CID PCB Materials & Manufacturing Processes 3 — Questions and Answers
Question 1: What is the primary function of a prepreg (pre-impregnated) layer in multilayer PCB construction?
- To provide electrical conductivity between layers
- To act as an adhesive dielectric bonding inner cores together (Correct answer)
- To serve as a solder mask over copper pads
- To act as a drill stop for controlled-depth routing
Correct answer: To act as an adhesive dielectric bonding inner cores together
Prepreg is B-stage (partially cured) resin-impregnated glass fabric that bonds inner core layers under heat and pressure during lamination.
Question 2: Which copper foil type offers superior high-frequency electrical performance due to its smoother surface profile?
- Electrodeposited (ED) standard copper
- Rolled annealed (RA) copper
- Reverse-treated (RTF) copper
- Ultra-low profile (VLP) copper (Correct answer)
Correct answer: Ultra-low profile (VLP) copper
VLP (very low profile) copper has minimal surface roughness, reducing skin-effect losses at high frequencies compared to standard ED or RA foils.
Question 3: In sequential lamination for HDI boards, what process step immediately follows inner-layer imaging before the next lamination cycle?
- Via drilling and plating (Correct answer)
- Solder mask application
- Component placement
- Final board routing
Correct answer: Via drilling and plating
After inner-layer imaging and etching, micro-vias are drilled (laser or mechanical) and plated before adding the next dielectric and copper layer.
Question 4: What is the primary advantage of using a back-drill (controlled-depth drilling) on high-speed PCBs?
- Increases copper plating thickness in the barrel
- Removes the unused stub portion of a PTH to reduce via resonance (Correct answer)
- Improves mechanical pull strength of the via
- Reduces drill cycle time by using larger drill bits
Correct answer: Removes the unused stub portion of a PTH to reduce via resonance
Back-drilling removes the non-functional stub below the last connected layer, eliminating the transmission-line stub resonance that degrades signal integrity.
Question 5: Which laminate property is most critical when selecting a material for lead-free solder assembly processes?
- Low dielectric constant (Dk)
- High decomposition temperature (Td) (Correct answer)
- Low surface resistivity
- High peel strength at room temperature
Correct answer: High decomposition temperature (Td)
Lead-free soldering peaks at 260°C+, so a high Td ensures the resin does not decompose and delaminate during the assembly thermal cycle.
Question 6: What does the term 'z-axis CTE' refer to in PCB laminate specifications?
- The dielectric expansion in the x-y plane during reflow
- The coefficient of thermal expansion perpendicular to the board surface (Correct answer)
- The rate of copper oxidation through the laminate thickness
- The impedance variation along the board thickness
Correct answer: The coefficient of thermal expansion perpendicular to the board surface
Z-axis CTE describes how much the laminate expands through its thickness per degree of temperature rise, directly stressing PTH copper barrels.
Question 7: Which PCB process step uses potassium permanganate or plasma to improve resin smear removal after drilling?
- Desmear / etchback (Correct answer)
- Pattern plating
- Soldermask cure
- Oxide treatment
Correct answer: Desmear / etchback
Desmear (and etchback) removes resin smear deposited on inner-layer copper pads during drilling to ensure reliable PTH electrical connections.
CID PCB Materials & Manufacturing Processes 3
What is the primary function of a prepreg (pre-impregnated) layer in multilayer PCB construction?