Copper plays a conductive role in a printed circuit board (PCB). As an active chemical substance, copper tends to be oxidized when exposed to the atmospheric humidity, thereafter leading to issues in high-temperature soldering, detrimental to components mounting on PCBs and reliability of end products. Surface finish has two key functions: to protect copper from being oxidized and to provide a surface for soldering when components are being assembled on PCBs.

Board finishes can be classified into different classifications based on different technologies and chemical substances: HASL (hot air soldering leveling), Immersion Tin/Silver, OSP, ENIG (electroless nickel immersion gold), ENEPIG (electroless nickel electroless palladium immersion gold) etc. Among all the finishes, OSP is becoming increasingly prevalent owing to its low cost and environment-friendly features but there must be still some places you haven't been to which will be discussed in this article.