The surface-bond assembly process, especially for fine-pitch components, requires constant monitoring and system inspection. For example, in the United States, solder joint quality standards are based on IPC-A-620 and the National Solder Standard ANSI/J-STD-001. Only by understanding these standards and specifications can designers develop products that meet industry standard requirements.
Mass production design
Design for volume production includes all process, assembly, testability and reliability for volume production and is based on documented requirements.
For a series of conversions from design to manufacturing, complete and clear assembly documentation is absolutely necessary and successful. Relevant files and CAD data lists include bill of materials (BOM), qualified manufacturers list, assembly details, special assembly guides, PC board manufacturing details and Gerber data or ipc-d-350 program contained on disk.
CAD data on disk is very helpful in developing test and machining tools and programming automatic assembly equipment. Including XY axis coordinate positions, test requirements, outline drawings, circuit digrams and XYa coordinates of test points.
PCBA quality
Take samples from each batch or specific lot number to test for solderability. The PCBA will be compared to the product information provided by the manufacturer and the quality specifications calibrated on the IPC. The next step is to print the solder paste onto the pads and reflow them. If organic flux is used, it will need to be cleaned again to remove residue. When evaluating solder joint quality, the appearance and dimensional response of the PCBA after reflow should also be evaluated. The same testing method can also be used for wave soldering processes.
Assembly process development
This step involves continuous monitoring of each mechanical movement with the naked eye and automated vision equipment. For example, it is recommended to use a laser to scan the amount of solder paste printed on each PC board.
After the sample is attached to the SMD and resoldered, quality control and engineering personnel need to check the soldering conditions of each component connector one by one. Each member is required to document the alignment of passive components and multi-lead components in detail. After the wave soldering process, you also need to carefully inspect the uniformity of the solder joints and identify potential locations of solder joint defects due to pins or components being in close proximity.
fine pitch technology
Fine pitch assembly is an advanced construction and manufacturing concept. The density and complexity of components are much higher than the mainstream products currently on the market. If we want to enter the mass production stage, we must modify some parameters before putting it on the production line.
For example, fine-pitch components have pin pitches of 0.025" or less and can accommodate both standard and ASIC components. For these components, industry standards have very wide tolerances, as shown in Figure 1. This is because of the component supplier's Tolerance errors vary, so pad dimensions must be customized or modified to improve assembly yield.
The size and spacing of the pads generally follow ipc-sm-782a. However, in order to meet process requirements, the shape and size of some pads will be slightly different from this specification. For wave soldering, the pad size is usually slightly larger to allow for more flux and solder. For some components that usually remain near the upper and lower limits of process tolerances, the pad size needs to be adjusted appropriately.
Consistency of surface adhesive component placement orientation
While it is not necessary to design all components in the same orientation, consistency will help improve the efficiency of assembly and inspection of components of the same type. For complex boards, components with pins often have the same orientation to save time. The reason is that the fixture for mounting components is usually fixed in one direction, and the mounting direction can only be changed by rotating the board. As for general surface mount components, there is no such problem because the clamping jaws of the placement machine can rotate freely. However, in order to pass through the wave soldering oven, the orientation of the components needs to be uniform to reduce the exposure time to the solder flow.
The polarity of some polarized components is determined early in the entire circuit design. After understanding the circuit function, the process engineer can decide the order in which components should be placed to improve assembly efficiency, but efficiency can be improved by having the same directionality or similar components. If the placement directions can be unified, it will not only shorten the writing speed of placement component programs, but also reduce the occurrence of errors.
Consistent (and sufficient) component distance
Generally speaking, fully automatic surface adhesive placement machines are quite accurate. However, designers often overlook the complexity of mass production while trying to increase component density. For example, when tall components are placed too close to components with small lead spacing, not only does it block the view to inspect the lead solder joints, it also blocks the tools used for rework or rework.
Wave soldering is generally used for low-profile components such as diodes and transistors. Small components such as SOICs can also be used in wave soldering, but it should be noted that some components cannot withstand the high heat of direct exposure to the solder furnace.
To ensure consistent assembly quality, the distance between components must be large enough and evenly exposed to the tin furnace. To ensure that the solder can reach each contact, high components should be kept at a certain distance from low and low components to avoid a shielding effect. If the distance is insufficient, it will also hinder component inspection and rework.
Industry has developed a set of standard applications for surface adhesive components. If possible, use standard components as much as possible so that designers can build a database of standard pad sizes and engineers can better understand process issues. Designers can find that some countries have similar standards and the appearance of the components may be similar, but the pin angles of the components vary from country to country. For example, SOIC component suppliers in North America and Europe can meet the Eiz standard, while Japanese products use EIAJ as the design standard. It should be noted that even if they comply with EIAJ standards, components produced by different companies may differ in appearance.
Designed to increase productivity
Assembling a board can be very simple or very complex, depending on the shape and density of the components. A complex design can make production efficient and reduce difficulty, but if the designer does not pay attention to the details of the process, it will become very difficult. The assembly plan must be considered at the beginning of the design. Generally, mass production can be increased by adjusting the position and orientation of components. If the size of a PC board is small, the shape is irregular, or the components are close to the edge of the board, mass production in the form of connecting boards can be considered.
testing and repair
Using small-scale test tools on your desk to detect missing components or processes is highly inaccurate and time-consuming. Test methods must be considered in the design. For example, if you want to use ICT testing, you should consider designing some test points on the lines that the probe can touch. There are pre-written programs in the test system that can test the function of each component, indicate which component is faulty or misaligned, and determine whether the solder joints are intact. Detection errors should also include short circuits between component contacts and empty soldering between pins and pads.
If test probes cannot reach every common node on the line, it is impossible to measure each component individually. Especially for fine-pitch assemblies, automatic test equipment probes are needed to measure the connection points on all traces or traces connected between components. If it can't be done, it must pass functional testing, otherwise it will have to wait for customers to wear it out after shipment.
ICT testing is to develop different tools and testing procedures according to different products. If testing is considered in the design, the quality of each component and contact can be easily inspected. (Figure 2) Solder joint defects can be seen with the naked eye. However, insufficient tin and very small shorts can only be checked through electrical testing.
Since the density of components on the first and second sides may be the same, traditional testing methods may not detect all errors. Although there is a small via pad on the PC board for the high density and fine pitch of probe contacts, there is still a desire to increase the use of via pads.