PCB Materials & Manufacturing Processes Flashcards

What is the primary function of a prepreg (pre-impregnated) layer in multilayer PCB construction?

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.

Which copper foil type offers superior high-frequency electrical performance due to its smoother surface profile?

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.

In sequential lamination for HDI boards, what process step immediately follows inner-layer imaging before the next lamination cycle?

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.

What is the primary advantage of using a back-drill (controlled-depth drilling) on high-speed PCBs?

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.

Which laminate property is most critical when selecting a material for lead-free solder assembly processes?

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.

What does the term 'z-axis CTE' refer to in PCB laminate specifications?

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.

Which PCB process step uses potassium permanganate or plasma to improve resin smear removal after drilling?

Answer: Desmear / etchback

Desmear (and etchback) removes resin smear deposited on inner-layer copper pads during drilling to ensure reliable PTH electrical connections.