Introduction to PCB Assembly for Low Volume Production
Printed Circuit Board (PCB) assembly is the process of soldering or mounting electronic components onto a PCB. For low volume production runs, the PCB assembly process has some unique challenges and considerations compared to high volume production.
In this ultimate guide, we’ll cover everything you need to know about low volume PCB assembly, including:
- Designing for manufacturability and assembly
- Choosing components and materials
- PCB fabrication options for low quantities
- Manual vs automated assembly methods
- Testing and inspection processes
- Cost saving tips for low volume assembly
Whether you’re a startup bringing a new electronic product to market, a maker building a small batch of devices, or an engineer prototyping a new design, understanding the low volume PCB assembly process is critical to the success of your project. Let’s dive in!
PCB Design Considerations for Low Volume Assembly
Keep the Design Simple
When designing a PCB for low volume assembly, it’s important to keep the design as simple as possible. Avoid using complex shapes, tight tolerances, or non-standard hole sizes that could make the board more difficult or expensive to manufacture in low quantities.
Stick to standard board thicknesses (e.g. 0.062″, 0.093″) and common PCB materials like FR-4. Simplifying the design will make it easier to find a manufacturer willing to produce lower quantities of your board.
Consider Component Selection Carefully
Component selection is another key factor in low volume PCB design. When possible, choose common, readily available components to avoid supply chain issues or long lead times that could delay your assembly.
Keep in mind that some components may have high minimum order quantities (MOQs) that make them impractical or too expensive for a low volume run. Work with your contract manufacturer to identify any components that could be problematic to source in lower quantities.
Follow Design for Manufacturing (DFM) Guidelines
Even though you’re producing a low volume of boards, it’s still important to follow industry-standard design for manufacturing (DFM) guidelines. This includes things like:
- Maintaining proper trace width and spacing
- Avoiding acute angles in traces
- Providing sufficient clearance between components
- Following the manufacturer’s capabilities for minimum hole size, aspect ratio, etc.
Adhering to DFM guidelines will help ensure your boards are manufacturable and reduce the risk of defects or assembly issues down the line.
PCB Fabrication Options for Low Volume Production
In-House Rapid PCB Prototyping
For very low quantities (1-10 pieces), you may be able to fabricate PCB Prototypes in-house using a rapid PCB prototyping machine. These desktop CNC machines can mill, drill, and cut custom PCBs from Copper Clad laminate material.
In-house prototyping allows you to quickly iterate on your design without the lead time of outsourcing fabrication. However, the process is limited to simple 1-2 layer designs and may not be suitable for all applications.
Quick-Turn PCB Prototyping Services
If you need a few prototypes (5-50 pieces) fabricated quickly, you can use a quick-turn PCB prototyping service. These online services specialize in producing low quantities of Prototype Boards in 1-5 day turnaround times.
Quick-turn services typically offer a variety of PCB specifications and materials to choose from, so you can closely match your production intent. Most also provide instant online quoting and ordering to streamline the process.
The main drawback is that quick-turn prototypes are often significantly more expensive per board compared to ordering larger production quantities. But for fast prototyping, they can be a great option.
Low Volume PCB Fabrication Houses
For production quantities of 50-1000 pieces, your best option is usually a dedicated low volume PCB fabrication house. These manufacturers specialize in producing smaller lot sizes that may be too low for a typical volume manufacturer to accommodate cost-effectively.
Low volume fab houses generally have quicker turnaround times and lower minimum order quantities than volume manufacturers. They’re also more willing to work with you on custom specifications and materials.
The trade-off is that your cost per board will be higher than mass production. But for most low volume applications, the benefits of speed, flexibility, and lower minimum orders are worth the added cost.
PCB Assembly Methods for Low Volume Production
Once your bare PCBs are fabricated, the next step is assembling the components onto the board. There are two main methods for low volume PCB assembly: manual assembly and automated assembly.
Manual PCB Assembly
For very low volumes (<100 pieces), manual assembly is often the most cost-effective option. In manual assembly, a skilled technician hand-places and solders each component onto the PCB using tools like tweezers, soldering iron, and microscope.
Manual assembly offers the greatest flexibility in terms of component types and placements. Odd-form components, through-hole parts, and components that require special handling can all be accommodated. Last-minute design changes are also easier with manual assembly.
The main drawbacks are that manual assembly is slower and potentially less consistent than automated methods. It also requires skilled labor, which can be expensive in high cost regions.
Automated PCB Assembly
For higher volumes (100-1000+ pieces), automated assembly using pick-and-place machines and reflow ovens becomes more cost effective. Automated assembly can place and solder components very quickly and with high repeatability.
Most automated PCB assembly lines are optimized for surface mount technology (SMT) components. Through-hole and odd-form components may require secondary operations like selective soldering after the main SMT process.
Automated assembly generally requires more upfront engineering compared to manual assembly. The design needs to be optimized for the machines and the components need to be packaged properly for automated handling.
