Backup Material for pcb drilling
1 Purpose. The purpose of backup material is defined by the following criteria.
An ideal backup material will:
● Provide a safe medium for drill stroke termination
● Prevent copper burrs
● Not contaminate the hole or drill bit
● Minimize drilling temperatures
● Improve hole quality
2 Types. Numerous materials are available that are sold as backup material. Few of the materials used as backup materials are actually specifically engineered for circuit board drilling. Many of the popular backup products are composites with a variety of surface coatings or skins bonded to several different core materials. Available backup products include the following:
● Epoxy-paper-clad, wood-core composite utilizing a bonding agent with lubricating properties
● Aluminum-clad, wood-core composite
● Epoxy-paper-clad, wood-core composite
● Urethane-clad, wood-core composite
● Solid phenolic
● Aluminum-clad phenolic composite
● Plain wood
3 Performance. Desired qualities in a backup material are minimal thickness variations,flatness (no bow, warp, or twist), no abrasives or contaminants, a smooth surface, low cutting energy (minimizing drilling temperatures), and a surface hardness that supports the laminate copper surface (to prevent burrs) yet does not cause damage or extensive wear to
the drill bit.
Backup materials with lubricating properties have been proven to significantly reduce drilling temperatures by as much as 50 percent or more, often resulting in temperatures below the Tg of the laminate product being drilled.This advantage greatly reduces hole wall defects such as roughness, smear, and nail-heading and often allows increased drill stack heights and/or greatly increased drill bit maximum hit counts.The importance of these benefits is significant
reduction in drilling cost per hole and improved productivity and yield.Remember that drilled backup debris exits the stacks by passing through the holes in the laminate material and that therefore contamination (from the backup material) is of great concern. Materials containing phenolic, or composed of solid phenolic, are not suitable for circuit
Phenolic materials or phenolic composites (i.e., aluminum-clad phenolic) often warp and under most drilling conditions contaminate the hole wall, which results in problems with adherence of the plating because desmearing chemicals are not designed to remove phenolic resin. Hardboard types of materials cannot be maintained to thickness variation tolerances acceptable for circuit board drilling and are a source for a great variety of contaminants (e.g., oils crystallized on the surface for hardening purposes).