1. A method for inspecting solder joints, the method comprising the steps of: illuminating solder joints on a printed circuit board and obtaining sample images of the solder joints with a charge coupled device (CCD) camera; classifying the sample images by an inspector into a number of classes according to the soldering quality; inputting a specific sample image belonging to each class to a neural network to divide the class into a predetermined number of clusters; determining a synaptic weight of each class by training each cluster; adjusting the synaptic weight according to a boundary condition between the clusters belonging to neighboring different classes and determining the adjusted synaptic weight as a confirmed synaptic weight; and selecting a similar class by comparing an output of the neural network based on the confirmed synaptic weight and outputs an of inspector's class, wherein the step of determining the synaptic weight includes the steps of generating a new output neuron when the similarity between a current input and a preset synaptic weight based on a previous input is beyond a predetermined range, generating a new cluster when a new output neuron is generated, and causing the current input to belong to the new cluster, and wherein in the step of adjusting the synaptic weights, only when an input of one class takes the synaptic weight of another class, the synaptic weight of the latter is altered in a direction moving away from the input of the former.