Blister Free PTH Processes for High Tg and Exotic Base Materials - Sponsored Whitepaper

Introduction PCB manufacturers must permanently contend with difficulties associated with production of PCBs with ever more complex designs, technology requirements and increased lifetime / reli- ability requirements. Issues pertaining to the laminate material used, via sizes, copper layer thickness and electrical requirements, which significantly affect the manufacturing process are often the prerogative of PCB designers with extremely limited if at all any consultation of manu- facturers. Most importantly introduction of the Restriction of the use of Hazardous Substances (RoHS) di- rective in Europe from July 2006, has obliged PCB fabricators to use RoHS compliant materials. RoHS affects the metals Lead, Mercury, Chrome (VI), and the bromine containing resins PBB and PBDE. The main concern for PCB fabricators are the bans on Pb and Cd as these affect the choice of laminate materials and consequently the performance and requirements for the desmearing and metallization processes. Other countries most notably China from the 1st of March 2007 are set to follow with similar direc- tives.

Consequences of Lead Free Soldering New lead free soldering processes mainly those based on Sn/Ag/Cu (SAC) alloys have emerged. These have melting points that are about 30 °C above that of the conventionally used Tin Lead Alloy (Sn63Pb37). Lead free soldering, not only results in higher temperature but also results in longer exposure at the higher temperature. This combined effect results in significantly higher thermal stress of the base material and suitable laminate materials must be used. The logical reaction of many laminate manufacturers has been to develop new epoxy based halogen free materials with higher Tg and higher Td. Another typical trend for these materials is the inclusion of filler materials mainly to reduce CTEs. The result is laminates with lower CTEs and the ability to withstand higher temperatures, thus reducing the risk of delamination. This leads to less stress on copper in through holes and significantly reduces the probability of barrel cracks and inner layer connection defects occurring. Experiments have shown that high Tg laminates exhibit a higher resistance to chemical attack expressed through reduced weight loss and less roughening tendency during permanganate at- tack. As a consequence high Tg materials require more aggressive permanganate etching for the same cleaning effect as with conventional laminates The filler materials often have a different response to permanganate attack, compared with the laminates in which they are imbedded. Desmear parameters must therefore be finely adjusted for optimum attack of both the resin and filler materials to prevent unacceptable geometrical defor- mations of features such as BMVs.

Consequences of Halogen Free Laminates The removal of bromine from the basic formulation of laminate materials means that alternative flame retardant systems have to be implemented. Typical alternative flame-retardants are based on phosphor compounds included in the resin system or added separately. Also a significant number of modern halogen free materials tend to include fillers. It appears that there is again a significant change in the formulation of materials entering the laminate market so that they are able to fulfill the halogen free directive. This means that the PCB manufacturer has to assess the impact on the production process and make suitable adjustments.

Implications for Process Qualification The proliferation of new laminates means that many PCB manufacturers and their suppliers must modify and re-qualify their production processes. Thermal cycle testing, interconnect stress testing and thermal shocking are the predominant test methods for thermal robustness. More complex test vehicles which are designed to ensure more stressful conditions through the inter-