The present inventors have observed that in some applications of reactive composite joining there is escape of a portion of the molten joining material through the edges of the joining regions. Such escape is not only a waste of expensive material (e.g. gold or indium) but also a reduction from the optimal thickness of the joining regions. In some applications, such escape also presents risk of short circuits or even fire. In this invention, two approaches are taken toward preventing damage to surroundings by the escape of molten joining material. First, escape may be prevented by trapping or containing the molten material near the joint, using barriers, dams, or similar means. Second, escape may be reduced by adjusting parameters within the joint, such as solder composition, joining pressure, or RCM thickness.