It’s more appropriate to term my interest in electronics a general interest in electricity – from the naturally occurring to the human-made, any field tangentially associated with the phenomenon garners my attention. A family friend and I once fell deep into a discussion on the topic before he revealed his past working on electrical towers fresh out of college. While he’s much happier with his feet on the ground (literally), our talk covered the importance of safety and three points of contact while working at a height of several stories anchored only by a safety harness.
A reliable three-point contact can also make a difference in PCB fabrication. The plating of through-holes during fabrication needs an adequate amount of copper surface to cling to for reliability, and mechanical drilling can significantly reduce this area. PCB etchback following desmearing can add additional copper surface area through targeted removal of one of the types of board layers.
Negative vs. Positive Etchback at a Glance
The Lead-In to PCB Etchback
PCB etchback is an integral part of multilayer board fabrication, wherein the manufacturer uses a chemical agent or plasma to recede conductive or dielectric layers post-drilling for better plated through-hole (PTH) barrel formation. Primarily, etchback improves through-hole plating outcomes by providing clean and continuous access to the board’s conductive material. However, the etchback process also can roughen the through-hole and offer greater adherence for the plated copper, strengthening the PTH.
It’s worthwhile to quickly review the PCB fabrication process up to etchback to understand its purpose better. After layer etching, circuit board fabrication begins by fusing the alternating layers of conductive material (often copper) and dielectrics in lamination. A lay-up step has a technician place the materials vertically to match that of the stackup (the layer-by-layer materials of the board) before a high-heat press causes the resinous component of the dielectric to reflow and encase the board. After cooling fixes the board structure, the laminated product resembles the familiar board shape and form sans a crucial design element: drilled through-holes for vertical layer-to-layer connections.
Unlike in-plane routing (which only requires an etching of the layer), plated through-holes are as much mechanical elements as they are electrical. Different drilling options are available depending on the circumference and depth of the hole; standard drilling options use drill bits to bore out the board material at the specified location before a plating bath solution forms the conductive barrel. PTHs (specifically vias) are arguably the most common source of long-term reliability issues from the board fabrication, as differences between the CTE of the barrel and surrounding materials can develop significant internal stresses due to thermal cycling during the board’s service life. Therefore, manufacturers invest considerable attention and resources to ensuring the robustness of their plated through-hole barrels.
Weighing Different Etchback Factors and Techniques
Mechanical drilling creates high temperatures, which can cause the resinous material of the dielectric to remelt and reset in the drilled hole, wholly or partially covering the conductive material. Desmearing removes the resin from the hole, which is closely associated but technically distinct from etchback (so much so that they are often used interchangeably as a single continuous procedure). Finally, etchback strengthens the PTH during plating by providing better adhesion to the copper layers. Etchback operates under two different methodologies:
- Negative etchback is the cheaper option that etches the copper layers so they are recessed relative to the dielectric layers. Quality control makes this process highly repeatable, but there can be some issues with long-term board reliability. Boards can be more liable to experience delamination, causing a continuity failure at the PTH barrel junction. Additionally, gasses can become trapped, which can cause outgassing later in the manufacturing process or during operation if conditions are suitably hot.
- Positive etchback is the more expensive process that removes dielectric from the laminated board instead of copper. The advantage of this method is greater exposure of the copper layer: instead of plating from the PTH barrel to one side of the layer, three sides of the layer become exposed during processing. Greater surface area and a three-point attachment mean a more reliable interface between copper layers and the PTH barrel, improving the long-term reliability of the board. Positive etchback is not without its potential defects: excessive hole roughness can lead to continuity errors due to cracking at the plated hole or copper foil.
To further drive home the reliability point, IPC-6012 restricts any negative etchback on Class 3A electronics (typically, boards fitted for aerospace); however, a small amount of negative etchback is permissible on the more general Class 3 boards. Negative etchback certainly has its place in manufacturing, and most production lots of appreciable volume will prefer it over positive etchback to limit per-board expenses.
Selecting a chemical agent or plasma treatment for etchback will impact cost, but board material is generally the distinguishing criterion. For instance, plasma is more suitable for polyimide-based flex-printed circuits than a chemical etchant. A manufacturer may also employ combination plasma-and-chemical processes for superior desmearing-etchback results.
Your Contract Manufacturer’s Got Your Back
PCB etchback is an additional fabrication process following desmearing to improve the surface adhesion between the copper layers and PTH barrel. Depending on the final application of the board and practical concerns, manufacturers can instruct designers on the etchback options available. An experienced PCBA manufacturer can assist product development teams in this process: we here at VSE are a team of engineers committed to building electronics for our customers. With over 40 years of experience operating in the Bay Area and our new Reno facility, we can provide prototyping, NPI, and higher-volume production solutions.