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68 PCB007 MAGAZINE I JULY 2022 From the table, the first reaction is the simple etch reaction where the etchant (Cu +2 (NH 3 ) 4 Cl 2 ) reacts with copper (Cu 0 ) to provide the spent form of alkaline etchant (Cu +1 (NH 3 ) 2 Cl). e etch reaction is rela- tively simple, unlike the regeneration reac- tion. e regeneration reaction tends to be the primary cause for concern because it is com- plex yet essential to your process. e regen- eration reaction is important because it allows you to etch your panels and not see a decline in PCB quality or production rate. In the regen- eration reaction, the spent form of the etchant reacts with ammonia (NH 3 ), ammonium chlo- ride (NH 4 Cl), and oxygen (O 2 ) to convert the etchant back to its original form. Water (H 2 O) is a by-product of this reaction. All the reaction components (ammonia, ammonium chloride, oxygen, and water) play a vital role in maintain- ing the etchant. Ammonia plays a critical role in pH control, ammonium chloride contributes to chloride content, oxygen acts as the limiting reagent in regeneration, and water helps with specific gravity control (copper concentration control). ese components to the reaction help you stay within the parameters alkaline etchant needs to function. e main factors you will need to watch out for are pH, chloride levels, and air flow. ese should be the focus of your efforts because these tend to be the most high-risk or difficult things to control. pH Control Holding a steady pH value in alkaline etchant is the biggest challenge. One of the main rea- sons it is difficult to manage is because pH is not a value that is easily measured. Alka- line etchant can easily damage pH probes and cause inaccurate readings. Since pH dropping can cause sludge-out, having inaccurate pH readings cannot be acceptable. To monitor pH, it is recommended to use pH probes along with routine titrations. e pH should be kept within the range of 8.1-8.2 (may vary slightly depending on chemical supplier). 1 If you start to notice the formation of dark blue crystals in your etchant, that is also a sign that your pH is too low. Another reason pH is difficult to control is because it can fall out of range easily. is is because pH is maintained with ammonia gas, and sufficient pH levels rely on having ammo- nia dissolved in the etchant. With ammonia being highly volatile, keeping it dissolved in your etchant can be difficult. ere are many variables that can affect how ammonia remains in the etchant. Temperature, spray pressure, and ventilation are the main parameters that affect whether ammonia escapes or stays in the etchant. Even if you are not using the etchant at a given time, you still need to worry about main- taining pH. If alkaline etchant is allowed to sit for a long time in an unsealed environment, the ammonia will bleed out of the etchant, drop the pH, and sludge-out will occur. To prevent this, it is best to pump it out of the etching machine and place it in drums if you don't plan to use it within three days. By placing the etchant in drums, you are sealing it and preventing the ammonia from escaping. Chloride Control It is also necessary to sustain chloride levels to prevent sludge-out. Like pH control, it is a good practice to perform routine titrations to check chloride levels. ankfully, maintaining your chloride level is easier than maintaining pH. is is because the only way to lose your chloride content is by dilution or insufficient input to keep up with the regeneration reac- The regeneration reaction tends to be the primary cause for concern because it is complex yet essential to your process.