Authored By:
Jigar Patel, ZESTRON Corporation, Manassas, VA
Umut Tosun, ZESTRON Corporation, Manassas, VA
Summary
Today, printed circuit boards used within electronic assemblies for high reliability applications are typically subjected to cleaning or defluxing processes. As assembly complexity has increased, that is, more densely populated with greater use of stacked and leadless components and with ever reducing standoff heights, effective defluxing is increasingly challenged.
Copper traces and pads are integral to PCB design. In order to protect these from corrosion and oxidation, the PCB is covered by a solder mask. This prevents performance degradation by providing a barrier between soldered joints and other conductive elements on the PCB. As detailed in IPC SM-840D, solder mask materials applied to the printed board substrate shall prevent and/or minimize the formation and adherence of solder balls, solder bridging, solder build-up and physical damage to the printed board substrate. The solder mask material shall help impede electromigration and other forms of detrimental or conductive growth [1].
The solder mask is necessary for long term reliability of PCBs, but can its presence also impact cleaning process effectiveness? When incorporating a solder mask, the designer can specify the solder mask as either Solder Mask Defined (SMD), Non-Solder Mask Defined (NSMD) or No Solder Mask (NoSM). Although there are design considerations for using either solder mask approach depending upon component details, in general with SMD and NSMD, the component standoff height is slightly less when compared with NoSM which could impact the cleaning process effectiveness.
For this study, the authors wanted to assess the impact of different solder mask options on under component cleanliness. The solder mask specification for the substrates used in this study included SMD and NSMD as well as NoSM for comparative purposes. The solder mask used on the test vehicles employed for this study was liquid photo-imageable (LPSM or LPI) solder mask. The test vehicles were populated with numerous chip cap components with four solder paste types: no-clean tin-lead solder paste (old generation), no-clean tin-lead solder paste (new generation), no-clean lead-free solder paste (old generation) and no-clean lead-free solder paste (new generation).
All test vehicles were cleaned in a spray-in-air (SIA) inline process utilizing two different water-based engineered cleaning agents, one alkaline and the other pH Neutral. Additional variables considered were wash exposure time and wash temperature. Thus, for each solder paste used, variables included solder mask type, cleaning agent type, wash exposure time, and wash temperature. The test plan employed full factorial analysis.
Cleanliness assessment was conducted by visual inspection per IPC TM650. All components were mechanically removed from the test vehicle thereby enabling thorough under-component inspection. Localized extraction and Ion Chromatography analyses were also conducted in accordance with current IPC standards.
Conclusions
Solder mask is the most critical factor impacting under-component cleanliness. NoSM option is significantly easier to clean as compared to SMD and NSMD options.
Wash temperature & wash exposure time are critical factors which also impact under-component cleanliness.
By increasing wash temperature, we can also increase conveyor speed and achieve complete under-component cleanliness.
Initially Published in the SMTA Proceedings
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