Acid Traps: Embrace Optimal Etchant Usage by Fabricators
If you have looked under your bathroom or kitchen sink, you will see a waste water plumbing fixture called a P-trap. Usually made from PVC pipe, p-traps consist of two 90o degree joints that combine with a horizontal overflow pipe to form the letter “P.” Water exits from the sink drain through one 90o joint and then into the other 90o joint before reaching the overflow pipe.
The components that make up the P-trap and the trap configuration prevent waste water and any debris from back flowing into the sink. Preventing backflow also keeps any sewer gas from leaking back into the house. The small pool of water that forms at the bottom of a P-trap serves as an additional barrier for preventing the backflow of sewer gas.
It’s an Acid Trap!
Although plumbing systems require traps, PCB designs stay away from traps--especially acid traps. An acid trap usually occurs when traces meet at sharp angles and etching solution becomes trapped at the junctions. These junctions could be if a smaller-than-normal gap that exists from the trace to the via in a high-density or multi-layered PCB, or if a tiny pocket that forms during the etching process. If the manufacturing process relies on acidic etchants, the acid can accumulate in the corner created by the angles. When acid traps exist, the possibility of open circuit conditions increases.
Fabrication will necessitate some smart awareness from the fabricators.
An acid trap can destroy the copper trace as well as nearby components and lead to a defective board. Acid traps can become particularly harmful for multi-layer boards if the acid travels from the trap through an open via and other sections of the board. In addition, leaking etchant acid can harm component connectors and surface-mount components.
Traditional methods for avoiding acid traps include eliminating all 90o angles in the routing design and observing a minimum 3 mil gap between connections. However, the desire of customers for products--and as a result PCBs--that have smaller and smaller footprints makes achieving those solutions more difficult.
Good Chemistry Works for Fabrication
Acid traps have become a thing of the past due to advances in etchant materials. Most--if not all--fabricators have transitioned away from etchants such as sulfuric acid + hydrogen peroxide, sulphuric and chromic acid, and other types of acidic etching solutions. The use of non-acidic, photo-activated etchants prevents acid traps from forming and also responds to the need for environmentally friendly etchants. As the name implies, photo-activated etchants become active when exposed to light. When ultraviolet light irradiates the board, etching occurs. Fabricators then use water to wash the etchant way. Because photo activated etchants produce a precise edge, the possibility for developing an acid trap becomes fairly remote.
Fabricators have also begun to embrace moving away from the traditional subtractive methods for etching a PCB to additive PCB manufacturing methods. Those methods use the capabilities of modern PCB design tools to fully integrate mechanical and electrical computer-aided design (MCAD and ECAD). The digital technologies used for additive PCB manufacturing eliminate the many stages of the subtractive method and help to conserve resources without polluting the environment.
Use a Design Mindset
The combination of acidic etchants and high density PCB designs was problematic because of the reduced distance between vias, the need for fine lines and tight spacing, and product requirements to use as much board area as possible for routing. Design teams can use PCB design rules to control conductor widths, hole sizes, and the layout. Using and applying the design rules allows teams to utilize acute angles for routing without the risk of building an acid trap.
Managing the fabrication of surface mount components can also be difficult.
Modern PCB design software includes Design for Manufacturability (DFM) tool suites that eliminate the chance for developing an acid trap and increase the efficiency of the design. For example, Every Layer Interconnection (ELIC) assists design teams with reducing or eliminating mechanical holes, minimizing line widths, and maintaining the uniformity of traces.
Along with achieving the efficiencies seen with ELIC, the use of dense Ball Grid Arrays (BGA) and fine-pitched BGAs requires a precise design approach that improves the reliability of interconnection points. While this approach improves signal and power integrity, precision in design allows design teams to reduce plane perforations by placing power and ground planes on the top layer of a PCB. Other tools in the DFM suite reduce the number of vias that connect to the inner layers of a multi-layer board.
Using the combination of design rule checks (DRC), error checking, and Design for Manufacturability also assures the fabricator that the design team has taken the necessary steps to submit a PCB design that does not have flaws. With those essential steps in place, fabricators can rely on their precise methods for applying photoresist and controlling etchants to ensure the production of a high-quality board.
Whether you’re looking for a smart DFM option for your board production, or for an advanced suite of design and analysis tools, those offered by Cadence are sure to assist you. Allegro PCB Designer is the integrative and customizable tool to work with your designers every step of their circuit production process.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.