Base board for printed circuit board

A base board for a printed circuit board, which has an insulating layer containing 5.0 to 0.00001% by weight of at least one compound selected from the group consisting of compounds of the formulae (1), (2) and (3), ##STR6## wherein R.sub.1 is hydrogen, alkyl, substituted alkyl, aryl or substituted aryl,

• each of R.sub.2 and R.sub.3 is, independently of the other, hydroxyl, amino, carboxyl, sulfone,
• a salt of hydroxyl, a salt of amino, a salt of carboxyl, a salt of sulfone,
• hydroxyl to which alkyl or substituted alkyl is bonded,
• amino to which alkyl or substituted alkyl is bonded,
• carboxyl to which alkyl or substituted alkyl is bonded,
• sulfone to which alkyl or substituted alkyl is bonded,
• hydroxyl to which aryl or substituted aryl is bonded,
• amino to which aryl or substituted aryl is bonded,
• carboxyl to which aryl or substituted aryl is bonded,
• sulfone to which aryl or substituted aryl is bonded,
• hydroxyl to which acyl or substituted acyl is bonded,
• amino to which acyl or substituted acyl is bonded,
• carboxyl to which acyl or substituted acyl is bonded,
• sulfone to which acyl or substituted acyl is bonded,
• a salt of alkyl, a salt of aryl, a salt of acyl, halogen, alkyl, substituted alkyl
• aryl or substituted aryl,
• and each of j and k is an integer of 0 to 3, ##STR7## wherein each of R.sub.4 and R.sub.5 is, independently of the other, hydroxyl, saccharide, substituted saccharide, alkoxy, substituted alkoxy, acyl, substituted acyl, acyloxy, substituted acyloxy, alkyl, substituted alkyl, aryl or substituted aryl, and each of m and n is an integer of 0 to 4, provided that m and n are defined by m+n.gtoreq.1, and ##STR8## wherein each of R.sub.6 and R.sub.7 is, independently of the other, hydroxy, amino, sulfone, nitro or a salt of any one of these, provided that at least one of R.sub.6 and R.sub.7 is sulfone, nitro or a salt of these and each of m and n is an integer of 0 to 4, provided that m and n are defined by m+n.gtoreq.1;
• wherein said insulating layer comprises a laminated product of a prepreg formed from a reinforcing substrate and a thermosetting resin, and said thermosetting resin comprises an alkylthioxanthone of the formula (4), ##STR9## wherein each of R.sub.8 and R.sub.9 is alkyl, each of p and q is an integer of 0 to 4, and p+q.gtoreq.1.