${\mathrm{PH}}_3$ and ${\mathrm{BF}}_3$ form an adduct readily because they form

 ${\mathrm{BF}}_3$acts as Lewis acid and ${\mathrm{NH}}_3$ acts as Lewis base. Hence ${\mathrm{NH}}_3$ forms an adduct readily with ${\mathrm{BF}}_3$ through coordinate bonding. So, they do not form covalent, ionic or hydrogen bond.