Bare Board PCB: Everything You Need to Know

Introduction

The bare printed circuit board is  the foundational component of a printed circuit board, upon which other surface mount or SMT components are mounted and connected. The relative emptiness of the board affords technicians the opportunity to conduct several tests to ensure suitability to the desired PCB design, and proactively correct errors where necessary.

What is a Bare Board PCB?

A bare board PCB refers to a printed circuit board that is devoid of through-holes or electronic components. Empty as it usually is, it is regarded as the skeleton of the conventional PCB, before it becomes modified with through holes or electronic components. A bare board consists of the PCB substrate, metal coating, vital conductive pathways and patterns. Even though it is considered empty, in this state, it has undergone photo-resist lamination, etching, drilling, solder mask application, and silkscreen printing.

In the industry, bare boards are often called bare PCB, blank PCB or simply PCB. It is also commonplace to refer to a bare board as a PCB while calling an assembled board a PCB assembly; often called PCBA.

The bare PCB board provides electrical and mechanical support for circuit production, and it is this circuitry of electronic components that distinguishes the already mounted PCB from the bare board. The blank board slate also offers flexibility and versatility, making bare boards viable for mass production instead of making unique ones for a specific order.

Recommended reading: PWB vs PCB: Differences and Similarities

When to Use a Bare Board

Bare boards are designed to be configured, so they can be used to test its design, concept, and functionality before more expensive components are assembled to make a PCB. These bare boards offer mechanical support to the components, traces, and pads as they are the substrates that house all other components. This means that bare boards are a foundational element, essential for PCB fabrication and design, and by extension, electronics production.

The bare board is useful for carrying out testing in order to preempt possible issues that may occur at a later stage of the PCB fabrication process which would affect the usability of an electronic device. [1]

Advantages and Disadvantages of Bare Boards

The bare PCB board offers a lot of benefits such as the earlier mentioned mechanical support to the components, traces, and pads that are mounted and designed onto the board; more so, the circuit connections on the board allow the routing of electrical current between the components it carries. As the base of all printed circuit board assembly, the bare board is indispensable to PCB fabrication, and has proven advantageous for the following reasons:

1. Early Detection of Defects: Without components, boards are easier to test and appraise. The board’s layout and surfaces aren’t occupied by electrical components or other obstructions, so they can be examined on the basis of the structure and design. Any issues in the circuitry or selected components can be assessed and rectified before the full circuit is completed.

2. Cost Saving: The foundational process of board design, testing and basic fabrication ensures that the board is functional and durable. This gives room for early detection of problems that might be expensive to fix later on, if at all possible. When problems are identified early, erroneous assembly with expensive components can be avoided, thereby reducing the risk of money loss.

3. Time Saving: Early prototyping and testing ensures satisfactory and design-specific functionality, so there is no time wasted building a full, but ineffective PCB. Also, the streamlined, automated process compensates for the upfront time investment.

Nevertheless, at any point in board manufacturing, even at the bare board design stage, there is always a chance of  accuracy and orientation errors, as well as etching mistakes such as over-etchings, under-etchings, and holes that may cause severe defects. Consequently, thorough inspection is paramount, and the best time to ensure the board is error free is when it is bare, so that mistakes can be intercepted and corrected. This is why bare board testing is important and why bare PCB boards have no real drawback. [2]

What is Bare Board Testing?

Bare board testing is the electrical testing of the unpopulated circuit board to evaluate the continuity and isolation of the board’s connections. The continuity testing process is to ensure that the circuits have no open points that will hamper the current, while isolation testing determines the resistance between two distinct electrical connections. This test ensures the board circuit correctness by testing to the electrical footprint of the intended design.

Bare board testing can save time, money, and labor as it helps to prevent the cost of repair and maintenance of a board. When testing is carried out,  PCB manufacturers can detect any errors on the bare board early on in the manufacturing process.

Testing bare boards helps to guarantee reliability. PCBs are often densely populated during miniaturization. Therefore, it is vital to ensure that the bare board can solder the expensive components. The slightest error in a blank board will result in an electronic assembly malfunction, being that it is the basis of an electronic system. Therefore if it fails, the entire system does. This is why the importance of building a good foundation cannot be overstressed.

Bare board testing is especially beneficial in the early stages of board development, when it can save the most money and time. By introducing bare board testing into the manufacturing process from the start, many aspects of production can be efficiently streamlined. [3]

Types of Bare Board Testing

The old fashioned way of testing bare boards is by using a digital multimeter, one connection at a time. But the complex multilayer board designs of today have phased out such methods to be replaced with more feasible ones suitable for high volume and modern day production.

1. Pinned Fixture Test: This is also known as Bed of Nails and In-Circuit testing, and is suitable for large scale manufacturing. The method tests every connective surface on the PCB board at the same time, using spring-loaded pins or probes. Top and bottom plates apply pressure to a custom pin fixture to ensure a connection across the board.

The pinned fixture test is remarkably efficient and only takes a few seconds to complete. It has however shown some downsides such as high expenses and lack of flexibility, because each PCB design requires a new fixture created, together with a custom array of pins.

2. Flying Probe Test: This type of testing uses two or more robotic arms equipped with poles that glide across the surface of the board to test each net and verify a PCB’s connections. These flying probe testers  travel across the connections using instruction from a software program, checking for opens, shorts, resistance, capacitance, inductance, and diode issues.

Flying probe test machine

The flying probe method offers better affordability and flexibility than the pinned fixture test, although it has a longer testing period. This makes it suitable for smaller production quantities where slower testing speed will not restrict the design process.

3. Automated Optical Inspection (AOI): 

AOI is a method that applies a single 2D camera or two 3D cameras to photograph the board and compare it to a schematic. If the board doesn’t match the schematic, it can be flagged to undergo visual inspection by a technician. AOI is useful for discovering issues early and altering production plans, but it does not power the board and is not independently fail safe, so it is usually combined with other testing methods.

4. Burn-In Test: This type of testing is used to detect failures early and establish the load capacity of the board. This is done by running a power supply through the electronics at an elevated temperature, usually at the highest capacity. This is done continuously for 48 to 168 hours, with a goal to eradicate any latent defects activated by the conditions of the test.

Due to the intensity of its method, this type of testing can be destructive to the board, but it may spot serious defects before the product is launched.

5. X-Ray Inspection: This method of testing is used more for board inspection, as radiographs can be used to identify defects like solder connections, barrels, and internal traces, in the early stages of PCB manufacturing. This way, defective elements that may be hidden from the naked eye can be discovered for correction. [4]

Key Takeaways

PCB manufacturing is a complex process where copper layers are etched and copious holes are drilled according to a particular design. Even with the aid of machines, the outcome isn't always error free. Consequently, bare board use and testing have become significant for many benefits in terms of cost, risk management and production value. As the testing technology advances so also does the possibility of a more automated board design and fabrication process increases.

References

1.Cadence. Understanding Bare Board PCB Testing. 2022. [cited 2022 Jul 9]. Available from: https://resources.pcb.cadence.com/blog/2022-understanding-bare-board-pcb-testing 

2. Candor Circuit Boards. What is a Bare PCB Board? Testing and Uses. 2020. [cited 2022 Jul 9]. Available from: https://www.candorind.com/what-is-a-bare-pcb-board/ 

3. Millennium Circuits Limited. What is a Bare PCB Board | Bare Board Testing 2022. [cited 2022 Jul 8]. Available from: https://www.mclpcb.com/blog/when-use-bare-board/ 

4. Gerber Labs. What is a Bare PCB Board. 2020. [cited 2022 Jul 8]. Available from: https://www.gerberlabs.com/what-is-a-bare-board/