In simplest terms, a faster printer logically means higher throughput. Overall speed, however, is dependent upon a number of factors including the individual cycle times for various steps in the print cycle (e.g., stencil wiping frequency, board separation and printing speed).

Process traceability and verification has become increasingly important as electronics assembly has moved to higher throughputs and volume requirements with tighter process windows. Ensuring repeatable, high yields requires an optimized process that is constantly in control. Knowing and tracking every parameter relative to how a PCB has been assembled is critical to maintaining control and traceability. If and when something goes wrong, it’s a lot easier to get to the source of the problem with comprehensive traceability.

Stencil printing today requires a higher degree of printer accuracy than ever before. Printer accuracy becomes increasingly difficult to maintain as demands for higher speeds and throughput test the limits of a printer’s capabilities, particularly with tiny apertures and tight land patterns. Today’s equipment design engineers are tasked with building machines with tighter performance tolerances and accuracy.

Paste-in-Hole (PIH) printing, a.k.a. through-hole printing, pin-in-paste, intrusive reflow, etc., has always been a way to accommodate traditional through-hole components on a mixed-technology SMT assembly using reflow soldering rather than wave to make the through-hole connections. Paste is printed in such a manner to as to fill the through-holes, the through-hole components are inserted, and the assembly is reflowed.

Fine pitch/fine feature solder paste printing in PCB assembly has become increasingly difficult as board geometries have become ever more compact. The printing process itself, traditionally the source of 70% of all assembly defects, finds its process window narrowing. The technology of metal blade squeegees, with the aid of new materials, understanding, and settings such as blade angle, has kept pace with all but the smallest applications, e.g., 200µ - .50 AR and 150µ - .375 AR, which have been pushing blade printing technology to its limits.

Stencil printing technology has come a long way since the early 80’s when SMT process gained importance in the electronics packaging industry. In those early days, components were fairly large, making the board design and printing process relatively simple. The current trend in product miniaturization has led to smaller and more complex board designs. This has resulted into designs with maximum area utilization of the board space. It is not uncommon, especially for hand held devices, to find components only a few millimeters from the edge of the board.

Stencil printing is a critical first step in surface mount assembly. It is often cited that the solder paste printing operation causes about 50%-80% of the defects found in the assembly of PCBs. Printing is widely recognized as a complex process whose optimal performance depends on the adjustment of a substantial number of parameters. It is not uncommon to hear that stencil printing is more of an “art” than “science”. In fact, the process is so complex that sub-optimal print parameters usually end up being used.

The techniques described have proven to be robust and particularly well suited for detecting troublesome bridge and bridge-like features that span the gap between pads. This paper describes part of a research effort currently under way in the field of print defect detection.

Increasingly fine pitch component patterns on PCBs, and ever-smaller passive components such as 03015’s continue to narrow the process window for PCB assemblers. The drive toward ever-finer pitches is driven by a number of factors, particularly miniaturization in mobile devices such as Smartphones, where consumers are demanding greater functionality in compact, hand-held devices. As a result, material handling and component placement becomes more difficult, and solder paste printing is especially impacted.