe may directly act on the endpoint or pin of the component due to the bias of the probe caused by the positioning error, which may cause damage to the component.
What manufacturability issues should be considered in PCB design?
3. Closing remarks on PCB Design
The above are some of the main principles that should be considered in PCB design. In the manufacturing design of PCB oriented to electronic assembly, there are quite a lot of details, such as the reasonable arrangement of the matching space with the structural parts, reasonable distribution of silkscreen graphics and text, appropriate distribution of heavy or large heating device location, In the design stage of PCB, it is necessary to set up the test point and test space in the appropriate position, and consider the interference between the die and the nearby distributed components when the couplings are installed by the pull and press riveting process. A PCB designer, not only considers how to obtain good electrical performance and a beautiful layout but also an equally important point which is manufacturability in PCB design, in order to achieve high quality, high efficiency, low cost.