Electronics 3

Question 1

What are the properties of a PCB substrate?

Answer 1

Good dielectric properties
Must withstand soldering
FR4 - fire retardancy
Typical thickness 1.6mm
Conductor material almost always copper foil/film.

Question 2

What are the properties of PCB drills?

Answer 2

Usually solid tungsten carbide with standard length and shank diameter. 500µm to 3.15mm diameter

Question 3

What is required for PCB drilling?

Answer 3

Small holes mean high spindle speeds. Often multi-spindle CNC machines used Entrance and exit material (sacrificial) is used to minimise burring and improve accuracy. Layers stacked to improve throuput. Although the hole aspect ratio limits the stack height.

Question 4

When is punching used in PCB manufacture?

Answer 4

high volume, low quality, low complexity product (high tooling cost)

Question 5

When is moulding used in PCB manufacture

Answer 5

high volume, low complexity, but can be 3D product (high tooling cost)

Question 6

What other processes are used in PCB Manufacture

Answer 6

Photolithography and laser drilling

Question 7

What is photolithography?

Answer 7

A process to create tracks on a circuit board using light.

Question 8

How does the exposure vary with sensitivity and intensity?

Answer 8

exposure = sensitivity(J/m2)/intensity(W/m2)

Question 9

How is resolution increased in photolithography?

Answer 9

Collimated UV (parallel rays) - angled light can lead to partial exposure.
Mask put very close to photoresist to prevent diffraction

Question 10

What is proximity? lithography and why is it used?

Answer 10

Mask is a distance away from photoresist. Requires not as much cleaning of photoresist but accuracy is reduced due to light diffraction

Question 11

What is projection lithography and why is it used?

Answer 11

Uses a lens to focus light mask. The mask can be a different size to the output image meaning very small patterns can be created. Used when high volume and high tolerances are necessary.

Question 12

What are alignment marks and how do they affect the pattern?

Answer 12

Marks to indicate where to align mask. Often a box fits over a slightly smaller feature to align them. The difference in size indicates the tolerance to misalignment

Question 13

What is a rule for good alignment mark design?

Answer 13

Both features change with the same tolerance to misalignment.

Question 14

What is isotropic etching?

Answer 14

Process used to remove metal under resist layer

Question 15

Positive vs negative resist

Answer 15

Positive - resist becomes soluble when exposed to light. Negative - resist becomes insoluble when exposed to light

Question 16

What is anisotropic etching?

Answer 16

Wet etching of a single crystal material
Dry etching (gas/plasma)

Question 17

electroplating vs electrolust?

Answer 17

Electroplating- product must be a conductor, but a thick layer can be built quickly.
Electrolust- product can be an insulator but layer formation takes a long time

Question 18

What can be used instead of lithography in single-sided PCB manufacture?

Answer 18

Silk screening. Faster and cheaper process but only used for low resolution/cheap parts

Question 19

What are 2 other types of PCBs and what are their properties?

Answer 19

Multilayer - diminishing returns as requires more via holes with more layers. Sequential build can be used for maximum area utilisation
Flexible - provides wiring between parts of a system. Allows for movement e.g. disc drives.