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74 PCB007 MAGAZINE I SEPTEMBER 2020 whereas with a Type 1 ICD, a thermal excur- sion is required to cause the separation of the electroless copper from the post (Figure 2). One can easily surmise that a Type 1 ICD and D-sep are the result of a highly stressed cop- per deposit. However, there are several other potential causes of these issues that must be considered. Deposit stress can be introduced in a variety of ways. Since D-sep occurs without the application of a thermal excursion, a skilled troubleshoot- er would look at the following areas. This ex- ercise includes both chemical and mechanical parameters related to the electroless copper process. The main cause of D-sep can be sum- marized as a change in the deposition condi- tions of the copper deposit. This is not due to drill smear remaining on the interconnect post. The change in key chemical parameters, in- cluding the hydroxide concentration, solution specific gravity, and solution temperature are outlined as follows: 1. Low caustic level: Depleting the caustic stops the plating reaction. 2. High specific gravity: By-product components are produced that have an inhibiting effect on the chemical plating, most notably sodium sulfate and sodium formate. 3. Vibration: Vibration removes air bubbles that may be trapped in the hole as it enters the tank. Hydrogen gas is produced during the reaction and needs to be removed from the holes. 4. The temperature of electroless: Higher temperature increases the plating rate. If solution is not transferred fast enough, the reaction stops. As the following reaction shows, the elec- troless copper plating solution ages over time with the buildup of formate, carbonate, and chloride as by-products. These by-products are measured with specific gravity but are not Figure 2: Type 1 ICD.

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