Frequently Asked Questions

FR-4 glass-reinforced epoxy laminate is the most common material family used for conventional rigid PCBs. However, FR-4 covers a wide range of materials with different thermal and electrical properties.

Tg is the glass transition temperature of the laminate resin system. It indicates the region in which the material changes from a relatively rigid state to a softer state and its thermal expansion behaviour changes significantly.

No. Tg is only one characteristic. Td, Z-axis CTE, T260/T288, Dk, Df, moisture performance and other characteristics may be equally important depending on the application.

IPC-4101 is the IPC specification covering base materials used primarily for rigid and multilayer printed boards. Its individual specification or "slash" sheets define requirements for different laminate and prepreg material categories.

For some standard PCB applications, yes, but "FR-4" alone may not sufficiently define the performance required. More demanding products should specify the necessary material characteristics and/or applicable IPC-4101 classification.

Potentially, but it should never be assumed that two materials are equivalent simply because both are described as FR-4. Thermal, mechanical, electrical, processing, UL and qualification requirements should be reviewed before approving an alternative.

Tg describes the transition in the physical behaviour of the resin system, whereas Td relates to thermal decomposition of the material. A material's Tg therefore does not, by itself, tell you how well it will withstand high-temperature PCB processing.

There is no single answer based solely on Tg. The PCB construction, thickness, layer count, via structure and assembly profile should be considered alongside properties including Td, Z-axis CTE and time-to-delamination performance.

Low-loss materials should be considered where signal frequency, data rate, insertion loss or signal-integrity requirements exceed what can reliably be achieved with conventional FR-4. Dk and particularly Df become increasingly important as frequency rises.

Via plugging is a manufacturing process in which material is applied to partially or completely close a PCB via. Different processes include tenting, plugging, filling and filling followed by copper capping.

A plugged via uses material to close the via with partial penetration, whereas a filled via targets filling and encapsulation of the hole. The exact finished construction should be defined rather than using the terms interchangeably.

A tented via has solder mask or another specified mask material bridging over the via opening. It is a relatively simple and economical form of via protection but should not be treated as a guaranteed method of completely filling a hole.

Via-in-pad is a PCB design technique where a via is located directly within a component's solderable pad rather than alongside it. It is commonly used for fine-pitch BGAs and high-density PCB routing.

VIPPO stands for Via In Pad Plated Over. The via is filled and planarised before copper is plated over it, creating a flat solderable pad.

For conventional via-in-pad used as a solderable component pad, the via normally needs to be filled and plated over. Leaving the via open can allow solder to wick into the hole during assembly.

The filling material isn't, but the via still conducts electricity. Electrical connection is provided by the copper plating around the wall of the via.

Not necessarily. Non-conductive epoxy is widely used for via-in-pad applications. Conductive filling should normally be selected only where the application has a specific electrical or thermal requirement.

IPC-4761 provides design guidance and classifications for protection of printed board via structures, including tented, plugged, filled and filled-and-capped structures.

Generally, yes. Additional processes such as dedicated plugging, epoxy filling, planarisation and copper capping add manufacturing operations compared with standard open or solder-mask-tented vias. Cost increases with the complexity of the required treatment.

For mechanically drilled vias requiring a filled and metallised surface suitable for via-in-pad, a filled-and-capped Type VII structure is commonly appropriate. The complete PCB construction and assembly requirements should still be reviewed before finalising the specification.