PCB pad oxidation issues on production lines typically manifest as cold solder joints, solder bridges, tombstoning, and dull solder joints. The root cause lies in the formation of Cu?O or CuO oxide films on the bare copper surface; once the thickness exceeds 5 nm, it significantly inhibits the spreading and wetting of tin-lead or SAC305 alloys. Since the organic acid activators in conventional Solder Paste cannot effectively penetrate this dense layer at room temperature, targeted pre-treatment of oxidized pads must be implemented before printing. Otherwise, no matter how high the solder paste activity grade (e.g., J-STD-004B Class L3) or how precise the reflow peak temperature (235℃±3℃), it will be difficult to salvage Solder Joint Strength.

During the preparation phase, it is necessary to confirm whether the PCB storage time exceeds the exposure limit. For FR-4 substrates under 40%RH humidity and 25℃ temperature, the solderability window of bare copper pads is typically 72 hours. If inventory exceeds this period, nitrogen-protected storage or factory return for OSP rework is required. For on-site emergency treatment, prepare dilute hydrochloric acid solution (volume ratio) with a concentration of 8%±0.5%, 150-mesh nylon brushes, lint-free cloths, ionizing air blowers, and a constant-temperature oven (set to 120℃±2℃). All tools must be wiped with IPA and air-dried in advance to avoid introducing silicone oil or halogen residues, which would otherwise induce corrosion and leakage after reflow.

In terms of core parameter control, the pickling time is strictly limited to 8 to 12 seconds. Too short a duration fails to remove the CuO layer, while excessive time causes over-etching of the copper surface and leads to micro-hole defects. The brushing pressure is controlled within the range of 1.8N to 2.2N, corresponding to a brush head compression deformation of approximately 0.35mm. During operation, brush in a single direction at a constant speed with a 30° inclination angle; reciprocating friction is strictly prohibited to avoid scratching the edges of the solder mask. After pickling, immediately rinse 3 times with deionized water with conductivity below 2μS/cm, then dry for 15 minutes under 60℃ hot air circulation, and transfer to a 120℃ oven for 30 minutes of heat preservation to remove microscopic moisture. Omitting this step will cause steam-burst-type voids inside solder joints during reflow.

During the printing step of the operation procedure, synchronously adjust the stencil tension to 45N/cm2±3N/cm2, and set the squeegee angle to 58°±2° to enhance peel force. When using REL-600 series solder paste, lock the ambient temperature and humidity at 22℃±1℃ and 55%±5% relative humidity, because the sebacic acid derivatives in this solder paste will hydrolyze and fail prematurely when humidity exceeds the standard. After printing, component placement must be completed within 30 minutes; otherwise, Flux volatilization will cause oxidized pads to passivate again. The placement pressure is set to 0.12MPa±0.01MPa to ensure initial mechanical contact between component leads and pads, providing an effective stress transmission path for subsequent reflow.

During anomaly troubleshooting, if solder joints exhibit annular solder shrinkage with exposed copper in the center, it indicates incomplete pickling. Check whether the hydrochloric acid concentration is below 7.5% or the brushing time is less than 10 seconds. If solder balls accumulate at the four corners of the pad and the solder joint height is low, it is caused by the drying temperature not reaching 120℃ or the holding time being less than 25 minutes. At this point, residual hydroxyl groups on the pad surface hinder solder flow. If batch solder joint spikes occur and the thrust test is below 50gf, verify whether the heating slope in the third zone of the reflow oven exceeds 2.8℃/s. An excessively high parameter will cause the flux to be blown away from the interface before the oxide layer is completely decomposed.

For practical tips, it is recommended to use XRF to detect the oxygen content on the copper surface after pickling the first piece of each batch. A reading above 0.8wt% is judged as cleaning failure. For daily testing, stack 10 PCBs of the same model, clamp them in a stainless steel fixture, simulate aging in a 120℃ oven for 24 hours, and then conduct sampling inspection to reverse-estimate the actual solderable life of inventory PCBs. Additionally, when designing stencil apertures, add a 10μm trumpet-shaped chamfer to easily oxidized areas (such as BGA peripheral pads) to improve solder paste release rate and compensate for partial wetting loss. Finally, be sure to verify the activity of the pickling solution with a copper mirror test piece before each shift. When the mirror surface shows a uniform rainbow film instead of mottled white fog, it indicates that the hydrochloric acid concentration is still within the effective range.