What is the BOTTOM side of the PCB?

A printed circuit board (PCB) is a essential component of most electronic devices and equipment. It provides the mechanical structure and electrical connections between components using conductive tracks, pads and other features which are etched from copper sheets laminated onto a non-conductive substrate.

PCBs have two sides - a TOP side and a BOTTOM side. Components are mounted and soldered onto one or both sides of the PCB during manufacturing. The TOP side is the side where most or all components are mounted, while the BOTTOM is the side with few or no components.

Typical features on the TOP vs BOTTOM of a PCB

What is found on the BOTTOM of a PCB?

While the TOP houses most of the action, the seemingly empty BOTTOM side also serves some key functions.

Copper layers and traces

The BOTTOM will contain copper layers and tracings that connect to traces and components on the top. Multi-layer PCBs in particular can have complex inner-layer circuitry sandwiched between TOP and BOTTOM outer layers.

Bottom side showing traces connecting to inner layers

Even double-sided PCBs with just TOP and BOTTOM layers require tracings on both sides for full connectivity.

Text and markings

The BOTTOM often contains:

• Text and logos: Branding text, logo, copyright notices, product info, model numbers etc. These help identify the device or manufacturer.
• Board thickness markings: Multi-layer boards have thickness markings (e.g. "1.6mm") on the BOTTOM. This aids quality checking.
• PCB color: May have a colored soldermask (e.g. green, blue) or be left as bare FR-4 substrate.
• Tooling holes: Holes used for positioning during PCB manufacturing.
• Fiducials: Copper marks used by automated assembly machines to align components.

Text and markings on PCB bottom

Less soldermask

The BOTTOM can contain large areas with no solder mask coating. Solder mask provides an insulating protective layer over traces, preventing short circuits. Less mask is often needed on the BOTTOM since there are fewer components.

Vias and mounting holes

BOTTOM also frequently contains:

• Vias: Plated through-holes connecting traces on different layers.
• Mounting holes: Allow screws or standoffs to secure PCB.

Vias visible on BOTTOM side

Heat transfer features

Certain features aid heat dissipation from TOP components:

• Thermal reliefs: Cutouts around pads preventing OVER soldermask trapping heat.
• Thermal vias: Plated vias conducting heat to other layers. These help dissipate heat from hot components.

Why does BOTTOM look different than TOP?

There are some sound reasons for the different appearances of TOP vs BOTTOM:

• More space on BOTTOM since fewer components mounted. This allows room for text, logos, markings without obstructing circuitry.
• Text is kept OFF TOP to avoid interfering with silkscreen legends vital for assembly.
• Bare FR-4 offers lowest cost finish if product branding allows.
• Masking bare minimum BOTTOM area cuts cost. As long as insulation functionality maintained.

So while TOP contains complex precision circuitry, the BOTTOM fulfills simpler branding and connectivity roles.

How is BOTTOM side of PCB designed and manufactured?

PCB BOTTOM shares the same core design and fabrication process as the TOP side:

PCB design stage

PCB layout software allows BOTTOM features to be designed along with TOP:

• All conductor (copper) layers are designed as a unit including inner layers
• BOTTOM mask openings, silkscreen, markings etc positioned appropriately
• Thermal relief cutouts and heat dissipation features included

PCB layout design showing TOP and BOTTOM layers

This stage ensures the PCB BOTTOM interconnects properly with the overall board circuit topology.

PCB fabrication stage

The same fabrication process applies to both sides, with key steps being:

1. Copper layers bonded to substrate
2. Photoresist coating
3. Masked exposure to UV light
4. Chemical etching to remove unwanted copper
5. Stripping of resist to leave desired conductor pattern
6. Soldermask coating
7. Silkscreen printing
8. Surface finish plating
9. Electrical testing
10. Depaneling

This sequence builds up both TOP conductive network AND BOTTOM features. Automated optical inspection after fabrication checks for defects on both sides.

What tests are done on BOTTOM of PCB?

While most circuit testing focuses on the TOP conductive network, some quality checks do target the BOTTOM specifically:

Reliability is improved by eliminating potential failure points on the less critical but still important BOTTOM side.

What defects could occur on BOTTOM of PCB?

With less circuit complexity than the TOP side, defects on the BOTTOM are rarer but can still slip through:

Exposed traces visible as a BOTTOM defect

While less mission critical than the TOP circuitry, it is still prudent to minimize BOTTOM defects and avoid reliability impacts over product lifetime.

Applications where BOTTOM is as important as TOP

For most PCBs, the TOP bears the brunt of functionality burden but there are exceptions where even BOTTOM requires complex, high-precision design:

High density interconnect (HDI)

HDI boards have conductors spaced less than 100 microns apart necessitating very fine multi-layers. This means no compromise on BOTTOM completion.

LED lighting PCBs

Lighting PCBs mount LEDs on both sides to distribute intensity and heat. This necessitates BOTTOM perfection matching the TOP side.

Double-access or bottom-terminated components

Components like speakers, buzzers, some connectors etc need traces accessing pads on both sides simultaneously. BOTTOM accuracy becomes vital here.

Cooling enhancement

Some boards sink heat to external surfaces. This requires thermal vias and reliefs on outer BOTTOM for maximum effectiveness. No imperfections can be tolerated here.

High voltage boards

Voltage handling depends on the combination of conductor spacing and insulation quality on both TOP and BOTTOM. Defects like exposed traces or soldermask voids would compromise safety.

In such situations the BOTTOM must match TOP standards to deliver product functionality and safety.

Summary

In summary, while the TOP side handles active electronics, the PCB BOTTOM plays a subtler yet still vital supporting role:

• Interconnecting with TOP circuitry through traces and vias
• Providing branding, product information markers
• Aiding automated assembly using fiducials
• Allowing safe heat dissipation via thermal features
• Delivering insulation using soldermask where needed

A defect free BOTTOM side is vital for electrical, thermal and mechanical integrity over the PCB lifetime. So while out of spotlight, BOTTOM merits just as much design and fabrication diligence as TOP!

FAQ

What are the key differences between TOP and BOTTOM of a PCB?

The TOP side has most or all the electronic components and complex circuitry while the BOTTOM has minimal components. TOP contains the silkscreen legends vital for assembly. BOTTOM often has product text, brand markings and board thickness information.

Can components be mounted on the BOTTOM side?

It is possible but unusual to have components on PCB BOTTOM. Some exceptions are lighting or double access boards needing wiring access from both sides. As a rule components are mainly on TOP while branding text goes on BOTTOM.

What PCB fabrication processes apply to the bottom?

PCB BOTTOM layers go through the exact same fabrication sequence as the TOP, including photolithography, copper etching, resist stripping and plating. High reliability standards apply equally to top and bottom construction.

What test and inspections are done on BOTTOM?

Bare board electrical testing and automated optical checks ensure BOTTOM integrity. Special tests like soldermask clearance checks, insulation resistance and breakdown voltage are critical for BOTTOM safety even though it has fewer active circuits than TOP.

When does BOTTOM require precision matching TOP complexity?

For most PCBs the TOP carries more functionality than BOTTOM so only needs simple interconnects and markings. But in certain cases like HDI, high power or double sided boards, the BOTTOM requires equivalently complex circuitry and precision as TOP. Defects can't be tolerated on either side here.