Printed circuit board etch-back is a technique used to maintain optimal transmitting signals between many layers in PCBs. It is specifically used with PTH procedures, where producers add a copper coating to the interior sides of the via. PTH enhances conductivity by linking the circuits in multilayer printed circuit boards.
Understanding PCB Etch Back Processes
PCB etch-back is a crucial process to maintain optimal routing signals between multiple layers within printed circuit boards. It involves removing the epoxy glue from the edges of the drilled via to allow electroplating and maintain the high link between layers. This process is especially important in high-reliability appliances. It is subject to standards set by an IPC-6013, which dictate that etch-back must expose a minimum of 0.003 mm and 0.08 mm of copper as specified in the Procurement records.
To improve the adhesion via plating, micro-etch procedures are used, which involve submerging the base laminate & drilling PTH into a multilayer printed circuit board. The dielectric debris and excess smears from a plated sidewall are then removeable using a plasma etch in the process known as de-smearing.
Moreover, PCB manufacturers utilize PCB etch-back in conjunction with de-smearing processes. PCB etch-back removes material from a via hole and extends copper layers to improve the electrical conductivity of the printed circuit board. This process allows producers to plate 3 edges of a copper layer & promotes the sustainability of the PCB design.
Creating PCB Etch Back
Two common processes in flexible circuit production, micro-etch & plasma etch, can impact etch-back. To enhance Coating adherence, the laminate substrate is often treated through a micro-scale etching, where a small copper is removed by submerging or spraying the substrate. After laminating the circuit layers together and drilling through holes, a plasma etch is performed to remove any organic resins or debris, also known as desmearing. It ensures the surface of copper in the via hole is lead-free. The plasma etching is carefully controlled to remove a specified amount of dielectric, revealing the copper coatings.
Causes of Epoxy Resin Smears
When the temperature of a drilling process surpasses the glass transition temperature of a substrate, it can melt the dielectric resin and its dispersion on the copper texture of interior layers, causing smears. This melted resin may obstruct the terminals of inner layers, leading to a decrease in the effectiveness of electrical connectivity.
Type of PCB Etch Back Processes
Two primary types of printed circuit board etch-back exist, namely negative & positive, and they yield various appearances & outcomes.
· Negative PCB Etchback
In negative printed circuit board etch-back, the inner copper coatings gradually recede from the walls of the via hole to varying degrees depending on the configuration of the PCB.
Negative etch-back is less aggressive and is more suitable for the production of high-durability & long-term installations.
Polished copper cylinder walls by negative etchbacks, which do not have stress points within the inner plane. But if we use these processes too much, it can cause gaps between layers because of dirt and air bubbles.
· Positive PCB Etchback
On the other hand, positive PCB etch-back results in copper pads sticking out from the intersecting edges of the via hole, which provides a sturdy Tri-terminal connection. This type of etch-back is commonly useful in highly reliable multilayer printed circuit boards, particularly in the aerospace, medical & military fields.
Positive etch-back is also useful to remove smears of dielectric substance aggressively. However, one significant disadvantage of this method is the potential formation of Pth barrels & foil cracks by strain accumulation on a fabric. Despite this drawback, positive etchback is still available.
PCB Etch Back Techniques
PCB manufacturers have several options for etch-back techniques, each utilizing different solutions and yielding varied outcomes depending on environmental conditions. These options include chemical and plasma etch technologies.
· Chemical Etch
Chemical etch processes typically use a basic etching solution of potassium permanganate, usually removed via plating. Another option is sulfuric acid, which removes water from the air and has a limited period, changing resin-reducing rates based on purity.
Another chemical etch option is chromic acid. It gives a uniform resin removal rate but comes with water pollution hazards.
· Plasma Etch
Plasma etch-back is an expensive technique useful in PCB manufacturing that offers fast and effective stain removal. The process involves creating volatile radicals that corrode epoxy glue buildup, making gases that producers eliminate at the last step of the procedure. However, plasma etch-back can increase the total price of PCB production due to reduced manufacturing throughput and longer processing time. Additionally, after the plasma etch-back, you still require a chemical desmear procedure to get the glass to etch & remove any fiberglass.
Difference between Chemical & Plasma PCB Etch Back Processes
Etching is the process of removing copper & unprotected areas, leaving the intended board behind. There are two main methods of etching: chemical etching and plasma etching. Although both techniques serve the same purpose, they differ in several ways. The following are some of the differences between chemical etching and plasma etching.
· Efficiency
The most significant difference between chemical etching and plasma etching is their efficiency. Chemical etching, which removes unprotected areas by dissolving them, is effective, however inefficient, similar to digging a ditch. In contrast, plasma etching has set new standards by greatly improving efficiency.
· Etch-back
Another difference between chemical etching and plasma etching is the occurrence of etch-back. During the chemical etching procedure, etch-back or undercutting always occurs, which requires increased trace widths. The thickness of the copper also affects the etching time required to eliminate copper, which is present between the pads. On the other hand, the new procedure of plasma etching eliminates the occurrence of etch-back. Therefore, the occurrence of etch-back is another distinguishing factor between chemical etching and plasma etching.
· Etch factor
Plasma etching is crucial as it reduces imaging errors by utilizing an imaging process that transfers the layer to the substance/material, thereby eliminating an etching factor. But the chemical etching procedure is the traditional method that results in a distorted side of the pad. With chemical etching, there is a certain slope amount for every wire height increase.
Plasma Etching: A Safer, More Reliable Solution
Unlike chemical etching, plasma etching does not use a hot chemical solution like a manganate (VII) solution. It is a dry technique that is eco-friendly and has been proven effective in completely removing any leftover epoxy resin from the printed circuit board drilling process.
Compared to permanganate solutions, which rely on wet chemistry & chemicals that lose effectiveness over time, plasma ashing is a cleaner and more consistent process. It does not generate any hazardous liquid chemical waste. Additionally, the equipment needed takes up less area on the fabrication floor. Additionally, the plasma treatment tool is easy to operate and does not require highly-skilled personnel.
Wrap Up!
The terms “de-smearing” and “etch-back” may be used interchangeably by some manufacturers, but these two processes are slightly different. In particular, it is possible to perform de-smearing without requiring resin etchbacks.
De-smearing, or smear removal, removes resin from the via hole wall side to ensure proper interior layer electrical connectivity. Etchback, on the other hand, improves conductivity by extending or etching the copper layers. While some manufacturers may use the terms interchangeably, there are slight differences between the two processes. Etchback is more effective in multilayer printed circuit board configurations, while you cannot use it for PCBs with double sides.
Furthermore, chemical etch-back strategies are generally more cost-effective than plasma methods. Still, they are less efficient & involve caustic chemicals that may pose hazards during handling. On the other hand, plasma cleaning treatments offer greater precision, allowing for better customization and fine-tuning of the drilling process.
PCB manufacturers can opt for plasma processes as a safe & eco-friendly alternative to chemical etch-back methods. Plasma processes produce by-products that discard without gathering hazardous waste. By partnering with a specialized printed circuit board etch-back professional, manufacturers can achieve higher adaptability, optimized repeatability & precision for even the most complex fabrication processes. It leads to more reliable, more sustainable printed circuit boards, reduces waste & improves reliability. Although plasma etching eliminates the need for additional time to rebuild boards, resulting in low cost.
