The term "single-layer PCB formed as copper on one side" refers to a single layer of copper with base materials, solder mask, and silkscreen. Copper layers should always be formed with a surface finish. Surface finish is the new word at the moment. We will discuss the surface finish later in this page. The basic use of surface finishing is to provide protection for the copper layer from the external environment's effects, and it is also used to achieve better solderability while assembling components on the PCB.
Let's take a look at how a single-layer copper PCB is made and what kind of layer forms on it. We did not show the surface finish layer separately to facilitate understanding of single-layer PCB formation.
A surface finish is always applied to exposed copper, which means a surface finish is applied where no solder mask ink is present. If solder masking is not required on copper PCBs, then a surface finish should be applied to the entire copper circuitry to protect copper from environmental effects. Surface finish also improves solderability when assembling components on a PCB. Many different types of surface finishes are applied on copper, i.e., standard HASL (HASL-hot air solder leveling), lead-free HASL, silver, immersion gold (ENIG- electroless nickel immersion gold), etc.
Sometimes we can assume which surface finish is applied on PCB by visually inspecting the colour of exposed copper on PCB (light grey should be HASL or lead-free HASL, goldish colour should be ENIG, etc.). However, assuming surface finish by colour for PCB assemblers is not standard practise. They must confirm with the PCB manufacturer or customer about the used surface finish.
Refer below to Fig. 5 as an illustration example of PCBs without solder mask but with surface finish (HASL or ENIG) and Fig. 6 as an illustration example of PCBs with solder mask and surface finish.
