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PCB-June2015

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12 The PCB Magazine • June 2015 tion containing hydrazine [4] . While this can be effective at improving adhesion of electroless copper films, hydrazine is extremely hazardous and challenging to handle safely. In addition, many material types, such as those that contain adhesive bonding layers, are incompatible with strong alkaline solutions. Due to the fact that most surface treatments are ineffective, or not practical or compatible in some situations, it is critical that the electro- less copper process provides a significantly wide processing window to alleviate blistering defects and accommodate a variety of substrate types. The most common commercially available elec- troless copper plating solutions are not designed to meet these requirements. It is known that blistering and peeling of the copper deposit is also a function of the internal stress and strain of the deposit and that additives can be includ- ed in an electroless copper solution that affect the properties of the resulting electroless copper deposits (5–7). However, inclusion of additives may affect PCB reliability and careful selection is necessary. In this study, we evaluate select ad- ditives in an electroless copper system for their influence on the deposit stress and, ultimately, their effect on the reliability of a PCB by thermal shock and interconnect stress test (IST). Experimental Electroless copper plating solutions com- prised of 0.03 M copper sulfate, 0.15 M form- aldehyde, 0.08 M metal chelator, 0.1-0.3 M so- dium hydroxide and select stress reducing addi- tives were used for electroless copper metalliza- tion. The substrates were activated with palla- dium prior to electroless copper metallization. One to 2 microns of electroless copper was de- posited onto the substrates of interest. Substrate was also processed through the aforementioned solution to increase chemical byproducts of the electroless copper reaction, represented by an increase in specific gravity, from a specific grav- ity of 1.03 to 1.10. These solutions were evalu- ated at various points within this range. PI sub- strates were used for blister evaluation. Internal stress was evaluated using a Ya- mamoto JIS-H8626 spiral contractometer with a 0.15 mm nickel and teflon coated spiral per ASTM B 636-84 (2001). A 1.57 mm thick interconnect defect (ICD) solder shock coupon comprised of 8 layers of al- ternating 1 oz. and ½ oz. copper and 1.02 mm plated through holes (PTH) was used for evalua- tion. Substrate of construction was an epoxy FR-4 with a 180°C glass transition temperature (T g ). after electroless copper metallization these cou- pons were electroplated in a commercially avail- able sulfuric acid based copper plating solution to increase the total copper deposit thickness to 28–30 microns prior to ten thermal shocks at 288°C for 10 seconds each in accordance with IPC-TM-650 2.4.13f. cross sectional evaluation was performed on seven PTHs per coupon. Reliability was also evaluated using an IST testing system from PWB Interconnect Solu- tions Inc. (Ottawa, ON CA). IST coupon design GM40001A with a thickness of 3.18 mm com- prised of 14 ½ oz. copper layers was used for evaluation. The coupon was constructed using A HIGH-RELIABILITY, STRESS-FREE COPPER DEPOSIT FOR FPC, POLYIMIDE AND RIGID-FLEx continues Figure 1: IST GM40001a. FeAtuRe

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