In PCB manufacturing, hole metallization is a key process to ensure interlayer electrical connections. Its process control involves multiple links. The following is an introduction from aspects such as core processes, key parameter control, common problems and solutions, quality inspection and control, etc.
First, the core process flow and control points of hole metallization
Drilling and removing drilling stains
Drilling: Use CNC drilling or laser drilling technology to ensure the accuracy of the hole diameter and the quality of the hole wall, and avoid burrs and residue of glue.
Removal of drilling stains: Use strong oxidants such as potassium permanganate to remove the epoxy resin drilling stain layer on the hole wall, ensuring a reliable connection between the inner layer of conductors and the hole wall layer.
Pretreatment for electroless copper plating
Degreasing: An alkaline degreasing system is adopted to remove oil stains, fingerprints, oxides and dust inside the holes on the board surface. The polarity of the hole walls is adjusted to facilitate the subsequent adsorption of colloidal palladium.
Roughening (micro-etching) : Remove the oxide on the plate surface through a system of sulfuric acid and hydrogen peroxide, increase the surface roughness, and enhance the adhesion of electroless copper plating.
Pre-impregnation, activation and de-gelation: Adsorb a layer of discontinuous heavy metal particles (such as colloidal palladium) on the pore wall, inducing a self-catalytic reaction of chemical copper precipitation to form a uniform chemical copper layer.
Electroless copper plating
Plating solution formula: Optimize the proportion of copper salts, complexing agents, reducing agents (such as formaldehyde), pH regulators and additives to ensure the quality of the coating.
Process parameters: Strictly control the temperature (24-26 ℃), pH value (12-13), stirring speed and plating time (10-20 minutes) to obtain a uniform and dense electroless copper plating layer.
Thickened electroplated copper
Plating solution formula: Increase the concentration of copper ions, sulfate ions and additives to improve the surface quality and thickness distribution of the coating.
Electroplating parameters: Precisely control the current density (1-3A /dm²), voltage, temperature and time, and adopt pulse electroplating technology to enhance the uniformity of the coating.
Second, control of key process parameters
Temperature control
The temperature of the electroless copper plating solution is stabilized at 24-26 ℃ to ensure the rate of the chemical reaction and the quality of the coating.
The temperature of the electroplating solution should be controlled within the process range to avoid affecting the performance of the coating due to excessively high or low temperatures.
pH value control
The pH value of the plating solution is monitored and automatically adjusted in real time through an online pH meter to keep it within the optimal range (for example, the pH value of the electroless copper plating solution is 12-13).
Stirring and oscillation
Vertical stirring or jet stirring is adopted to ensure that the plating solution can fully and evenly contact the hole wall, thereby enhancing the uniformity and thickness consistency of the coating.
For Wells with a high thick-to-diameter ratio, ultrasonic oscillation can be added in the swelling, oxidation and neutralization tanks to enhance the microscopic ion exchange rate within the Wells.
Maintenance of plating solution
Regularly analyze the composition of the plating solution and make supplementary adjustments to ensure the stable performance of the plating solution.
An online electroplating solution circulation and filtration system is adopted to remove suspended impurities and metal ion impurities in the electroplating solution.
Third, Common problems and solutions
Copper-plated cavities on the hole wall
Cause: Bubbles, foreign substances or poor back-and-forth flow of the aqueous solution of the medicine in the hole.
Measures: Enhance the stirring intensity and oscillation frequency, add ultrasonic oscillation, and strengthen the material exchange between the solutions inside and outside the holes.
The adhesion of the coating is poor
Reasons: Incomplete pretreatment, non-dense crystallization of the electroless copper plating layer or improper control of the micro-etching thickness.
Measures: Optimize the pretreatment process and strictly control the process parameters of electroless copper plating and micro-etching.
The coating thickness is uneven
Reason: Uneven current density distribution or insufficient electroplating time.
Measures: Optimize the design of the electroplating tank and the electrode layout, adopt a multi-anode and multi-cathode layout, and precisely control the electroplating time and current density.
Fourth, quality inspection and control
Backlight test
The copper plating effect and the quality of the copper plating layer were determined by checking through the backlight test process method and referring to the light transmission degree image (divided into 10 levels).
Metallographic sectioning test
After thickening the copper plating, metallographic sectioning tests were conducted in accordance with the process requirements to check the thickness, uniformity and adhesion of the coating.
Online monitoring
High-precision temperature control equipment, online pH meters and other devices are adopted to monitor process parameters in real time to ensure the stability of the process.
Fifth, production environment and equipment management
Production environment control
Establish a clean workshop, control the temperature (20-25 ℃), humidity (40%-60%), and dust level (below million-level) within the workshop, and prevent dust and impurities from affecting the quality of the coating.
Equipment maintenance and upkeep
Establish a complete equipment maintenance and upkeep system, conduct regular inspections, repairs and upkeep of the equipment to ensure its normal operation.
Regularly replace and calibrate the key components of the equipment, such as the electrodes, agitators, and temperature control systems of the electroplating tank.
