Complete DFM analysis guide

I have a good friend who has a joke about planning a new PCB design for manufacturing: he often asks, “Did you contact your manufacturer today?"”To emphasize that you should cooperate with your manufacturing partners many times during the design process. This is something that designers often forget, and it may cause major headaches before full production. In fact, your circuit board should go through multiple rounds of DFM (design and manufacturing matching) analysis to ensure its manufacturability, both in terms of manufacturing and composition.

Now, when should you start DFM analysis of your design? Another important question may be: what is the best way to speed up the DFM analysis process?Any circuit board has areas that need to be inspected, and a comprehensive manufacturability inspection of the design can be very difficult, especially in complex layouts. Here is the expected content of the quick DFM analysis and how to complete the process.

What does the DFM analysis of PCB include?

Broadly speaking, DFM (design manufacturability) analysis is applicable to any product that requires large-scale production. Manufacturing products needs to be designed to be suitable for large-scale production processes, so the design needs to be checked to ensure that there are no factors in the design that will cause the problem. Your PCB manufacturer and PCB assembler may be located at the end of the earth, ensuring that they have access to a single, controlled project information store for DFM analysis is critical.

For PCBs, DFM analysis involves checking whether the design complies with the manufacturer's manufacturing and assembly processes. Any experienced designer knows that the list of design choices that may affect quality is long. I still don't remember every possible manufacturability problem that may be known in the design, so I often rely on my manufacturer to check my circuit board when I am ready for manufacturing batches.

Constantly check the project design

This raises an important question: when should you perform some DFM checks on your design? If you are working on some simpler circuit boards, it may be enough to rely on your manufacturer to perform a final DFM inspection before production; repeated in-depth DFM inspections will only take up too much time, and your manufacturer can quickly complete the work. For more advanced designs, such as high-level digital mixed signal boards, with tight gaps and multiple signal standards, multiple DFM analysis operations are necessary in order to detect potential quality problems as soon as possible.

The best way to prevent unnecessary design changes before manufacturing is to perform DFM analysis at several different points in time：

• When choosing components: This mainly involves passing the size of the components, especially 0201 and 01005.If you have to use these small components, just make sure your manufacturer can handle these.
• During planning and planning: At this time, we will determine some basic aspects of the circuit board, such as the number of possible layers, the range of trace widths, the size of through holes, whether we need to turn to HDI, which PCB laminate to use, and which IPC manufacturable grade is suitable for the design.
• After placing the components: After placing the components, consider the process, especially regarding the welding of the grounding SMD board. Also consider how any grounding elements will be welded to their reference plane and whether they require heat release.
• When planning to modify it for a while: you will be surprised how much it needs to be in the design before it can enter manufacturing. This is as simple as asking your manufacturer for some verified tables.
• After generating the Gerber file: Some defects in your Gerber file are easier to see. It is best to scan your Gerber to find problems such as overlap and through-hole aspect ratio.
• When working with the MCAD team: In some cases, the placement of weldable connectors or other mechanical components may cause the gap to be too tight.

There are a few points that deserve to be explained in detail, because they may not be discussed often in other articles.

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Some points that apply to connectors also apply to other components, but there is one point about gaps that is worth checking. Make sure you have left enough space for expansion during assembly, especially for connectors with plastic housings or bases. If the two components are too close, they expand during the installation process, and they may both rotate from above during the installation process.

Checking the gaps in the DFM analysis can help us predict the lift-off of components during the most recent FAB operation.

View footprints

Obviously, you should work hard to ensure that your footprints are verified. This can be done manually, or only use verified components from the direct manufacturer (if they are available). However, once the footprints enter the layout, you need to check the solder paste opening, the gap with the through hole, the gap with other components, the aspect ratio of the through hole, and so on. If the software you are using does not have the correct rule checking function, you may float the hot pad, or you may put the posture too close to the solder wire. You can directly view the PCB layout, but it is also completely possible to generate an overview Gerber file and compare your layers (see below).

You can find components that require solder resist openings and tears from the temporary Gerber file.

Process inspection

This may sound simple, but if you ask your manufacturer directly about the number of layers you need and the way the layers are arranged, you will easily pass this level. They have completed the required DFM (design) manufacturing matching) analysis to ensure that specific layers can pass through their processing process. They provide you with the trace width, trace accuracy (for differential pairs) and layer thickness you need to use, as well as the laminate you need. In some cases, you may be surprised to find that the laminate you need is not available, and you need to use a close corresponding material.

If you contact the manufacturer as soon as possible, they will send you a qualified stacking table.

For 4-layer spectacular, you may receive a standard 8mil/40mil/8mil S/P/P/S spectacular with a total thickness of 62 mil. More complex applications may require customized tables, especially when you have a board that controls the wiring when needed. If you get some information in advance, you won't risk using the wrong traces and dispersion to control impedance, and everything will have been verified.

DFM analysis before manufacturing

Once you have completed the circuit board and have sent it to the manufacturer for manufacturing, your manufacturer should use your final Gerber file for their own DFM (design and manufacturing matching) analysis. Note that I use “should” here because not all manufacturers will be present; for some manufacturers, after you upload your Gerber file, they will not doubt produce circuit boards as shown in your manufacturing file. For manufacturers, you need to clearly require this service level, because different service levels may only be provided as additional services.

Once you get the DFM analysis from the manufacturer, you will see many results in the following two areas: gap inspections for process capabilities, and inspections for specific industry requirements.

Check the characteristic size according to the process ability

When you submit the design file to the manufacturer and they perform DFM analysis, you may see a lot of results about the gap inspection. The manufacturer should have checked the above series of areas, but they also need to compare your characteristic size and transparency with their process capabilities. Even if you have already used the preliminary Gerber file for this process at the time of quotation, it is best to continue running because you may miss something.

The following is an example of a DFM analysis report provided by an ITAR manufacturer I prefer. In this table, we can see the radius, the size of the ring pad, and the gap between the vias and the copper. As can be seen from the bottom row, the gap between my trace and the copper is set too low, and the pad size of some components has increased.

The sample DFM analysis report shows the gap compared to the process capabilities.

In this example, we found multiple errors on a specific package, which happened to be a TO-92 package. In this case, the aperture in the built-in library is too large, so that the ring pad at the edge is too small to maintain the gap. After adjusting the aperture size, we were able to leave room for the level 2 ring pad while still retaining enough clearance to prevent short circuits.

For a large and complex design with thousands of networks, how does your manufacturer check every possible feature in the PCB layout? There are some applications that can help automate the process and compile a report listing any errors. Some manufacturers have their own applications for internal use, while other manufacturers also provide a download program that allows you to check the design before manufacturing.

IPC category reliability review

Another area of design requirements that requires more experience is IPC-level audits. One important point pointed out in the quotation process is, if any, which level of IPC qualification are you seeking. This involves checking the teardrop shape, ring pad size, width and pad diameter and copper weight, the ability to plating holes and holes, and the thickness requirements of the dielectric layer. Only a few main reliability requirements are met. The physical layout will be compared with the manufacturer's capabilities to ensure that the final design meets the eligibility and performance requirements defined in the IPC standard and must be changed before manufacturing.