1 Introduction
As market competition communications, electronic products continues to increase, the product life cycle continues to shorten, upgrade existing products and launch companies speed new products to the survival and development of enterprises play an increasingly critical role. In the manufacturing sector, how in the production of import less time to get used to manufacture higher quality and manufacture of new products has increasingly become the pursuit of insight core competitiveness.
In the manufacture of electronic products, with the miniaturization, product, complicating the assembly of increasingly high-density circuit boards, consequential and get a new generation of SMT equipment associated process widely used, requires the designer at the outset, it must be Taking into account the manufacturability. Once the cause of ill-considered in the design for manufacturability poor, is bound to modify the design, will inevitably prolong the time of import of the product and increase import costs, even for minor changes PCB layout, re-making boards and SMT solder paste printing network plates cost up to thousands or even million or more, even for analog circuits re-commissioning. The delay time may be introduced into the market so that enterprises in the missed opportunities, the strategy in a very disadvantageous position. But if you do not make changes and barely production, the product is bound to a manufacturing defect, or manufacturing costs soared, the price to be paid will be greater. Therefore, when designing new products enterprise, the sooner considering the design for manufacturability issues, more conducive to the effective introduction of new products.
2, PCB design considerations content
PCB design for manufacturability divided into two categories, one refers to the production of printed circuit board processing technology; the second refers to the circuit and the structural components and printed circuit board mounted linking manufacturability. For the production of printed circuit board processing technology, the general PCB production factory, due to the affected manufacturing capacity, will be very detailed to provide designers with the relevant requirements of the application in practice is relatively good, according to the author's understanding of the real in practice has not been enough attention is the second category, namely for the electronics assembly of design for manufacturability. The focus of this paper is to describe it in PCB design stage, designers must consider manufacturability issues.
For the electronics assembly of PCB design for manufacturing requirements early in the design of the PCB designers to consider the following:
2.1 Select the appropriate assembly methods and component layout
Assembly methods of selection and component layout is a very important aspect of PCB manufacturability, the impact on efficiency and cost associated equipment, product, quality is great, and in fact I had quite a lot of contact with PCB, considered in some very basic principles There are also lacking.
(1) Select the appropriate assembly methods
Usually associated loading densities for different PCB
As a circuit design engineer should be installed on the PCB design process associated processes have a correct understanding of this principle can be avoided to make some mistakes. When selecting assembly methods, in addition to considering the density PCB assembly, wiring ease, but must also assembled according to the typical process of this approach, taking into account the level of process equipment business itself. If the companies do not have a good wave soldering process, then select the table in the fifth assembly methods may give yourself a great deal of trouble. Also worth noting is that if you plan to implement the welding surface wave soldering process, the surface should be arranged to avoid welding caused a few SMD process complicated.
(2) Component Layout
PCB layout components have a very important impact on production efficiency and cost, is a measure of the PCB design can be installed linking important indicator. In general, the components as much as possible, there are rules, a neatly arranged evenly, and in the same direction, the polarity of the distribution arrangement. There are rules of order to facilitate inspection, will help improve patch / plug-in speed, evenly distributed heat and help to optimize the welding process. On the other hand, in order to simplify the process, PCB designers always have clear, either side of the PCB, reflow and wave soldering using only one group received the welding process. This greater density in the assembly, PCB soldering SMD components surface must be distributed more time, especially noteworthy. The designer should consider the use of surface mount components soldered group which welding process, the most preferred is the use of wave soldering process patch after curing, can simultaneously element surface piercing device pin welding; But crest soldering SMD components have relatively strict constraints, only welded 0603 and larger chip RC SOT ﹑ ﹑ SOIC (≥ 1mm pitch and height of less than 2.0mm). Distribution of components on the solder side, when Fang Xiangyi transport direction perpendicular to the wave soldering PCB pin to ensure that both sides of the weld end components simultaneously or lead solder dip, adjacent Priorities and spacing between components should meet Wave soldering is required to avoid "shadowing" effect, as shown in Figure 1. When using wave soldering multi-pin SOIC and other components should last two (each one on each side) is set at the foot solder tin tin burglary flow direction pad to prevent even welding.
