Residue Formation Mechanisms in BGA Ball Mounting on OSP-Coated Cu Pads : Influence of Acid Value of Flux, Thermal Impact, Ball Mounting Process Design, and Incoming Pad Condition

Organic Solderability Preservatives (OSP) are widely used as surface finishes for Cu pads in Ball Grid Array (BGA) packaging due to their cost-effectiveness and compatibility with fine-pitch designs. However, during the BGA ball mounting process, elevated thermal exposure can trigger complex chemical interactions between the OSP film and water-washable flux, leading to the formation of persistent residues. These residues are often difficult to remove via conventional water jet cleaning, posing risks to surface cleanliness, interconnect reliability, and downstream process compatibility.

 This study investigates the mechanisms of residue formation on OSP-coated Cu pads during BGA ball mounting, focusing on four key process parameters:

1. (1) Flux chemistry, using commercial water-washable fluxes with varied acid values and activity levels; 2. (2) OSP thickness, ranging from 0 μm (bare Cu) to 0.2 μm; 3. (3) Thermal exposure, with emphasis on differences in cumulative heat history from upstream assembly stages from wafer grinding to ball mounting(in multiple chip packaging); and 4. (4) Ball mount process flow, comparing one-step versus two-step mounting approaches to assess the impact of multiple reflow for OSP coatings.

Additionally, this work investigates the visual and chemical characteristics of incoming Cu pads, which were categorized by color—pink, light yellow, and dark yellow. These color differences are considered qualitative indicators of surface oxidation state or prior chemical exposure, which may influence flux interaction, OSP decomposition, and residue formation tendency.

Surface analyses using Focused Ion Beam (FIB) cross-sectioning, Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM/EDX) revealed that residue formation is primarily driven by the thermal-induced chemical conversion of the OSP layer, with incomplete residue removal as a secondary contributor. Under prolonged thermal exposure or repeated heating/reflow, the OSP film undergoes polymerization, forming a Cu-rich “pearl layer” structure that entraps or react with flux decomposition products and becomes increasingly water-insoluble. Extended thermal pre-conditioning, such as die-attach and wire bonding heating prior to ball mount, was found to accelerate this OSP degradation mechanism.

Importantly, this study also observed that residues were present both inside the solder joint (Cu pad) and on the surrounding substrate surface after pre-cleaning process in ball mounting flow(two-step ball mounting porcess). EDS analysis of external residues which out of ball pads revealed organic decomposition compounds as well as detectable Cu signals, suggesting that OSP breakdown products reacted with Cu may extend beyond the intended soldering area during thermal processing and after water jet clean. Furthermore, pads subjected to repeated or long-duration thermal exposure showed a higher likelihood of both internal and external residue formation, especially in regions with minimal clearance.

Interestingly, the one-step ball mount process design was found to consistently yield lower residue levels compared to two-step sequences, likely due to solder ball has been placed in system, induce the reaction between Cu and Sn, to form IMC(intermetallic compound) which acquire Cu element in ““pearl layer”” fast than OSP decomposition products. Cleaning outcomes were notably improved under this configuration, with significantly fewer residues observed under optical and SEM examination.

These findings highlight the intricate interplay between OSP stability, flux activity, thermal history, and process design in BGA ball mounting. The study confirms that thermal-induced degradation of OSP, especially after extended pre-conditioning from die attach and wire bonding stages, plays a pivotal role in forming insoluble, Cu-rich residues both inside and outside the solder joint area. Furthermore, the one-step ball mount process demonstrates a clear advantage in reducing residue formation, likely due to the rapid consumption of exposed Cu by solder alloy to form intermetallic compounds (IMCs), thereby minimizing the opportunity for OSP decomposition products to interact with Cu and form persistent residues. This integrated approach—careful control of thermal exposure, flux selection, and optimized mounting sequence—provides a practical pathway for improving soldering cleanliness and reliability in high-density packaging environments.