2.1 Material Science, Stackups & Via Architecture

The bare printed circuit board is a complex composite component that acts as the mechanical and thermal backbone of the product. Its specification must account for the harsh thermal excursions of lead-free reflow and the mechanical stresses of operation. This section moves beyond basic material selection to define the critical thermomechanical properties required for yield and reliability.

Base Material Selection

Standard "FR-4" is an insufficient designation. The transition to Lead-Free (SAC305) soldering requires materials capable of withstanding peak temperatures of 260 C.

• Tg (Glass Transition Temperature): High-Tg materials (Tg > 170 C) are mandated for all multilayer boards (>4 layers) or boards undergoing multiple thermal cycles (e.g., double-sided reflow + wave).
• Td (Decomposition Temperature): Must exceed 340 C to prevent weight loss and delamination during rework.
• CTE (Coefficient of Thermal Expansion): Z-axis expansion must be controlled (< 3.5% from 50 C to 260 C) to prevent barrel cracking in plated through-holes.

Impedance Control and Stackup

Impedance requirements must be explicit. Specifying trace width alone is inadequate due to process variations in etching.

• Target Definition: The Fab Note must specify the Target Impedance (e.g., 50 Ohm Single Ended, 100 Ohm Differential) and the specific layer reference.
• Etch Compensation: The PCB fabricator is granted a +/- 10% tolerance on trace geometry to adjust for their specific process etch factors to meet the target impedance.

Via Protection Architecture

Unprotected vias are a leading cause of assembly defects.

• Tented Vias: Covered with solder mask (Type VI). Sufficient for non-critical areas but prone to breaking.
• Plugged Vias: Partially filled with non-conductive epoxy.
• Filled & Capped (VIPPO): Required for Via-in-Pad designs. The via is plated, filled with epoxy, and plated over with copper to create a flat surface. Placing an open via in a component pad is strictly prohibited as it drains solder paste (wicking), leading to open joints.

Final Checklist