Lead-free Reflow for High-layer-count PCBs


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This article is an update of the Holden-Carano article originally published in the February 2013 issue of The PCB Magazine.

Abstract

One of the most difficult printed circuit boards to adapt to Pb-free assembly processes is the high-layer count multilayer. Often, these multilayers have through-hole and hand-soldered components, and requirements for two or more rework cycles. The higher reflow temperatures and slower wetting of lead-free solders place an enormous strain on the laminate and copper-plated hole barrel. In many cases, the boards cannot be assembled reliably even with newer, higher thermal performance FR-4s. 

One solution to this problem is to redesign the multilayer using current design rules and newer innovative fabrication technologies. This article will review four of these new and enabling technologies:

  • Laser-drilled microvias;                                                        
  • Routing BGA using channels;
  • Contribution of new SMT connectors; and                               
  • Layer assignment changes (architectures).


Microvias offer the most significant opportunity to reduce not only the layers and thicknesses of multilayers, but also their cost while improving their electrical performance and density. Several examples will illustrate these new opportunities. Since blind vias are surface phenomena, to get the maximum benefit from them, layer assignment for signal, ground and power need to be reviewed and alternative constructions considered. These blind vias, by reducing the number of through-holes, contribute to increase routing density that allows the lower layer usage. Finally, by replacing through-hole connectors with surface mount connectors, higher connector density and improved electrical performance can be realized.

The resulting new multilayers are not only thinner, cheaper, and easier to design, but are less costly and suitable for lead-free assembly.

Read the full article here.


Editor's Note: This article originally appeared in the April 2014 issue of The PCB Magazine.

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