Mixtures for attaching a semiconductor chip to a substrate. The semiconductor chip has an array of joining material bumps, such as C4 solder balls. The substrate has an array of conductive pads corresponding to the array of joining material bumps. In a first embodiment the fixture has a body having a first cavity containing the semiconductor chip and a second cavity for containing the substrate. The substrate is placed over the semiconductor chip with the conductive pads opposing and in contact with the joining material bumps, such that during reflow of the joining material bumps, the weight of the substrate acts against the joining material bumps and aids in the attachment of the semiconductor chip to the substrate to form electrical connections therebetween.