Cave Markings and Circuit Boards: How to Read a PCB Assembly Drawing
“The devil is in the details.” Ever think about the meaning of that phrase? It tells us about the importance of details and that overlooking details will cause problems. Such a brief sentence also speaks to us about PCB assembly drawings. Accurate component placement makes the PCB layout easier. If we gloss over the small details, many different problems can damage circuit performance.
PCB assembly drawings provide a master, controlled print of information needed to completely assemble a Printed Circuit Board. Whether produced in a .pdf or .jpg format, the drawing can include component outlines, surface mount and through-hole pads, polarity marks, reference designators, the board outline, and titles.
What Does the Assembly Drawing Accomplish?
In reading the assembly drawing, reference designators, polarity markings, and the pin 1 position for integrated circuits give an assembly machine operator the information needed to check the component position and reduce mistakes. Along with component information, assembly drawings also reference the current revision of the schematic, the quality standard used for assembly and inspection, and a reference the Bill of Materials (BOM).
In addition, assembly drawings may include any special instructions for the assembly operator. For example, an fabrication drawing for a printed circuit board used in a harsh environment may provide instructions and call-outs for specific components that require different bonding methods. Other special instructions may cover masking information or measures taken to avoid short circuits.
Reference Designators and the PCB Assembly Drawing
Whether directly assembling to a board or programming a pick and place machine, you can use the PCB assembly drawing and reference designators to clarify your layout. Each reference designator consists of letters and numbers that represent different types of components. Pick and place robots rely on this consistent method for referencing components when placing components on the PCB layout.
Reference designators consist of a letter and a number. This standard practice seems simple enough and easy to maintain. As an example, we mark resistors with a letter designation of “R,” capacitors with a designation of “C,” and diodes with a “D.” However, some companies apply variations of standard practices when referencing other components. While standard practices designate a Zener diode with a “D,” design teams may decide to use “Z.”.
When we read an assembly drawing, our human intelligence allows us to understand the meaning of a variation. Toss in multiples of different types of components along with the application of multi-board designs within family panels, though, and wading through the variations becomes a tougher chore.
The family panel approach requires continuous numbering schemes from one board to the next. For example, the numbers for resistors on PCB#1 begin with “R101” and continue through “R125.” When we move to PCB#2 in the family panel, the resistor numbers start with “R126 and continue through “R143.” The resistor numbers for PCB#3 begin with “R144.” You may want to consider starting the numbering scheme with the bottom and proceeding through your Printed Circuit Boards.
With smaller projects, you may not run into any issue with your designators
As the process of building the printed circuit board moves from human eyes to assembly robots, any inconsistencies will cause numerous problems. If we use two different styles of reference designators for resistors, the robot will conclude that two completely different components exist and reject the PCB layout as an error.
In a previous paragraph, we briefly touched on capacitors always having a reference designation of “C.” Assembly drawings provide value in showing that a reference designator of C1 associates with a particular type of capacitor on an individual Printed Circuit Boards.
When the assembly involves a family panel consisting of several PCBs, the precision seen with “C1” becomes murkier because of different capacitor types. PCB #1 may use “C1” as a reference designator for one type while PCB #2 associates “C1” with a different type. A pick and place robot that sees the circuit board as one unit—and all capacitors as part of that single unit—will become confused unless the assembly diagram uses an organized designator scheme.
Polarity Marks and Diode Symbols
Polarity marks should embrace a standardized approach. Unfortunately, the opposite rings true. While standard diode symbols use an arrow to show the direction of forward current, dots, stripes, or arrows may mark diode polarities. Or suppliers may decide that pin one of an LED should represent the cathode and then change later to denote the anode as pin one.
To make matters even trickier, variations of those schemes may or may not mark the polarity of surface-mount diodes. Some of the confusion occurs because of the different types and polarities of diodes. For instance, reverse-biased Zener diodes have a positive cathode. Rectifiers and LEDs have negative cathodes.
To clear these problems, you can use the proper reference designator for the diode and either a letter “C” or a letter “K” to mark the cathode in both the assembly drawing and the silkscreen.
The same type of issues for marking polarity occur with electrolytic and tantalum capacitors. Electrolytic capacitors have the negative terminal marked, most tantalum capacitors mark the positive terminal. Using tantalum capacitors as an example, the assembly drawing ensures the proper placement of the polarity-sensitive capacitors. A reverse biased tantalum capacitor will have dielectric oxide break down that results in a short circuit and thermal runaway.
Keep Your Product on Course
Anyone can get lost in the details. An assembly drawing keeps ICs from becoming lost by using the standard convention to mark pin one of the device. The drawing will use either a dot or a number to show the location of pin one. From there, pins always number around an IC in a counter-clockwise direction.
Finding a pin should be as easy as reading a map.
Without the assembly drawing, finding pin 1 can present a problem. While some manufacturers may indicate pin 1 on the chip with a dot, others bevel the pin 1 corner or use a band to show pin 1.
Design teams often place special notes and instructions within an assembly drawing. Those notes and instructions help assembly operators and others avoid mistakes and delays. For example, you may want to specify the use of a solder/flux type if the circuit needs to meet certain standards the Restriction of Hazardous Substances (RoHS). A solder mask does wonders!
You may also want to add notes about any International Traffic in Arms Regulations (ITAR) compliance requirements that may apply to the final product. Notes may also include how the product responds to Institute for Printed Circuit (IPC) classifications for workmanship and reliability.
With the right circuit board design software, you’ll be able to add and create everything you need to ensure that your pcb fabrication goes through the design stages to manufacturing painlessly. With great design rule checking and easy generation of manufacturing output files, Altium Designer® seems like a smart choice.
If you’d like to know more about how to manage your assembly drawings, consider talking to an expert at Altium Designer today