DRILLING - HOLE METALLIZATION
From: Author: Publish time:2021-09-17 15:49 Clicks:0
DRILLING
The drilling of the holes is performed on a wide range of CNC machine types. Simpler boards may be drilled with older machine designs that work quite satisfactorily. More complex and denser boards are typically drilled with newer machine designs which have higher rpm spindles, drill extremely fast and are much more accurate. In addition, they may have a tool belt that can hold thousands of tools, a CCD camera for panel alignment and an auto loader and unloader for improved throughput. The panels are drilled for connection to the inner layers, through hole components and mounting holes.
HOLE METALLIZATION
After drilling the holes need to be metallized in order to make electrical connection to the inner layers. The first step in the process is to desmear the holes. The desmear process is designed to remove any resin smear from the drill operation which may electrically insulate the connection to the inner layer or reduce the strength of the plating adhesion to the inner layer. The desmear operation will also, clean the hole of loose debris and add topography to the hole wall for increased adhesion of the initial metallization layer. The second step is to add a thin layer of copper or carbon to the hole wall that will allow for subsequent plating operations.

OUTER LAYER IMAGING
Prior to electroplating the circuit image must be transferred to the outer layers. For the outer layers a reverse outwork is applied. The unwanted areas will be covered by the resist and the designed pattern will be exposed.

FINISH PLATING
There are various types of plating metallurgies and types of plating options. The most common is pattern plating with a copper plate followed by a tin plate. The copper is required for through hole reliability and current carrying purposes. The tin is required as an etch resist during a subsequent etching operation. Plating of copper in the holes is typically 0.001” followed by a tin plating of 0.0003”. Copper is by far the most common plated metal due to its low cost, easily controlled, and the mechanical/electrical properties are both good.
TIN PLATING
The panels are then plated with tin, protecting the traces, pads and holes from being etched away when the unwanted copper is removed.

RESIST STIP/ETCH/TIN STRIP
After electroplating the photoresist is stripped from the panel which exposes the background copper for etching. The etching process etches away the exposed copper and the tin protects the copper traces from the top. Some sidewall attack take place on the traces but is compensated for by CAM. After etching the tin is stripped away leaving the copper traces and the plated holes.

SOLDER MASK/LEGEND
Solder mask has several purposes ranging from protecting the traces during assembly operations, defining pads during soldering operations, electrical isolation and protection from the environment. Solder mask and legend inks come in a variety of colors and finishes with green being the most common. Solder mask is typically applied by either screening or spraying. The most common solder masks are photo imageable. After the solder mask is applied to the panel, the process is like dryfilm photoresists exposing and developing processes.
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