But once the process is dialed in, automated assembly can produce high quality boards very efficiently, making it an attractive option even for relatively low volumes.
Testing and Inspection Processes for Low Volume PCBs
Visual Inspection
No matter how your boards are assembled, the first step in ensuring quality is always visual inspection. A skilled inspector will check each board for common defects like:
- Solder bridges or shorts
- Insufficient or excessive solder
- Lifted or tombstoned components
- Damaged or missing components
- Reversed polarity on polarized parts
For low volumes, visual inspection can be done manually with magnification. Higher volumes may utilize automated optical inspection (AOI) machines to speed up the process.
Electrical Testing
After visual inspection, each board should be electrically tested to verify functionality. This can be done with:
- Flying probe testing: A robotic “flying probe” tester checks for short circuits and continuity on each net of the board.
- In-circuit testing (ICT): The board is placed in a test fixture that makes contact with each node and runs a battery of tests.
- Functional testing: The fully assembled board is powered up and put through its paces to verify all functions work as intended.
The type and extent of electrical testing will depend on the complexity of your board and the volumes you’re producing. More extensive testing adds cost but can save money by catching defects earlier.
Burn-In Testing
For some high-reliability applications, boards may also undergo burn-in testing. In this process, the boards are powered on and subjected to heat, vibration, and other stresses for an extended period to weed out “infant mortality” failures.
Burn-in testing is relatively expensive and is usually reserved for critical applications where reliability is paramount, such as aerospace, medical, or military projects. For most low volume commercial products, it’s overkill.
Cost Saving Tips for Low Volume PCB Assembly
Panelize Boards to Share Setup Costs
PCB manufacturing has significant fixed setup costs regardless of volume. One way to save money on low volume runs is to panelize multiple copies of your board onto a larger panel.
By running several boards at once, you can amortize the setup and tooling costs over a larger number of units. Just be sure to include breakaway tabs or mousebites between boards for easy depanelization.
Consider Consigned Components
Some PCB assemblers allow customers to consign some or all of their components rather than procuring them through the assembler. If you can source components more cheaply on your own, consignment can be a way to reduce your overall assembly cost.
Just be sure you’re providing parts that meet the assembler’s quality and packaging requirements. Cheaply sourced components that cause defects and rework can quickly eat up any upfront savings.
Optimize Your Design for Automated Assembly
Even if you’re planning on manual assembly for prototypes, it’s a good idea to optimize your design for eventual automated assembly. Avoiding the need for custom fixturing or special handling will make your design easier and cheaper to assemble in the long run.
Some automated assembly considerations include:
- Ensuring adequate component spacing
- Avoiding unusual component packages
- Minimizing through-hole and odd-form parts
- Providing good access for testing points
- Including fiducials for machine vision alignment
Consider a Turn-Key Contract Manufacturer
For the lowest possible volume production, managing the supply chain and coordinating multiple suppliers for fabrication, assembly, and testing can be a major headache. In this case, choosing a turn-key contract manufacturer to handle the entire process under one roof may be the most efficient option.
A good low volume contract manufacturer will work closely with you to understand your needs and optimize your design for their processes. They can handle everything from component procurement to final testing and packaging, freeing you up to focus on other aspects of your business.
Frequently Asked Questions
What is considered “low volume” for PCB assembly?
There’s no hard and fast rule, but in general, “low volume” typically refers to production runs of less than 1000 pieces. Below that threshold, the setup costs and economies of scale for mass production make it difficult to compete on a cost-per-board basis.
How much does low volume PCB Assembly Cost?
The cost of low volume PCB assembly can vary widely depending on factors like the size and complexity of the board, component costs, testing requirements, and turnaround time. As a very rough ballpark, expect to pay anywhere from $10 to $100 or more per board for quantities under 1000 pieces.
How long does low volume PCB assembly take?
Turnaround times for low volume assembly can range from a few days to several weeks depending on the complexity of the job and the supplier’s workload. If time is critical, it’s best to work with a supplier that specializes in quick-turn prototyping and communicate your schedule requirements up front.
What are the most common defects in low volume PCB assembly?
Some of the most common defects include solder bridges, insufficient or excessive solder, component misalignment or damage, and incorrect component polarity. Most of these issues can be caught with thorough inspection and testing processes.
What file formats do I need to provide for PCB assembly?
At a minimum, you’ll need to provide Gerber files for the bare board fabrication and a bill of materials (BOM) and pick-and-place file for the assembly. Most assemblers will also request 3D CAD files, schematics, and other documentation to help with quoting and planning the job.
Conclusion
Low volume PCB assembly presents some unique challenges compared to mass production, but with proper planning and execution, it’s possible to get high quality boards produced quickly and affordably.
The key is to design with assembly in mind from the start, choose your suppliers carefully, and communicate your requirements clearly. By following the guidelines and advice in this article, you’ll be well on your way to successful low volume PCB production.
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