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Understanding PCB manufacturing - Part I (Imaging and Etching)

Our earlier article covered basics of drilling process. The next important process, any PCB designer should have good understanding of, is etching. Capabilities and limitations of PCB manufacturer’s etching process play an important role to set track width and track spacing constraints while designing PCB.

To understand the etching process, we also need to understand the imaging process. So, let us show you how imaging and etching is carried out in a typical PCB manufacturing plant.

Imaging Process

Step 1 A dry film is required to apply on a piece of rigid laminate, consisting of a woven glass epoxy base material clad with copper on one or both side.

Imaging Process

Step 2 Dry film is a dark blue colour light sensitive film (in another word, a photo image-able resist) that needs to be coated on to the panel. This film is sensitive to ultraviolet light. This is the main reason why “yellow light” is used in most image processing room. The room must also be clean to ensure that no dust gets onto the surface otherwise it could cause a short or open circuit on the finished PCB.

Imaging Process

Step 3 Track work on outer and inner layers are plotted positive and negative respectively. Plotted films are then aligned on to the panel before exposing to a high intensity ultraviolet light source. Clear areas of the film allow UV light to pass through and harden the film resist thus creating image of the circuit pattern.

Imaging Process

Step 4 Exposed panels are sprayed with a powerful alkali solution (developing process), which removes the unhardened resist. The track work we want is now covered with hardened resist.

Imaging Process

Etching Process

Step 1 Unwanted copper on to the panel is now removed using a powerful alkaline solution to dissolve (or etch away) the exposed copper. This is extremely critical process and operator pays lot of attention to ensure correct amount of copper is etched away otherwise the track work may have under-etch or over-etch problem. It is very important that correct conveyer speed is maintained throughout the etching process. Every manufacturer provides etch tolerance during CAD process so that etching is compensated and track width and spacing fall under a specific range, which is called allowable tolerance.

Etching Process

Step 2 Dark blue photoresist can now be stripped off, which protected the copper so far. PCB track work is ready with exact pattern designed by an engineer.

Etching Process

IPC standard for etching – don’t get caught by surprise!!

In the event of under etch, IPC 6012 allows minimum conductor width to be 80% of the conductor pattern. For SMT pads, allowable pristine area must consist of central 80% of the pad width and length. When deciding pad reduction criteria for SMT stencil, this allowance must be kept in mind.

IPC standard for etching

Track width/spacing constraints for etching Process

If you require PCB having non-standard copper thickness, you must consult QualiEco to verify allowable track width and spacing to suit our etching process. Ignoring this important fact could cost you time and money while you re-designed whole PCB.

Refer following table as an indicative guide for your PCB design. These figures vary from plant to plant so please discuss with us before you start designing PCB.

Cu Thickness Track Width Track Spacing
1oz (35-42 micron) 3.5 to 6 mil
(0.089 to 0.15mm)
3.5 to 6 mil
(0.089 to 0.15mm)
2oz (55-70 micron) 7 to 9 mil
(0.178 to 0.23mm)
7 to 9 mil
(0.178 to 0.23mm)
3oz (90-105 micron) 10 to 13 mil
(0.25 to 0.33mm)
10 to 13 mil
(0.25 to 0.33mm)
4oz (90-105 micron) 14 to 17 mil
(0.356 to 0.43mm)
14 to 17 mil
(0.356 to 0.43mm)

PCB Design Tips

When designing a PCB, try to limit the amount of draws you use. Draws use a lot of memory and slow the programming time. Use a flash for pads instead.

More tips...
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