PCB Fabrication

What are Peel Off, Countersunk & Impedance Control?

    Peel-off masks, also known as peelable masks, strippable masks, or blue masks, are mainly solder masks placed on a Printed Circuit Board (PCB) to act as protection during assembly, with the assembler removing them after processing.

    Earlier, assemblers used heat-resistant masking tapes for the job, but peel-off masks are now offering significant technical and economic advantages. Compared to adhesive tapes, peel-off masks consume less application time, are more cost-effective, and leave behind no adhesive residue, which may be difficult to clean.

    Why are Peel-Off Masks Required?
    When assembling PCBs, especially during wave/reflow soldering, it is often necessary to protect some areas to avoid wetting then with solder. There can be many such areas, such as Gold-plated rotary contacts, Gold-plated contacts, multi-point connectors, touch key contacts covered with conductive carbon, or other large areas that require multiple or selective soldering.

    Requirements of Peel-Off Mask
    Although peel-off masks are economical, they must meet specific requirements such as:

    • Resistance to Lead/Lead-free Solder
    • Resistance to Immersion Processes
    • Resistance to Vertical Hot-Air Levelling
    • High Temperature Stability
    • Good Peelability before and after Thermal Stress
    • High Tear Resistance
    • Capable of Achieving High-Definition
    • Capable of Tenting Plated Through Holes
    • Should not Change the Resistance of Carbon-Conductive inks
    • Should not Cause Discoloration of the PCB Base Material
    • Should not Cause Corrosion of Metallic Copper on PCB

    However, it is not possible for a single peelable solder mask to meet all the above requirements. For specific fields of applications, manufacturers offer various ink types.

    Application of Peel-Off Mask
    Assemblers cover the area with either heat-resistant tape or use peelable solder masks applied screen printing. The latter option is a very simple yet cost-competitive method compared to tapes. Lately, with the growing demands of Lead-free soldering, the requirement of peelable solder masks able to withstand higher processing temperatures is also on the rise. After soldering, the assembler peels off the masks manually.

    To meet most of the above requirements, assemblers apply peel-off solder masks in a thick layer. This is not a problem as most peelable inks are viscous, and they flow in a paste-like manner. Usually, a minimum coating of thickness between 250 μm (10 mils) and 300 μm (12 mils) should be adequate for coating areas of PCB without plated-through holes. When plated through holes are present, they require tenting, and hence, the coating thickness requirement rises to between 300 and 400 μm (12 and 16 mils). As a rule of thumb, assemblers consider higher coating thickness as more reliable.

    Whereas for covering small areas, thick film stencils made of 17 T or 18 T polyester fabrics perform the best, covering larger areas require stencils made of 12 T polyester fabrics. In all cases the screen tension of at least 18 N works best. For best results, it is necessary to use a rubber squeegee blade with a shore-A hardness of 50-65. To enable pulling the thick ink film, the edges of the rubber squeegee blade should preferably be slightly round and the operator should hold the squeegee at an angle of approximately 75 degrees. Semi- or fully automatic screen-printing machines can also process peel-off solder masks.

    A session of thermal curing is necessary to complete the degree of cross-linking within the printed ink to achieve high tear resistance. For best results, assemblers apply a brief but high curing temperature, and this offers the best peelability when removing the peel-off solder mask from plated through holes.

    Countersunk Holes
    When attaching external artefacts such as heat sinks to PCBs, sometimes it is necessary for the PCB to have countersunk holes to allow sinking the screw head into the PCB laminate. As the screw head does not project out, it makes the board look clean and flat. In general, there are two types of shapes for countersunk holes—V-shaped and T-shaped. Fabricators countersink the required holes after they complete the standard drilling process.

    Whether V-shaped or T-shaped, designing a countersunk hole in the PCB requires the following information:

    • Side of the PCB for the countersunk hole
    • Plating or non-plating the countersunk hole
    • Largest diameter of the countersunk hole
    • Through hole diameter of the countersunk hole
    • Depth of countersink
    • Angle of countersink or taper (only for V-shaped hole)

    The designer of the PCB must know the depth of the countersunk hole before routing the tracks. This is because the countersink may be deep enough to cut into one or more of the inner layers and affect the copper in those layers. Again, the hole may be PTH type or Non-PTH type.

    For Non-PTH type of countersunk hole, the designer must take care that copper at the inner layers where the hole passes should not leak into the hole shaft. Therefore, the designer must place antipads at these layers. For instance, if the through hole diameter of the countersunk hole has a diameter of 4.5 mm (177 mils), the anti-pad should have a diameter of 4.9 mm (193 mils), allowing for an anti-pad of 0.2 mm (8 mils) width. PTH type countersunk holes also need this anti-pad if copper from any layer is to remain insulated from the PTH barrel.

    The designer must also take care of the solder mask opening for the countersunk hole. For instance, for a non-PTH hole, the opening on the top surface should correspond to the largest diameter of the countersunk hole, while that at the bottom surface should correspond to the through hole diameter. For a PTH type countersunk hole, the solder mask opening should open according to the corresponding pads on the two outer layers.

    Impedance Control
    When a circuit operates at a high frequency (50 MHz and above), the combined effect of capacitance, inductance, and resistance within the circuit plays a significant effect on the signals passing through the circuit. Engineers call the combined effect as impedance, and it is an AC characteristic, related to the operating frequency.

    Controlled Impedance
    At high frequencies, the traces, vias, and stack up, all contribute to the impedance, and unless the designer and the fabricator control the impedance carefully, it contributes to the deterioration of signal integrity and clarity. If left uncontrolled, the impedance will vary in value at every point along the trace, and the effect on the signal is unpredictable. This is because at high frequencies, rather than behaving as simple connections, PCB traces, vias, and planes start contributing to degrading the signal as it passes through them.

    Essentially, controlling the impedance implies matching the properties of the substrate material with the dimensions of the trace and its location, thereby ensuring the impedance the signal faces remains within a certain percentage of a specific value. With a controlled impedance, a PCB offers repeatable performance at high frequencies.

    Using Controlled Impedance
    PCB designers decide to control the impedance of the signal path when they want a predictable outcome of the high frequency signal passing through the PCB. By matching the impedance of the PCB traces, the designer can maintain data integrity and signal clarity. This requires matching the impedance of the PCB traces to the characteristic impedance of the circuit. The following factors determine the impedance value of a specific PCB structure:

    • Thickness and width of copper traces carrying the high frequency signal
    • Thickness of the prepreg or core material on either side of the copper trace
    • Dielectric constant of the prepreg and core material
    • Distance from other copper features

    PCB manufacturers use impedance modeling software and impedance testing hardware to meet the controlled impedance specifications.

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