Coverlay in SMT Circuit Board Assembly

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SMT Circuit Board Assembly

SMT, or surface mount technology, is a method of assembling PCBs that eliminates the need for components to be attached via solder connections. It offers a more compact board design, lower production costs, and improved EMC performance. However, the small size and lead spacing of SMDs can cause solder connections to fail in thermal cycles, requiring expensive tools and specialized skill levels for manual repair and rework operations. This can cause component failure and circuit malfunctions. A better way to prevent these problems is to use a coverlay that protects the exposed copper and provides easy access for inspection, rework, and replacement of failed components.

Coverlay is a thin film material that covers and protects the bare copper of a flexible printed circuit (FPC). It plays the same role as solder mask does on rigid boards, but because conventional solder masks have limited bendability, coverlay is bonded onto the FPC to encapsulate its external circuit layers. It consists of a polyimide film coated with a thermoset adhesive.

To make a flexible PCB, the coverlay is cut or punched to create openings for pads and other features. These may be drilled, laser cut, knife cut, or punch and die pressed using metal stamps. Then, the coverlay is applied to the FPC with an adhesive layer, which can be either thermosetting or pressure-sensitive.

Coverlay in SMT Circuit Board Assembly

Once the coverlay is in place, a thin layer of solder mask is applied on top of it. This coating protects the copper from contaminants during assembly and end-use. It also prevents signal interference from the edges of adjacent traces. It is important to understand that solder mask will flow slightly beyond the edge of an opening, a process known as “adhesive squeeze out.” This amount depends on several factors, including the thickness of the copper and coverlay, and it is usually minimized by selecting a thinner, more flexible coverlay.

To help ensure good alignment between the conductors and connector pins, it is essential to check the smt circuit board layout for potential coupling problems. This can be done with a software program that can highlight each net and show how it could couple to or from the connector pins. This can make it obvious where a design change is needed to avoid compliance and emissions issues.

In flexible PCB designs with higher density SMT and/or PTH features, it is common to combine multiple feature openings into larger “ganged” openings. This allows for more precise positioning of the reflow oven and also helps reduce the risk of void formation in areas where there are numerous solder joints. This solution also minimizes the cost of fabrication as it requires less material. Another option is to use both coverlay and LPI solder mask, combining them in areas with higher density features. This allows the fabricator to apply the solder mask in more precise areas where it is necessary and use coverlay in the more flexible sections. The combination approach also allows the use of less costly and more durable thermoset adhesives.

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