Introduction

Printed circuit boards (PCBs) are essential components in virtually all modern electronic devices. They provide the mechanical structure and electrical interconnections for components like integrated circuits, resistors, capacitors etc. While the final mass produced PCBs are fabricated by large manufacturers, engineers often need to create prototypes during product development. This article provides a comprehensive guide on how to make a PCB prototype.

PCB Design Software

The first step is selecting suitable PCB design software. Popular options include:

• Altium Designer – Used by professionals for advanced designs and multilayer boards.
• Eagle – Widely used tool with free version suitable for simpler designs.
• KiCad – Open source software with full capabilities. Steep learning curve.
• DipTrace – Affordable tool good for 2-6 layer designs.
• EasyEDA – Online tool with free basic features.

Key software capabilities needed are schematic capture, PCB layout routing, design rule checks, library components, and Gerber file generation.

Schematic Capture

This involves using the PCB software to draw a schematic diagram showing the electrical connections between components. Standard symbols are used to depict parts like ICs, transistors, connectors etc. Unique references are assigned to each part for identification. Simulation can be done at this stage to test circuit operation before prototyping.

PCB Layout

The physical PCB layout is created next by placing and routing component footprints. Good layout practices include:

• Match schematic and PCB component references
• Place parts with shortest connections first
• Use grid for alignment
• Maintain clearance between tracks and pads
• Allow space for mounting holes
• Minimize vias and crossovers

Design Rules

The software’s design rule checks ensure the PCB layout meets fabrication requirements like:

• Minimum track width and spacing
• Via dimensions and pad sizes
• Copper clearance around pads and tracks
• Annular ring size for vias
• Silkscreen margins

Errors must be fixed before generating final Gerber files.

Gerber Files

Gerber files contain PCB fabrication data in a standard format understood by manufacturers. Below are typical files needed:<style> table { font-size: 15px; } </style>

File	Description
Copper layers	Copper tracks and pads for each layer
Silkscreen layer	Component outlines and markings
Solder mask layer	Insulating layer with openings for pads
Drill file	Locations and sizes of drill holes
Assembly file	Component locations and references

Footprint Libraries

Most PCB software rely on libraries of component footprints for placement. Creating custom footprints is needed for new parts lacking library data. This requires knowing dimensions from datasheets.

Choosing PCB Materials

Common PCB substrate materials include:

• FR-4 Glass epoxy – Most common, low cost material
• CEM-1 Cotton paper – Low cost, lower performance than FR-4
• Rogers RO4003 – High frequency circuits, costly
• Arlon 85N – Extremely rugged, high temperature PCB

Factors when selecting PCB materials include dielectric constant, loss tangent, thermal conductivity, CTE, Tg, moisture absorption, flammability, cost.

Layers and Thickness

2-layer boards suffice for simple circuits. Complex designs require 4-16 layers or more. Minimum trace/space width reduces with more layers. Standard thickness is 1.6mm but can range from 0.4mm – 3.2mm. Thicker boards provide better heat dissipation and rigidity.

Assembly Options

PCB prototype assemblers offer following options:

• Through hole assembly – For through hole components with leads
• SMT assembly – For surface mount components
• Mixed mode – Combination of through hole and SMT
• No assembly – Unpopulated bare PCB

Verify supported component sizes/BGAs when ordering assembly.

Solder Mask

Photoimageable solder mask coats copper to prevent solder bridging between pads during assembly. Green is the most common color but also available in blue, red, yellow etc. Matte finish helps reduce glare. Both sides should have solder mask.

Silkscreen Layer

Silkscreen printing displays component outlines, labels, board name etc. Using white or light colors provides good contrast against solder mask. Use minimum text heights of 0.7mm to 1mm. Allow margin between silkscreen and pads.

Surface Finishes

Common PCB surface finishes include:

• HASL – Hot air solder leveling, provides good solderability
• Immersion Silver – Lowest cost finish with good shelf life
• ENIG – Electroless Nickel Immersion Gold, excellent for components and durability
• Immersion Tin – Reacts with copper, short shelf life

ENIG is recommended for fine pitch components.

