How to Avoid DFM Issues

Before starting with PCB manufacturing, Rush PCB Inc insists on a clean and unambiguous data package. This not only decreases cycle time, but also ensures a clear understanding of requirements. To achieve this, we present a list of seven common PCB DFM issues for design engineers to address.

PCB Dimensional Discrepancy

One of the most common issues is dimensions in fabrication drawings not matching the actual dimensions in Gerber files. The dimensions stated on the mechanical drawings are different from the actual measurement data in Gerber files. We call this type of mismatch as PCB dimensional discrepancy.

Typically, this may be a documentation error on the mechanical drawing. In about 95% of the cases, the Gerber data is the correct one, and that stated on the mechanical drawing is incorrect. However, this slows down the process at the manufacturer’s back end, as we need to a first article inspection. Therefore, when the inspector is measuring dimensions, they verify it against the drawing, and the mismatch can hold up the process.

PCB Drill Chart Discrepancy

Similar to the discrepancy above, mismatches in the numeric control or NC drill files and mechanical fabrication drill chart can slow down the manufacturing process. Very often customers offer a drill chart along with the fabrication or mechanical drawing. The drill chart may contain data like 10 holes of diameter X1, 25 holes of diameter X2, 40 holes of diameter X3, and so on. However, this may not match with the actual data going into the machine because of the mismatch with the actual data.

In the majority of cases, the NC drill data files are the correct ones, and the designer may have just forgotten to update the mechanical or fabrication data to match with it. Whatever be the reason, the ambiguity holds up of the process.

PCB Fabrication Notes

Sometimes there are a conflict with specifications. Customers typically provide a written note where the stated specification differs from the actual Gerber data. This may be due to there being a broad-based specification or a master specification, which does not match with the actual specifications. For instance, the master specification may state that all printed circuit boards for that customer must have blue solder mask, but the fabrication drawing specifies a green solder mask. This is a conflict holding up the process until resolved.

Another common mismatch is the overall PCB thickness. The fabrication notes may state the thickness of the PCB must be 0.062” ±7 mils. However, adding up the dielectric and copper weight in their stackup in a separate document may exceed the dimension in the fabrication note. Most often, this is a result of cut and paste of fabrication notes on their drawing.

This can result in assigning a specification belonging to one PCB to another board, which may actually have different specifications. For instance, one board may have filled vias, whereas the other may not have any vias. Cutting and pasting of fabrication notes may specify filling vias with non-conductive paste where the board has no vias.

Such discrepancies make it mandatory for design engineers to review each and every note for every PCB and verify that the specification does indeed match that particular design.

Improper Copper Specification

Customer frequently confuse specifying copper with ounces and mils. They assume we plate with 1 ounce of copper, whereas that would result in 1.4 mils thickness of copper. In reality, Class III PCBs require 1 mil of copper, and Class II requires 0.8 mil. Frequently, customers state on their drawing that holes should have a plating of 1 ounce of copper, when they really mean is 1 mil—very rarely do customers actually need 1.4 mil of plating.

Again, customers do not properly specify copper weights for each layer. For a multi-layer board, a general statement for using double-sided, 1-ounce copper leads to a question mark about the copper thickness for the inner layers.

Most customers are also not aware that the copper on inner layers remains unplated. Therefore, the copper thickness of the finished layer is the starting copper thickness. On the other hand, we plate the external layers. For these two layers, the finished thickness is more than what we started out with.

Specifying copper weight as the finished copper weight always is the proper way. In a drawing, specifying only the copper weight generates a dilemma whether to start with the given copper weight, or to achieve it after plating.

Traceability Markings

Most customers require traceability and want several markings on their PCB. Most times, this consist of a date, a serial number, and maybe a panel number, and the customer is not sure of where these markings should appear to the best advantage.

We prefer to generate the traceability markings on the silkscreen. We use a laser jet printer for transferring the data onto the PCB. The operator will type in the required information in the printer prior to starting it. The machine is intelligent enough to increment the serial number for each board it prints.

Therefore, we prefer the specification of the traceability information to appear on the silkscreen, and on no other place.

Material Specifications

Often there is mismatch between material specification requirements. Customers often demand a conformation to industry-accepted IPC standard for material, while they also want brand-specific material. If the two do not match, this causes confusion and results in a delay until they sort out the matter.

We prefer customer to not go with brand-specific material. Rather, specifying the IPC standard is a better choice, as it gives us the freedom to select the material brand that conforms to the IPC standard. Only if there is a very specific requirement should customers go for brand-specific material, along with equivalents if possible. In the latter case, the fabrication notes should categorically mention the requirement for the brand-specific material.

Over Documentation

Once the customer mentions the IPC standards the board should conform to, many notes they place on the drawing may be unnecessary. Specifying an appropriate standard can cover a lot of different things and stating them again (and not correctly) on documents may only serve to increase the confusion.

For instance, we are familiar with IPC-6012 class 3 standard for boards, and there is no necessity to emphasize it will be a 1-mil plated through hole with a minimum diameter of 0.8 mils. Restating such requirements can often generate mistakes and lead to conflicts.

Often, the customer may want a lesser stringent specification than that of the standard, and they put both requirements on the document—the IPC standard, and their relaxed specifications. This causes a conflict because the manufacturer will not know what is more important to follow.

Conclusion

According to Rush PCB Inc, most documentation errors leading to DFM issues are a result of redundancy—stating the same thing in several places. When updating due to a change in specifications, it must change in all locations where it occurs in the documentation. Referencing everything only once has the advantage of updating it only at that one location, reducing ambiguity.