introduce

As a PCB designer, you have different requirements and expectations. There are electrical, functional and mechanical aspects to consider. In addition, PCB must be produced at the lowest possible cost and the highest possible quality. With all these requirements, you also need to consider DFM (manufacturing design). It is an important part of the PCB design process, if not handled correctly, it may lead to problems. Let’s take a look at the ten most common DFM problems you may encounter in PCB design, as well as some design solutions that can help you avoid these problems.

1. IPC based on pin geometry

Pad of components used in printed circuit board is a key component used to determine whether components can be reliably soldered. Through the pin design based on IPC, you can ensure that the components of PCB can be welded in the manufacturing process without errors.

Analysis of ten most common DFM problems in PCB design

Detailed customization in IPC pin Wizard

2. Even connection of element pads

For SMD assemblies of size 04020201 or smaller, it is important that the pads have uniform connections. This will help them avoid solder bumps – that is, lifting parts of the assembly partially or completely during reflow. It is also important to maintain a uniform connection with the BGA pad to ensure reliable welding results. The test procedures to ensure this are complex and expensive, usually involving X-rays.

Analysis of ten most common DFM problems in PCB design

The pad of the SMD element shall have a pad uniformly connected with the equipment to prevent solder blocking during welding

3. Through hole in SMD pad

This is the common point of PCB design wisdom. You should avoid using embedded devices at all costs. When welding, through holes may cause weak solder joints, which may eventually damage the circuit. However, there is a place in PCB design through pads, and it can be especially helpful for thermal management problems.

Analysis of ten most common DFM problems in PCB design

Through hole pads should be avoided; The vent should be separated from the pad

4. The copper foil is distributed on the copper layer

The process of creating copper images on a single sheet depends on many factors. If copper is removed, in one way, it’s unusual to keep a track standing. Therefore, it is suggested to keep the distribution of copper coating as much as possible.

Analysis of ten most common DFM problems in PCB design

Uniform copper distribution (bottom) creates the most reliable PCB

5. Component selection and layout

Many designers try to use as few through-hole Technology (THT) components as possible, usually only on one side of the circuit board. However, the use of tht is sometimes unavoidable. Depending on the combination of tht components on the top layer and SMD components on the bottom layer, all components must generally be placed as close together as possible. In some cases, this option does not include the option of using single side wave soldering. Instead, more expensive welding processes, such as selective welding, must be used.

When using the through-hole assembly, place it on one side and place the SMD in the opposite position

6. Layer or via offset

The creation of PCB output data is the last tolerance free process in the manufacturing chain. PCB manufacturing has tolerance, this is the copper layer image and through hole drilling. Then, PCB manufacturers can drill out PCBs in three or four groups instead of sorting them individually.

Layer and via offset is very important to keep PCB group drilling at the same time

If you imagine this level and via offset, drilling occurs in a three or four PCB, we see that things like the smallest ring and tears are important tools to help PCB designers increase manufacturing output. This, in turn, will help to reduce overall manufacturing costs.

Figure 5 – browsing hierarchy

The use of minimum ring and teardrop are tools to maximize production

7. Do not connect via pads

The through-hole pad or tht component pad removes the unconnected and unused inner layer, and the PCB manufacturer can keep its drilling tool for a longer time. However, PCB designers don’t like it. From an electrical point of view, this may have no effect on the product, but removing pads may weaken the possibility of physical enclosure. If the designer does not want to remove the pad, it is recommended to indicate it in the design specification.

The manufacturer’s notes about unused pads eliminate speculation during production

8. Solder mask

Many PCB designers use about 50 μ The actual value of M defines the perimeter of the pad, and the remaining coverage for the next tracking is at least 50 μ m。 However, if there is to be a solder mask bridge between two pads, it should be at least 75 μ m。 These factors should be considered during the preparation of components in the library and when the components are placed on the PCB. Otherwise, it may cause the distance to be too small and the solder mask may not work properly between pads.

The minimum distance between pads should be 75 μ M (or 3mil) to ensure enough space for the pad to be fully filled

9. Create graphic design and cleaning layer before output data

Placing through holes may cause some areas to be cut. However, as shown below, you can avoid this situation by making small changes to the placement of through holes.

If you don’t want to copper, you can put the vias as close together as possible

Also note that for PCB manufacturing, the sharp angle of the trace may be problematic. If possible, PCB designers should clean it at the end of the design.

If you don’t want to copper, you can put the vias as close together as possible

10. SMD direct connection

Direct connection of two SMD pads in or under SMD components may be an acceptable electrical shortcut at present, but it may cause problems in future tests. For example, during AOI (automatic optical inspection), the camera may not be able to detect a short circuit because the correct connection of the solder to the SMD pad interferes with the visual inspection process. However, small changes in PCB design can clear this out and make it easier for all participants.

The SMD pad is externally connected to facilitate AOI; The pad (left) or the connection below the SMD (not shown) causes the difference to be detected

conclusion

Designing today’s electronic devices is not an easy task, and electrical, mechanical and functional aspects need to be considered

In the whole design process. Manufacturing design presents another set of challenges for the first time to successfully manufacture circuit boards. Following the ten guidelines outlined in this white paper, you will have sufficient ability to determine the appropriate component placement, stacking, weld mask constraints, etc., in accordance with the manufacturer’s required guidelines.

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