Manufacturing Process of Double-Sided PCBs

May 2, 2025

Compared to single-sided PCBs, the manufacturing process of double-sided PCBs involves increased complexity, mainly in the following aspects:

1.Double-sided alignment between the front and back patterns.
2.Electrical interconnection between the front and back patterns.
3.Uniformity of surface copper thickness & etching uniformity.

Regarding the above three upgraded technical challenges, the following solutions are for reference:

1.The precision requirements for front-to-back alignment vary for different grades and types of products. Among them, products with front and back pattern coupling (such as suspended line filters, directional couplers, slot antennas, etc.) have the highest requirements, followed by Class 3 products with fine line width/spacing (no breakout allowed, minimum ring width 50μm). Butterfly clip manual alignment, CCD automatic double-sided alignment, and Laser Direct Imaging (LDI) are three commonly used pattern transfer methods. LDI has the highest double-sided alignment accuracy and can automatically calculate board expansion and contraction for maskless imaging.

2.Electrical interconnection between the front and back patterns of double-sided PCBs is generally achieved through plated-through holes (PTH). A few products also use conductive paste (copper paste or silver paste) filling, but their engineering applications are limited due to their relatively low reliability and large interconnection resistance. The indicators for evaluating the process capability of achieving metallized through holes include the Aspect Ratio and Throwing Power (TP).

*Aspect Ratio: Also known as the thickness-to-diameter ratio, it refers to the ratio of the hole's depth to its diameter. However, this value does not fully represent the process difficulty. The process difficulty coefficient is generally expressed as the ratio of the square of the hole depth to the diameter.

*Throwing Power: Under certain board thickness and hole diameter conditions, it is the ratio of the plating layer thickness inside the hole to the plating layer thickness at the hole opening after electroplating, reflecting the uniformity of the copper plating layer thickness inside the hole.

3.Surface plating uniformity (COV, coefficient of variation) is one of the indicators for evaluating electroplating capability and directly affects the uniformity of board surface etching. The PCB industry's electroplating production lines are divided into gantry electroplating production lines and vertical continuous plating production lines (VCP). Among them, the COV of gantry electroplating production lines is 8% to 10%, and the COV of VCP is ≤5%. Therefore, combining VCP with vacuum etching equipment with two-fluid function can achieve the best board surface etching uniformity and realize high-precision line width/spacing.

Plating Uniformity: The standard deviation of the data divided by the average value, expressed as a percentage.

The typical production process flow of PCB double-sided circuit boards is as follows: