Although the solder mask is widely used in current advanced organic substrate material in IC packaging, its application introduces an additional material with characteristics that are well inferior compared with the substrate resin. The solder mask Tg is ~105°C, well below the specification of "high" Tg substrates and within the specified operating range of many IC products. Moisture absorption is relatively very high, often limiting reliability of the entire package to MSL III or above. CTE at relatively low temperatures is very high, causing reliability failures like cracks in the conductive copper traces, especially when very fine pitch geometries are used.

UltraTera Corporation (UTC) is now introducing advanced organic substrates based on Driclad™ resin, where the same Driclad™ of the substrate core performs the masking function of solder mask type materials. The result is an organic substrate with preserved material properties and high dimensional stability.

The Rmask™  is a patented process that totally eliminates solder mask from the substrate, replacing it with pure Driclad™  resin. The conceptual cross sections of a general purpose PCB before and after the Rmask™  process are shown in Fig. 1. Vias filled with solder mask present very poor reliability due to voids. The more reliable way of filling vias with epoxy based paste is unfortunately quite expensive, requiring extra manufacturing steps, including chemical-mechanical polishing for planarization.

The superiority of Rmask™ substrate over the solder mask substrate is clearly depicted in Table 1.

Reliability
Rmask™ substrates have been successfully qualified at MSL I/II. The pre-conditioning flow of the samples is shown in Table 3.