Double Sided PCB Manufacturing

Double sided PCB board is a very important PCB board in circuit boards, it is suitable for a wide range of high-tech industries such as: telecommunications, power supply, computer, industrial control, digital products, scientific and educational instruments, medical equipment, automobiles, aerospace defense, etc.

Double sided PCB Production Process

The production of double sided PCBs is more complicated than that of single-sided PCBs. The main reasons are as follows:

(1) The uppermost and lowermost layers of the copper clad laminate must be wired.

(2) The wires on the top and bottom layers should be connected to the metallized vias.

Among them, through-hole metallization is particularly critical, which is also the core process of double sided PCB production. The so-called through-hole metallization is to coat a layer of metal on the inner wall of the through-hole to connect the top and bottom PCBs. Currently, electroless copper plating is mainly used for via metallization. There are two methods of electroless copper plating:

1. Electroless plating of thin copper first, then electroplating of the entire double sided PCB to thicken the copper layer, and then pattern transfer.

2. Electroless plating is performed on the crude copper first, and then the graphics are transferred directly.

Double Sided PCB Wiring Tips:

Use the autorouter to design double sided panels. In most cases, there will be no problem with the operation of automatic wiring for purely digital circuits (especially low-frequency signal and low-density circuits). However, when trying to use the automatic routing tools provided by the routing software to route analog, mixed-signal or high-speed circuits, some problems may occur, and may cause serious circuit performance problems.

There are many things to consider when it comes to double sided PCB wiring, but one of the more troubling issues is the grounding method. Double sided PCB boards use manual wiring, and the following design guidelines must be followed to ensure good results:

1. Design the ground as a ground plane as the current return path.

2. Separate the analog ground plane from the digital ground plane.

3. If it is unavoidable to put the signal wiring and grounding on the same layer, design the signal line and grounding line to be perpendicular to each other to reduce the interference of the signal line on the grounding current loop.

4. Put the analog circuit next to the double panel, and put the digital circuit system closest to the power supply. But it should be noted that these two double sided PCB boards have a ground plane on the lower layer of the circuit board. This is designed to allow engineers to quickly see the wiring when troubleshooting, and is often found on device manufacturers' demonstration and evaluation boards. But a more typical approach is to lay a ground plane on the upper layer of the double sided PCB board to reduce electromagnetic interference (EMI).

Double Sided PCB Material Types

Currently commonly used double sided PCB boards are FR4 boards and CEM3 boards. Both boards are flame retardant. The boards that are used more in actual projects are as follows:

1. Fiberglass epoxy board: FR4

2. Composite material epoxy board zd: CEM3

3. Paper fiber epoxy board: CEM1

In addition, ceramic substrate PCBs and aluminum substrate PCBs are often used in environments that require enhanced heat dissipation.

Surface Treatment of Double Sided PCBs

1. Bare copper

The cost of double-sided bare copper is low, the surface is smooth, and the solder ability is good (unless it is oxidized). It is easily affected by acid and humidity, and cannot be stored for a long time. It is easy to oxidize in the air; it cannot be used on double sided PCBs, because the second side has been oxidized after the first reflow soldering. If there is a test point, it must be printed with solder paste to prevent oxidation, otherwise it will not be able to make good contact with the probe.

2. HASL

The price of double sided PCB’ HASL is lower, and the soldering performance is better.

HASL is not suitable for soldering pins with fine gaps and too small components, because the surface flatness of HASL is poor. Solder beads are easy to be generated during double sided PCB processing, and short circuits are more likely to be caused by fine pitch components. When used in a double sided PCB SMT process, because the second side has undergone a high-temperature reflow soldering, it is very easy to remelt the tin spray and produce tin beads or similar water drops that are affected by gravity and become spherical tin spots that drip, causing the surface to be even worse. Flattening in turn affects welding problems.

3. OSP

OSP has all the advantages of soldering bare copper boards, and double sided PCB boards that have expired (three months) can also be resurfaced, but usually only once.

OSP is easily affected by acid and humidity. When used in the second reflow soldering, it needs to be completed within a certain period of time, and the effect of the second reflow soldering is usually relatively poor. If the storage time exceeds three months, it must be resurfaced. Use within 24 hours after opening the package. OSP is an insulating layer, so the test point of the double sided PCB board must be printed with solder paste to remove the original OSP layer in order to contact the pin point for electrical testing.

4. ENIG

The immersion gold process on double sided PCBs is not easy to oxidize, can be stored for a long time, and has a smooth surface, which is suitable for soldering fine-gap pins and components with small solder joints. It can be reflowed many times without degrading its solder ability. It can be used as the base material for COB (Chip On Board) bonding.

The cost of immersion gold is high, and the welding strength is poor. Because the electroless nickel plating process is used, the problem of black plate is easy to occur. The nickel layer oxidizes over time, and long-term reliability is an issue.

5. Immersion silver

Immersion silver is cheaper than immersion gold. If the double sided PCB has connection functional requirements and needs to reduce costs, immersion silver is a good choice; coupled with the good flatness and contact of immersion silver, then the double sided PCB should choose immersion silver process. Immersion silver is widely used in communication products, automobiles, and computer peripherals, and it is also used in high-speed signal design.

Because immersion silver has good electrical properties unmatched by other surface treatments, it can also be used in high-frequency signals. The immersion silver process is recommended for its ease of assembly and better checkability. However, due to defects such as tarnish and solder joint voids in immersion silver, its growth is slow (but not declining). It is estimated that about 10%-15% of double sided PCBs use immersion silver technology.

6. Immersion tin

Immersion tin has been introduced into the surface treatment process for nearly ten years, and the emergence of this process is the result of the requirements of production automation. Immersion tin does not bring any new elements into the welding place, which is especially suitable for communication backplanes.

Outside the storage period of the board, tin will lose its solderability, so immersion tin requires better storage conditions. In addition, the immersion tin process is restricted due to the presence of carcinogens. It is estimated that about 5%-10% of double sided PCBs currently use the immersion tin process.

How to Make a Double Sided PCB

As with single sided boards, there are a number of different ways to make double sided PCB boards:

1. Whole board electroplating, mask-etching

According to the process and material selected for the double-sided board, the whole board plating and etching process can be done reliably and economically. Ideally, start with very thin copper foil, which is easier to etch in the double sided PCB manufacturing process and the circuit is more delicate. This method avoids having to remove the metal resist layer before applying the solder mask.

2. Whole board electroplating and then graphic electroplating

The entire board electroplating and pattern electroplating process is similar to the above method except for the hole masking. Firstly, the resist metal is electroplated according to the circuit pattern, and then the circuit is etched. This approach avoids etching away all of the copper plated in the hole in the event of a hole in the resist or cracking of the resist over the hole. As above, it is easier to etch with very thin copper foils, and more precise traces can be obtained.

3. Graphic plating

Graphical electroplating is a very common method, and it is the most commonly used process in the double sided PCB manufacturing process. When using thin copper clad boards, the semi-additive process can be used. In this way, the processor can avoid the plating and removal of the metal resist layer.

4. Through hole printing

A special method for face-to-face interconnection is through-hole printing of conductive ink after the double-sided etching process is completed. The ink is usually screen printed or stenciled to the printed board placed on the vacuum table , which allows ink to be drawn down through the vias. This process is generally repeated from the other side to ensure adequate coating of the hole.

There are many ways to make a double sided PCB board, and there are also many different possible structures. There are many sequential and independent processes required in the double sided PCB manufacturing process, including: drilling, desmear, resist coating/removal, electroless copper plating, electroplating, and etching. Every process requires rigorous operation so as not to affect the quality of double sided PCBs.

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