Analysis on resin removal in laser drilling of printed circuit board

Abstract

On laser drilling of printed circuit board, one-dimensional non-steady heat conduction problem of two-layers composed of resin and metal was analyzed with the finite element method (FEM), considering resinous evaporation. Validity of analysis method was verified by comparing with experimental results. The main conclusions are as follows: (1) Proportionality between latent heat of evaporation and absorptivity can be obtained using the experimental results of removal quantity when resin is thick enough. These values can be estimated simultaneously by comparing analytical results by FEM and experimental results. (2) An equation for rough estimate of maximum removal velocity was derived. (3) Removal quantity is proportional to the number of shots, but its proportionality is lost after layer thickness reaches to absorption length, yielding smear in practical use. Residual smear is caused by heat conduction to substrate during pulse. (4) Removal quantity depends on fluence most, but hardly depends on repetition rate. When resin thickness is larger than absorption length, removal quantity per pulse is proportional to fluence. (5) Removal velocity becomes larger with shorter pulse width. When resin thickness is smaller than absorption length, residual thickness increases as pulse width becomes long.