Issue link: https://iconnect007.uberflip.com/i/291105
April 2014 • The PCB Magazine 15 LEAD-FREE REFLOW FOR HIGH-LAYER-COUNT PCBS continues layer-count multilayers. These multilayers have a much higher thermal mass as well as increased through-hole parts and copper planes. With the possible increased need for rework of complex parts and hand-soldering, the total thermal en- vironment may exceed what any FR-4 is capable of. When this occurs, the remedy is to reduce the thermal mass by reducing the mulitlayer's layers. Four main alternatives that can be used are: • Laser-drilled microvias • Layer assignment changes (architectures) • Routing BGA using channels • Contribution of new SMT connectors Moving to Microvias Microvias are nearly 30 years old now, hav- ing been used in high-volume by OEMs like Hewlett Packard for their first 32-bit computer— the FOCUS chip—in their HP9000 m845 desk- top computer starting in 1983. They employed laser drilling of blind vias in an 8-layer PTFE dielectric with copper used as the core layers for cooling (termed "finstrate" for this feature). Other OEMs like IBM and Siemens also devel- oped microvias technologies for their comput- ers, but the technology did not really take off until the need for miniturization in portable products like cellular phones increased. Today it is the fasting growing segment of intercon- nect packaging, used in portable products, IC and ASIC packaging and in large complex mul- tilayers for telecom and servers. Figure 1 shows the rapid growth of microvias during the early 2000s. Multilayer Cost-Density Tradeoffs In designing classical PWBs, there is a wiring barrier created by the size of component lands, traces and vias. If you look at a square inch of PCB real estate, there are only so many SMT land patterns, traces connected to the land and vias connected to the trace that you can put in that one square inch before it is full. Depending on the SMT land size, this barrier is called the TH wiring barrier [3] . HDI microvias provide the opportunity to reduce the number of layers of traditional Figure 1: the rapid increased use of microvias has been fueled by the popularity of cellular phones and the need to make them as small and as light as possible. now, all products use microvias.