Let\u2019s talk real stuff that actually works on the production floor.<\/p>\n\n\n\n
Why SMD Photodiodes Hate Moisture More Than You Think<\/h2>\n\n\n\n
Unlike regular resistors or capacitors, photodiodes have a large exposed die area or very thin plastic molding. That makes them extremely good at sucking water vapor out of the air. Once that moisture gets trapped inside the package, the rapid temperature jump in reflow (often 260\u00b0C peak) turns the water into steam. The pressure has nowhere to go, so it either cracks the package or delaminates the die attach.<\/p>\n\n\n\n
This is exactly what the Moisture Sensitivity Level (MSL)<\/strong> rating is trying to warn you about.<\/p>\n\n\n\n Most Si PIN photodiodes from major suppliers sit at MSL 3<\/strong> or MSL 4<\/strong>. That means they can only sit on the factory floor for 168 hours (MSL 3) or 72 hours (MSL 4) before they need baking again. In real production, those numbers are optimistic.<\/p>\n\n\n\n I\u2019ve personally seen MSL 4 photodiodes start showing voiding and die cracking<\/strong> after only 50 hours at 28\u00b0C\/65%RH in a poorly controlled SMT workshop in Southeast Asia. The spec said \u201cNo dew condensation,\u201d but reality didn\u2019t care.<\/p>\n\n\n\nUnderstanding MSL Levels for Photodiodes \u2013 The Practical Table<\/h2>\n\n\n\n
MSL Level<\/th> Floor Life (\u226430\u00b0C \/ 60% RH)<\/th> Safe Exposure Before Baking<\/th> Typical Reflow Cycles Allowed<\/th><\/tr><\/thead> MSL 2<\/td> 1 year<\/td> Very safe<\/td> 3+<\/td><\/tr> MSL 2a<\/td> 4 weeks<\/td> Good<\/td> 3<\/td><\/tr> MSL 3<\/td> 168 hours<\/td> Most common for photodiodes<\/td> 3<\/td><\/tr> MSL 4<\/td> 72 hours<\/td> High risk<\/td> 3<\/td><\/tr> MSL 5<\/td> 48 hours<\/td> Very risky<\/td> 2\u20133<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
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