Surface Mount International Conference Proceedings


Author: Joseph D. DeBiase
Company: Enthone-OMI Inc.
Date Published: 9/10/1996   Conference: Surface Mount International

Abstract: The acceptance and use of Organic Solderability Preservatives (OSPS) as replacements for Hot Air Solder Leveling (HASL) continues to grow. OSPS selectively protect and maintain the solderability of a PCB’S copper features (i.e. NUT pads, through-holes) by providing thermal protection against degradation during assembly. Historically one class of OSP, the benzotriazole inhibitors, has been successfully used by a number of major OEMS in select assembly applications requiring a single heat excursion. During the past five years a new upgraded class of OSP, the substituted benzimidazole, has proven to yield additional benefits to PCB fabricators while addressing a diversity of processing challenges encountered by PCB assemblers and OEMS. Environmental and economic factors, as well as advancements in PCB technology are the three major drivers which have ignited the change from HASL to OSPS. Environmental concerns include the desire to eliminate hazardous materials (e.g. lead) from the fabrication production environment, as well as the need to address potential legislation restricting the use of lead on the local, state, and national levels. Since OSPS are aqueous-based and do not produce hazardous waste, environmental compliance is made easier. Economically, the OSP process is substantially less expensive than HASL, which is a labor intensive and a high maintenance process. When one considers that the HASL process produces solder dross (considered a hazardous material), the disposal of the dross is both an economic and environmental burden that the PCB fabricator would welcome the opportunity to eliminate. When one compares actual processing cost, rework/repair cost, and the cost of rejects, current users of substituted benzimidazole-based OSPS have experienced a total cost savings of 25 to 50°/0 versus HASL. For benzotriazole-based coatings the savings are even more impressive. Finally, technology advancements evidenced by the advent of new and complex packaging designs such as fine-pitch, ball grid arrays (BGAs), chip scale, and flip chip have necessitated flat, planar surfaces. Another processing concern is the trend towards thinner pCBS. HASL has limited success in processing PCMCIA cards and thin PCBS. It is not uncommon for HASL to produce boards with inconsistent solder coating thicknesses, thus causing “crowned” SMT attachment areas and reduced clearance of through-holes. These deficiencies result in subsequent rework and rejects at assembly and reduce first-pass assembly yields. Conversely, OSPS are known to consistently produce thin coatings. Other problems associated with the HASL process include: intermetallic compounds formation (Cu/Sn) and the potential for poor solderabilty; warpage due to changes in temperature; and thermal shock which jeopardizes PCB integrity while escalating the potential for increased ionic contamination levels. This article will examine the considerations required in selecting benzotriazole and substituted-benzimidazole OSPS for implementation in PCB manufacturing.

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