PCB007 Magazine

PCB-June2016

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76 The PCB Magazine • June 2016 As the unexposed resist is developed away, pH will decrease. Thus it is critical—especially for higher density circuits—that the operating pH of the developer solution be controlled to a fairly tight range (typically 10.5−10.8). One practical approach consists of selecting a de- sired pH (e.g., pH 10.6) to be maintained in the developer. This pH value can be dialed into a pH-controlled feed and bleed system. The sys- tem may be set up to trigger feed at pH 10.5 and cut it off at pH 10.7. One must also prevent the re-deposition of the developed resist back onto the circuit traces as this will adversely affect the quality of the electrodeposited copper. Such concerns include pitting, mouse bites (more on this in a future column), peeling copper, domed copper, non-uniformly plated traces and in- complete etching. Figure 1 depicts an example of incomplete developing. Figure 2 depicts another issue often blamed on the developing process. The problem is a scumming on the board due to incomplete and/ or poor developing practice. In addition to underdevelopment, there are several other plausible causes for resist scum- ming. These include: • Phototools with low density in the opaque areas • Resist partially exposed to white light prior to development • Poor filtration in the developer chamber • Insufficient water rinsing after development • Resist build-up in develop—ineffective feed and bleed system • pH of developer too low • Plugged spray nozzles in the developer chamber One of the areas to investigate (if resist scumming is suspected) is the rinsing step after develop. Developing solutions are alkaline and need more water to rinse the resist developer residues from the copper surfaces and along the sidewalls of the exposed resist. Ensure there is sufficient water pressure in the rinse cham- ber to flush away the dissolved or partially dis- solved unexposed resist. Failure to remove all of the unexposed resist can also result in what is known as a positive resist foot. (More on the positive foot will be presented in a future col- umn.) Rinse water temperature should at least be 17°C. In addition, that rinse is more effective if the hardness of the water is between 150–350 ppm CaCO 3 equivalents. The hardness of the water acts as a "stop develop" by forming in- soluble binder salts. These salts are more easily rinsed away. As an aside, monitoring the pH of the resist rinse is an effective way to insure a more complete developing and rinsing opera- tion. PRIMARY IMAGING FOR PATTERN PLATING, PART 2: DEVELOPMENT Figure 1: Narrow traces due to incomplete developing. Figure 2: Example of resist scumming.

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