Stencil Printing-Inputs-Printed wiring
boards-circuit Board Pcb
Inputs to the process can be classified as: Design Input, Material Input, Process parameter Input. The output of the process is a printed wiring board that meets the process specification limits. These specifications usually are consistent solder paste volume and height, and printed solder paste aligned on the PWB pads. This determines the process yield.
Printed wiring boards
The PWBs are designed to have fiducial marks on them as reference points. Vision systems in the stencil printing machines use these fiducial marks for aligning the PWB. Without these fiducials the printer would not print the solder paste in exact alignment with the pads. There are two types of fiducials used namely global and local fiducials. Global fiducials align the printer to the PWB. Local fiducials are required for precise placement of fine pitched components, in the component placement stage. Local fiducial cannot be used in the printing process The entering PWB should have close dimensional tolerances so that it mates to the stencil. This is necessary to achieve the required alignment of solder blocks on the pads.
The required accuracy in alignment can also be achieved by controlling the flow of solder on the PWB during reflow soldering. For this purpose, the space between the pads is often coated with a solder mask. The solder mask materials have no affinity to the molten solder and hence, no positive bonding is formed between them as the solder solidifies. This process is often referred to as Solder Masking. The mask must be centered correctly. The mask protects the PWB against oxidation, and prevents unintended solder bridges from forming between closely spaced solder pads.
Also the height of the solder mask should be lower than the pad height to avoid gasketing problems as shown. If the height of the solder mask is greater than that of the pad, then some of the solder paste would settle in the empty space between the mask and the pad. This is what is referred to as gasketing. It is a seal that fills the space between two surfaces to prevent leakages. However, in this case, the gasketing is a problem as the excess solder paste around the pad may be more than a nuisance factor for circuits having very small line spacing.
As shown in the adjoining figure, if the height of the mask is greater than the pad, it could create the flow of solder paste around the pad.
The pads on the PWB are made of Copper and are susceptible to oxidization. Surface oxidization on the Copper will inhibit the ability of the solder to form a reliable joint. To avoid this unwanted effect, all exposed Copper is protected with a Surface Finish.