Lead-tin solder is a useful piece of evidence from a crime scene and may be examined for information related to the construction or source of an improvised explosive device (IED). A technique based on electrothermal vaporization into inductively coupled plasma optical emission spectrometry was improved for the direct quantification of trace and major elements in solder (Ag, As, Bi, Cr, Fe, Sb, Sn). NIST 1728, a tin-alloy certified reference material, was used for external calibration and achieve a direct, fully solid-sampling procedure using only 0.5–3.0 mg of sample. Point-by-point internal standardization with Ar 404.442 nm was performed to compensate for sample loading effects on the plasma, and a background correction technique was introduced to improve the overall efficiency of analysis. As solder was observed to change composition during some mock scenarios of IED preparation, which limits how solder can be examined in forensics, different soldering conditions (temperature, solder size, cleaning of the soldering tip or not between subsequent samples) were studied using a Fe-tip soldering device. Statistical analyses including Student’s t-tests and a one-way analysis of variance revealed that none of these conditions resulted in contamination of the melted solder sample, hence confirming the viability of the mock procedure used to replicate IED soldering in research. A new qualitative discrimination method is introduced and demonstrated in a blind trial for matching and discriminating lead-tin solders. This method represents an improvement from past research and has potential for use in evaluating other forensic evidence involving ferrous-alloys.