Soldermask Over Bare Copper (SMOBC)

SMOBC places soldermask directly over bare copper which acts as a ground or heatsink plane. It saves cost compared to a dedicated copper plane layer. Disadvantage is it cannot have traces or vias.

Copper Thickness

1 oz copper (35 μm) is standard but thicker copper is possible for high current applications. Thicker copper improves thermal performance. Common options are 1oz, 2oz, 3oz copper thickness.

Test Points

Include testpoints linked to important nets on the PCB to facilitate testing and troubleshooting. Use 0.9mm – 1.5mm circular pads with soldermask opening. Place testpoints along the edges or on non-component areas.

PCB Cost Drivers

Key factors impacting prototype PCB costs include:

• Board size – Measured in square inches (or cm)
• Number of layers
• Advanced materials like Rogers, Arlon
• Tighter tolerances on trace/space
• Gold ENIG surface finish
• Thicker 2oz or 3oz copper
• Smaller minimum hole size and pitch
• Special processes like impedance control
• DFM review requirements

Documentation

Maintaining schematics, datasheets, BOM, layer stackup diagram, assembly drawings, fabrication notes in a centralized location ensures proper archival and version control.

Prototype Assembly

After fabricating bare PCBs, assembly can be done inhouse or outsourced. Hand soldering suffices for a few boards. Higher volumes justify investing in a benchtop SMT machine.

Key steps in prototype assembly process:

• Obtain components
• Load BOM into SMT pick and place machine
• Perform solder paste stencil printing
• SMT component placement
• Reflow soldering
• Inspect for defects and rework if needed
• Through hole component insertion
• Wave or hand soldering
• Cleaning
• Programming and functional testing

DFM Checks

Running design for manufacturing (DFM) checks on the Gerber files can identify potential fabrication issues early to prevent delays and re-spins. This is highly recommended for 2+ layer boards.

Quality Control

Important QC tests conducted by PCB prototype suppliers:

• Netlist test – Checks for errors between schematic and layout
• AOI inspection – Automated optical inspection for defects
• Flying probe test – Electrical test of traces and vias
• ICT test – Tests for shorts and opens
• Impedance testing – For controlled impedance traces
• Cross-sectioning – To check layer alignment, via quality

Engineering Prototypes vs Production

Engineering prototypes focus on functionality rather than cosmetics, allowing faster turnaround and cost savings. In contrast, production boards emphasize repeatability, aesthetics, rigorous testing.

Conclusion

Creating PCB prototypes requires carefully executing multiple steps – schematic capture, board layout, design rules checks, generating Gerber files, material selection, assembly, quality testing. Using the right design software and prototype assembly services helps streamline the process. Investing time upfront in design reviews helps avoid costly re-spins.

Frequently Asked Questions

What are some key differences when making prototype vs production PCBs?

Prototypes focus mainly on functionality with less emphasis on aesthetics. They use simpler design rules, lower layer counts, andSkip rigorous testing that add cost. Production PCBs use more layers, automated assembly, and extensive testing procedures.

How many layers are typical for prototype PCBs?

2 to 4 layer designs suffice for most prototype PCBs. Higher layer counts add cost and longer lead times that may not be justified for early engineering prototypes. For complex circuits, advanced PCB software enables cramming more routing into fewer layers.

What are important checks to perform before submitting Gerber files?

Run design rule and electrical rule checks in your PCB software to fix errors upfront. Review layer stackup carefully. Get Gerber files reviewed by your board house, especially for 2+ layer boards. Verify all required files are included in correct format.

What are key considerations for selecting a PCB prototype supplier?

Choose a supplier with experience in quick-turn prototypes in your product’s volume range. Check capabilities in your required layer counts, technologies like impedance control or HDI. Evaluate quality certifications, customer reviews, ITAR registration if required, and supply chain transparency.

How can I reduce the cost of my PCB prototype?

Using less layers, standard FR4 material, 1 oz copper, immersion silver surface finish, relaxed design rules, and larger features when permissible can reduce prototyping costs. Compare pricing from multiple suppliers. Build incrementally, starting with bare boards then adding assembly and components gradually.