Similar type of element to be arranged on the board in the same direction, so that the mounting member, and weld inspection easier. Such as making all the right radial capacitors negative toward the plate, so mark facing the same direction so that all dual in-line package (DIP) gap, which can accelerate the speed of insertion and easier to find errors. Shown in Figure 2, the A-plate using this method, it is possible to easily find the reverse capacitor plate and B requires more time to find. In fact, a company can be all of its circuit board components manufactured standardized direction, some board layout may not necessarily be allowed to do so, but this should be the direction of an effort.
Also, similar to the type of component should be grounded as possible together all elements of the first leg in the same direction, shown in Figure 3.
But I really met a considerable number of PCB, assembly density is too large, the solder side of the PCB components must also be distributed higher ﹑ SMD tantalum capacitors and inductors and other fine pitch SOIC ﹑ TSOP and other devices, in this case, only After reflux for double-sided printing can patch welding paste, and the plug element, the element should be distributed as concentrated to accommodate manual welding, another possibility is that the piercing element as the element surface to be distributed in several major on the line to accommodate the latest selective wave soldering process, you can avoid manual welding and improve efficiency and ensure the quality of welding. Discrete distribution is a selective wave soldering joints taboo, will exponentially increase the processing time.
When the PCB file to adjust the position of components, must pay attention to components and silk symbol correspondence, if the mobile device without a corresponding movement of the element symbol next screen will become a major manufacturing quality problems because in the actual production, screen printing sign language is the industry's guiding role in the production.
2.2 PCB layout have to be used to automate the production of necessary gripping edges do ﹑ marking, positioning technology to locate holes.
Electronics assembly is currently one of the highest degree of automation industry, automation equipment used in the production of automatic transmission require PCB, so they asked the PCB in the transport direction (generally in the longitudinal direction), the up and down each road of no less than 3 - 5mm wide gripping edges to facilitate automatic transmission, to avoid near the edge of the board is not automatically installed components due to the joint holder.
Positioning Mark's role is now widely used for optical positioning assembly equipment, PCB needs to provide at least two to three anchor tag, optical recognition system for accurate positioning of the PCB and PCB processing error correction. Positioning markers commonly used, there are two marks must be distributed on the PCB diagonal. Anchor tag selection generally use solid round pads and other standard graphics for easy identification, around the tag should have a feature that no other circuit or open area labeled, preferably not less than the diameter size markers (Figure 4), marking the distance edge of the board should be more than 5mm.
In the PCB manufacturing itself, and in the semi-automatic plug-in installed ﹑ associated ICT test processes need PCB in the corner site offers two to three locating holes.
2.3 rational use puzzle to improve productivity and flexibility.
In the shape of an irregular shape or small size of the PCB mounted Union, will be subject to many restrictions, it is commonly used way to make the puzzle a few small PCB PCB spliced into the appropriate size be installed linking, shown in Figure 5. General unilateral size less than 150mm of PCB, can be considered puzzle way through two ﹑ four fight fight fight, Wednesday, etc., the large size of the PCB to fight to the appropriate processing range is usually wide 150mm ~ 250mm, length 250mm ~ 350mm The PCB is mounted automatic linking in a more appropriate size.
Another way is to double-sided puzzle are decorated with the SMD PCB of a positive and negative makes up a large plate, commonly known as yin and yang fight this puzzle, usually for cost savings stencil consider that through this puzzle, the original need for both sides of the screen, and now only need to open the side of the screen. Another technical staff in the preparation of the placement machine running the program, the use of yin and yang fight PCB programming more efficient.
The connection between puzzle board when the child can face carved double V-groove slots plus ﹑ long hole, etc., but the design must be considered as possible so that the separation line in a straight line, in order to facilitate the final scoreboard , but also can not be considered separate from the edge of the PCB traces too close, easy to damage the PCB when leaving the score board.
