Avoiding Holds in Printed Circuit Board Design, Manufacturing, and Assembly
Printed Circuit Board (PCB) design, manufacturing, and assembly companies make sure their boards adhere to the specifications of customers. In general, specifications determine the applicability and the general operating characteristics of a PCB. Therefore, before starting, the manufacturer makes sure they have understood all the requirements of the customer. Moreover, to avoid poor work output and discrepancies arising from wrong or incomplete design inputs, manufacturers will hold a job if they find differences between an order and the documents the customer provides.
The entire PCB design, manufacturing, and assembly industry follows this protocol, as almost all PCB designs demand definitive precision. Therefore, designers should explicitly avoid Design Rule Check (DRC) errors to make sure they maintain specifications such as copper-to-copper spacing and minimum hole diameter for vias. Necessarily, the recommendation is to double-check the orders to prevent delays due to holds during manufacturing. To ensure the product is delivered in time and with no errors, here is a discussion on some of the most common reasons leading to holds during design, manufacturing, and assembly of PCBs.
Broad Classification of Reasons for Hold
Both manufacturers and designers want PCBs to be delivered fast, error-free, and as per expectations. Usually, the entire sequence consists of the time taken for design of the PCB, time for its manufacture, and the time for its assembly, before it is delivered to the customer. A hold anywhere in this sequence adds to that time, as inconsistencies have to be sorted out before the sequence can be completed.
Usually, the designer proceeds to his next task after delivering the design documents for the PCB to the manufacturer. If the manufacturer has questions about the documents, the designer must take time off from his current job and return to his previous task—the time to switch depending on the urgency of each task. If this repeats a few times, apart from the holdup for the PCB, the next project of the designer also suffers a lag.
Common reasons for the manufacturer to ask questions broadly depend on receiving data that is bad, missing, or conflicting.
Primarily, this is the result of a bad design, as PCB design is rarely done manually nowadays. CAD software for PCB design has evolved greatly in the last two decades, and usually have sophisticated checks and balances built into them, called DRCs, to prevent errors from inadvertently creeping in. However, designers must be aware of and be properly trained to use the DRC rules to their advantage, thereby avoid generating bad data.
Manufacturers need four types of information to start work on a PCB. Hold ups can occur if any of these documents or information within them is missing, or ambiguous. These are:
1. Artwork Files—consisting of Gerber or ODB++ files related to component sides and solder mask details.
2. Hole Size & Board Dimensions—generally ASCII files related to drill details, aperture, and other mechanical details.
3. Fabrication Notes—related to how the designer wants the board to be built.
4. IPC Net List—mostly to compare with the actual connections on the PCB
Unless checked very carefully by the designer, PCB manufacturers often land up with conflicting data. For instance, Gerber files may differ from the IPC net list, Fabrication notes may cite information about two sides whereas only one side legend is available or different parts of the Fabrication notes offer conflicting instructions.
Other Reasons for Hold in Design and Manufacturing
Whether the data a manufacturer receives is bad, missing, or conflicting, there can be different reasons for their occurrences. While it is impossible to mention all of them, common reasons may be attributed to:
Mixup in Different Orders
A mixup in different orders can occur with equal probability at both the customer’s as well as at the manufacturer’s end. Customers placing orders for different PCBs with the same manufacturers or separate manufacturers can inadvertently mix up the documents. Likewise, manufacturers receiving orders from several customers can mix up documents or components in one order with another, unless they have procedures in place for handling such occurrences.
Updating Requirements of Recent Design Changes
Once a design is completed and documents delivered to the manufacturer, some changes may be necessary to make the design more effective or appealing. Depending on how far the manufacturer has progressed in the design, fabrication, or assembly process of the PCB, they may be able to incorporate the design changes immediately. If the manufacturer has progressed far into the process, the changes may cause a delay. Further, if the changes to the BOM, assembly files, instructions, and materials contain ambiguities, there may be a hold up as well.
Insufficient Inner Clearance
In any PCB with plated or non-plated holes, inner clearance refers to the minimal distance between the edges of a hole to the neighboring unconnected inner layer of copper. The designer must make sure there is sufficient inner clearance so that the drill will not short the inner copper layers. Failure to do so may result in a PCB hold.
Reasons for Holds in Assembly
This is a common reason for hold up for boards requiring assembly. Usually, customers provide components or tools, or a list of them, which they want the manufacturer to use when assembling their product. If any tool or component is missing, the entire assembly sequence may have to be held up until the missing part is available.
Ambiguous or Non-Marked Polarities
Capacitors, diodes, and batteries usually have polarities and this is important for their proper positioning during assembly and functioning. The designer may indicate the correct polarity by using differently styled copper pads, or placing appropriate markings on the screen documents. A mismatch in these documents or missing information may lead to a hold up in the assembly process.