In electronic design automation, the solder paste mask and thus the stencil is typically defined in a layer named tCream/bCream aka CRC/CRS,[1][nb 1] PMC/PMS,[2] TPS/BPS,[3] or TSP/BSP (EAGLE), F.Paste/B.Paste (KiCad), PasteTop/PasteBot (TARGET), SPT/SPB (OrCAD), PT.PHO/PB.PHO (PADS), PASTE-VS/PASTE-RS (WEdirekt),[4] GTP/GBP (Gerber and many others[5]).
For improved accuracy, stencils traditionally were often mounted in proprietary aluminum frames of various kinds.
The internal vision system aligns the stencil to the board, after which the squeegee prints the solder paste.
Misalignment of motion from the reference results in several defects, hence the board must be secured correctly before the process begins.
Vacuum holders must be carefully used, as they may affect the pin-in-paste printing process if not secured properly.
Vision systems in the stencil printing machines use global fiducial marks for aligning the PWB.
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.
Surface oxidization on the copper will inhibit the ability of the solder to form a reliable joint.
The core of a well printed PWB lies in the fill and release of solder paste into the aperture.
The amount of solder paste which is released from the stencil apertures and transferred to the PWB pads, determines whether or not the print is good.
Apertures which are not completely filled will not release paste onto the board, which results in clogged stencils and defective solder joints.
The laser cutting process results in trapezoidal apertures that can create better solder paste release characteristics.
With laser cutting, there are no photo films requiring precise alignment or protection from moisture.
The nickel has better wear characteristics than steel and electroforming creates smooth tapered aperture walls.
The process also creates a ridge along the bottom of the stencil that can improve stencil-to-board gasketing and result in more consistent solder paste release.
Also, molecular layer nano coatings are put on the stencil walls so that the solder paste does not stick.
In general, including stencils with tall and narrow apertures, an area ratio greater than 0.66 is recommended.
A PCB may need varying amounts of solder paste to be applied depending upon the design and size of components.
The stencil life is defined as a time period in which there will be no significant change in the solder paste material characteristics.
Actual stencil life for a paste should be determined from the manufacturers' specifications and on-site verification.
Stencils are typically identified by job numbers to reduce the risk of mishandling or misplacing.
Insufficient solder paste may cause poor bonds and contact between components and the board.
The common causes of insufficient solder paste are poor gasketing, clogged stencil apertures, insufficient solder paste bead size, paste/stencil being used beyond recommended life span, stencil not wiped clean, or low squeegee pressure.
Normally, solder paste printing equipment can handle warpage of 1.0 to 3.0 mm but beyond this limit needs some special jigs or fixtures to hold the PCB.
Processes are typically rated in terms of number of defects per million opportunities (DPM).
Statistical process control can then be used to continuously monitor and improve printing DPM levels.