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JUNE 2019 I PCB007 MAGAZINE 69 designing a product for a specific machine. This is feasible but means introducing a new solder mask onto the manufacturing floor that, most likely, does not work optimally on any other direct imaging systems at that facility. The direct imaging systems in the market can vary greatly. Available wavelengths start at 355 nm and go up to 420 nm. Some systems may feature just one wavelength, and others may have as many as four wavelengths. Generally, the light sources used include UV diodes or UV laser diodes. Alternately, some machines may feature solid-state lasers or special mercury va- por lamps. Additionally, the exposure optics vary from brand to brand. Seven Defects Practically speaking, these seven defects may be observed during direct exposure but gener- ally do not occur during contact exposure. 1. Solder Mask Surface Is Dull The solder mask surface could be more matte instead of shiny or satin due to missing wavelengths, oxygen inhibition, or the surface of the resist not being sealed. 2. Solder Mask surface Is Sticky After Development This defect could be due to oxygen inhibi- tion. 3. White Stains on the Mask Surface This defect occurs after ENIG and HASL or after wave soldering and cleaning. One side can be affected or both sides over the entire area. Sometimes, there are white spots of dif - ferent sizes visible. These stains are due to moisture absorption because of the missing wavelengths of 200–360 nm that seal the re- sist surface. By drying the PCBs at 100–120°C for 15–20 minutes, the spots generally disap- pear almost completely. In some cases, the defect can be avoided by better drying of the solder mask. Another option is a UV bump af- ter developing. Higher exposure energy is also helpful. 4. Spontaneous ENIG on the Solder Mask This defect is well-known and generally due to an overactive nickel bath. It also occurs in areas with poor rinsing. Due to the porous sur- face of the solder mask, palladium nuclei can remain behind and lead to nickel deposition even during good rinsing and normal activity of the nickel bath. 5. Solder Residues on the Resist Surface After HASL On the porous surface, more solder balls and solder spiders can be left. The defect is known but rarely occurs during contact exposure. Cer- tain configurations of solder mask, flux, and HASL parameters tend to form tin spiders. In a well-optimized HASL process, the defect does not occur. 6. Increased Ionic Contamination This defect highly depends on local condi- tions. If demineralized water is used through- out for rinsing, it is generally not a problem. If city water is mostly used, it can be a problem. 7. Solder Mask Residues on Soldering Surfaces This defect is observed when the solder mask is applied by spray coating on certain machines

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