PCB Component Placement Accuracy Needs for Various Assembly Types

May 26, 2020 Cadence PCB Solutions

Key Takeaways

  • Learn about the general functions of placing electronic components

  • Discover the accuracy requirements for component placement in different types of assembly

  • Understand why component placement accuracy is becoming more necessary in designs

Machine assembly of mass production circuit boards

When the Printed Circuit Board (PCB) replaced slow and cumbersome hand wired boards and systems of the past, it became a novel technology that allowed electronics engineers to assemble a complex electronic system relatively quickly, easily, and cheaply. The electronics industry, as a result, makes a substantial effort to make the making of these boards as easy as possible. A large amount of money is invested and spent to design software that aids the design, manufacture, assembly, inspection, and testing of printed circuit boards. 

These technologies have allowed design engineers in the electronics business to invest in improved processes that have reduced the size of parts, produced Surface Mount (SM) and Ball Grid Array (BGA) packages, produced reductions in the wire widths of PCBs and the sizes of other features, better inspection of cards with X-ray imagers, and more. 

The slow introduction of Artificial Intelligence (AI) in the design software means some amount of critical work like PCB component placement accuracy is still controlled by hand by a layout engineer. You’ll want to make sure, as a design engineer, that you maintain your knowledge on component placement accuracy to ensure the stability of your design. 

Evolution of Component Placement Throughout a Design Career

Back in school, I designed low density, single use circuit cards using wire winding methods. Then came the chance to make printed circuit boards. I began with simple PCBs – through-hole parts, reasonably sparse designs, and simple – even large – SMT parts. This means that the hand-soldering process, which follows the PCB design process, is in the early days also reasonably simple. 

However, the further in your career, the more high-density PCBs you get assigned to make using minuscule SMT, BGA and QFN parts in vulnerable design environments with thin wires and small separation between components in a layout. This makes 'assembling' a PCB a new variable that a design engineer must contend with. This may come as an unpleasant surprise to some, but I believe to be forewarned is to be forearmed.

Placing Parts on a PCB

Assembling a PCB can be achieved in one of many different ways. A design engineer should give some thought to specific assembly techniques before setting out to design a PCB – Machine Assembly, Hand Assembly, Hybrid Assembly (man and Infrared oven), etc. These choices are usually dictated by considerations like cost, time, quantity, and types of designs being considered by the engineer.

A number of considerations drive the process of placing parts on a PCB. Signal length, both minimum and maximum, ease of assembly, and ease of test are all vital to consider for the assembly of a PCB. Below, we’ll be going into the unique components for each aspect of assembly. 

Types of Assembly and Important Guidelines for DFA 

Throughout your career in electronics, you’ll almost certainly be dealing with many different varieties of assembly. It’s important to keep yourself in the know of what goes into each process, what the strengths and typical uses of each form of assembly are, and how you can best optimize your DFA process to coincide with the eventual production of your PCB.

PCB Component Placement in Machine Assembly

Machine assembly is usually available and used for high-volume manufacturing. The footprints of components on the PCB must be precise and all design rules must be followed. Machine assembly does not offer any flexibility that can be gained by using a human in the loop to fabricate the PCBs. 

Precision of a machine assembled card is absolute. It uses the most expensive industrial machines to complete assembly of a PCB as designed and as allowed by the rules. The need for non-recurring engineering time means this method is usually reserved for large volume soldering. 

Types of design rule violations on a circuit board

Instances of Design rule violations that might be rejected by a fully automated system (machine assembly).

Hand Assembly and Solder Bridge Problems

Hand assembly is the slowest technique. It is time consuming and requires the full time employment of a human. It is also prone to errors. Shorts caused by solder bridges have ruined the career of many an engineer. As a result, most institutions work with two technicians – the assembler and the Quality Assurance (QA) technician, who check the assembler's work to make sure solder bridges do not crop up in the PCBs.

An assembly technician must be a very gifted individual. They allow you to violate design rules that will not be permitted by a machine or even a hybrid assembler. As a result, if a design – preferably a low volume design will violate design rules to achieve compactness then hand soldering is the way to go.

IR Ovens or Wave Soldering in Hybrid Assembly

The rest of the market is taken up by hybrid assembly methods where a technician places components on a card and uses an IR oven or a wave soldering machine to complete the soldering process. 

A hybrid assembly usually uses a stencil and solder paste to create a solder coated PCB where the assembly technician merely places parts on the specified footprints and places the populated card in an oven to complete the re-flow/soldering process. The hybrid assembler must have enough space between parts to hand place them and re-work the soldered card if and when errors like vertical lifting occur in the oven soldering process. A QA technician is probably also a part of this production line.

Component placement in High-Density PCB designs

The need for making high-density PCBs creates new temptations that do not present themselves in sparse designs. When engaged in high density design activities do not forget that even in a high density system you must still follow the old design practices. 

Hand assembly of a circuit board

Hand assembly of a circuit board can be painstaking especially the more complex the board becomes. 

One of the most common practices that designers refuse to follow, and which produces a perceptible increase in density is to discard Reference Designators. However, an assembler still needs this information to assemble a card. Removing these designators means the engineer is now responsible for creating additional documentation to help and guide an assembly technician as they go about the business of assembling your PCB. 

Remember – that a PCB, like a schematic or a program, can pass through many hands while going through the development process before becoming reality and therefore each step must contain enough documentation to allow other people to interact with your creation. Always give thought to how the person who will interact with your design after you, will perceive your design choices. 

This next person – as we all believe – may not be the person who will pay big bucks for your gadget. It is likely to be somebody on your design and assembly team who has to turn your dream to reality by converting your PCB to a fully operable electronic system.

If you’re looking to learn more about how your design tools can assist you in your DFM or DFA processes, look no further than the suite of design and analysis tools available from Cadence. Furthermore, the layout tool you’ve been wanting to make your design and production workflows a breeze is found within Allegro PCB Designer. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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