The stability of themulti-layer PCB material standard is the primary factor affecting thepositioning accuracy of the inner layer. It is also necessary to consider theinfluence of the thermal expansion coefficient of the substrate and the copperfoil on the inner layer of the multilayer PCB. From the analysis of thephysical properties of the substrates used, the laminates all contain polymerswhose primary structure will change at a certain temperature, commonly known asthe glass transition temperature Tg. Glass transition temperature is a uniquefunction of large number of polymers, second only to thermal expansioncoefficient, it is the most important property of laminates.
Plated through holeshave a lower natural expansion rate than the surrounding laminate. Since thethermal expansion of the laminate is faster than that of the hole body duringthe processing of the circuit board, this means that the through hole body isstretched along the deformation direction of the laminate. This stresscondition produces a tensile stress in the through-hole body. When thetemperature increases, the tensile stress will continue to increase. When thestress exceeds the cracking strength of the through-hole plating, the platingwill crack. At the same time, the higher thermal expansion rate of the laminatesignificantly increases the stress on the inner layer wires and pads, causingthe wires and pads to crack, forming a short circuit in the inner layer of themulti-layer PCB.