There is also a very economical puzzle, does not refer to the PCB puzzle, but the graphics on the screen mesh conducted puzzle. With automatic solder paste printing applications, currently more advanced printing press (such as DEK265) has allowed the dimensions of 790 × 790mm steel online, open mesh graphical multi-faceted PCB can be used for multi-piece steel mesh printed products, is a very cost-effective approach, especially for the product is characterized by small quantities of many varieties of manufacturers.
2.4 design for testability considerations
SMT design for testability ICT equipment mainly for the current situation. The post-manufacturing testing issues in the circuit and PCB surface mount SMB designed to take into account. Improve testability design process to consider two aspects of the design and electrical design requirements.
2.4.1 Process design requirements
Accuracy, substrate manufacturing process, the size of the substrate, the type of probe is positioned to detect the reliability factors.
(A) the precise positioning holes. Set on a substrate precise positioning holes, positioning hole deviation should be within ± 0.05mm, provided at least two positioning holes, and the farther away the better. Using non-metallic positioning hole to reduce the thickening of the solder plating does not meet tolerances. If the substrate is then separated from the whole piece manufacturing test, the positioning hole must be located on the motherboard and each individual substrate.
(2) is not less than the diameter of the test points 0.4mm, the pitch of adjacent test points 2.54mm or more preferably, not less than 1.27mm.
(3) can not be placed over the test surface height * mm components, the components will cause excessive online test fixture probe test points is bad.
(4) The best place to test points around the outside of components 1.0mm, probes and components to avoid impact damage. Positioning ring around the hole within 3.2mm, can not have components or test points.
(5) test point can not be set within the range of 5mm PCB edge, which is 5mm of space to ensure clamping fixture. Usually Conveyor production equipment and SMT equipment also requires the same process side.
(6) All the best probe point tinned or choose soft texture, easy to run through, easily oxidized metallic conductive material, to ensure reliable contact to extend the life of the probe.
(7) test point can not be overwritten or text solder resist ink, otherwise it will reduce the contact area of the test point, reduce the reliability of the test.
2.4.2 Electrical design requirements
(1) which will keep SMC / SMD test points on the component side of the through-hole lead through to the welding surface, through-hole diameter should be larger than 1mm. This allows the use of single-bed line test to test, thereby reducing the cost of line test.
(2) Each node must have an electrical test points, each IC must have POWER AND GROUND test points, and components as close as possible, preferably within the range from IC 2.54mm.
(3) When setting the trace circuit test points which can be enlarged to a width 40mil wide.
(4) The test points evenly distributed on the printed circuit board. If the probe is concentrated in a particular region, the higher the pressure will deform the test plate or needle bed, resulting in a further portion of the probe does not come into contact with the test point.
(5) supply line on the board should be set up to test the breakpoint subregions, in order to supply decoupling capacitors or other components on the circuit board when the power supply short circuit, the more quickly and accurately find the point of failure. Design breakpoint, you should consider the carrying capacity of the power restored after the test breakpoints.
Figure 6 shows an example of a test point design. In part by extending the lead near the line set up a test pad or pads with vias test node test node is prohibited on election solder components, this test may make the probe Weld joints under pressure to squeeze an ideal position so that the Weld failure is masked, a so-called "fail shadowing effect." Due to the positioning error caused by the probe biasing Akira, may act directly on the probe end or pin components and cause damage to the components.
3, Conclusion
These are some of the main principles of PCB design should be considered, in facing the electronics assembly of PCB design for manufacturing, there are quite a lot of detail required, for example, with space for a reasonable arrangement and structure of a reasonable distribution ﹑ printing graphics When properly distributed and text ﹑ heavier or larger devices heat position set in place and test test points to consider in the use of space ﹑ ﹑ riveted rivet installation process couplings and other devices, the distribution of components and tooling nearby interference, etc., are in the design phase of the PCB should consider. A good PCB designers to consider not only how to get good electrical properties and aesthetic layout, as well as equally important point is that the PCB design for manufacturability, order high-quality, high efficiency and low cost.
