This month, we enter a new decade with great hope and impressive automated technologies. Maybe this new decade will provide a paradigm shift that will result in perfect data packages to manufacturing from design. The creation of a perfect PCB data package requires that the design/layout engineers create the PCB to be compatible with the fabricator and assembler processes. This is to ensure that the PCB will meet cost, reliability, and performance requirements that will ship in the committed cycle time.
One of the primary reasons that data packages aren’t compatible is the fabricator/assembler does not provide a complete set of design rules out of concern for giving away their intellectual property, or IP (Figure 1).
How are PCB fabrication design rules created? What is and is not considered IP? What information is required to be shared? Design rules present the capability that has been established to support specific customer requirements.
Equipment, Chemistry, and Materials
The base plant capability is established from the specific feature sizes that can be produced by each piece of equipment, chemistry, and support material that is procured. The standard capability and operating parameters and processes are provided by the suppliers. Process engineering may specify additional unique capabilities that may be considered as IP. Equipment and chemistries are intended to have greater capability than what is required to produce high yielding/reliable PCBs. This equipment list, but not the unique capability, is often provided on the customer required IPC-1710 “Qualification Profile” form during the qualification process.
Process engineering integrates the equipment, chemistries, and materials to define the capability that will be utilized. This may be tempered by the business, facility, operator skill/training, and environmental guidelines. Specific equipment operating and maintenance procedures are then generated. This knowledge is generally considered as IP.
The process “sweet spot” is then established to provide the capability/design rules with the highest yield and lowest cost path. This generally is presented as “typical” design rules. Deviations outside of this could impact the yield, reliability, and/or cost.
Process flow and traveler note requirements are documented and provided to the front-end engineering department to create tooling, inspection requirements, and travelers to fabricate and ship the PCB. There are many ways to define a process flow; thus, they tend to be unique. This is due to the differing skills/knowledge of process engineers and equipment/chemistry variations used by different suppliers and plants. Process flows are considered as IP and should not be required to be disclosed in PCB design rules.
One major exception to keeping this IP proprietary is when an 8D Corrective Action Report (CAR) is required to explain how a defective board was produced and shipped. The process flow and procedure updates are commonly provided along with any modifications that are required to reduce or eliminate the defect. This may also create 8D specific design rules. Generic process flows are often presented to customers during technical sales presentations, but these do not generally provide detailed process IP.
Design rules that are provided to a customer do not generally need to specify the equipment or process flow used to determine the rule; however, there are some exceptions. Surface finish chemistries and solder mask inks and characteristics may be required to ensure compatibility with assembly and secondary coatings applied after assembly, such as conformal coating. The designer may also require the post-processed final material stackup dimensions and supplier selections to ensure that they will meet electrical, mechanical, and thermal specifications.
Missing design rules will not be implemented and may create an engineering deviation or technical query (TQ) to be generated when the design is received because it will not be compatible with the manufacturing process. The planner may provide the protected IP design rule to the customer during the TQ to get approval to modify the dataset by the fabricator. In effect, this is completing the design by the fabricator.
What Should Be Included in an IP-protected Design Rule Document?
A matrix is commonly provided to customers and presents single-variable design rules (e.g., line width/space, hole annular ring, and solder mask registration). Multi-dependent rules should also be provided, such as:
- Minimum drilled hole size versus aspect ratio
- Via fill versus finished hole diameter and aspect ratio
- Preferred materials for low-cost and potentially for quick-turn stock materials
- Preferred solder mask inks and gloss
- Material-specific design rules
It is not practical to provide every new customer with design rules before the initial order because the board layout is already complete. However, it should be the goal of every fabricator to work with repeat customers to agree to a subset of the rules that are compatible with each other’s requirements. Reviewing the TQ communication is a good way to determine the gap and establish a plan to reduce them (I will discuss this process in more detail in a future column).
Providing designers with more in-depth design rules up front will reduce the true NPI cycle time and enable perfect data packages to be received and then directly tooled and released to manufacturing. This would be a great accomplishment in this new decade.
Dana is the principal consultant at Korf Consultancy LLC.