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PCBA Cleanliness Project Studies Impact of Cleanliness on Reliability of Bottom Terminated Components

iNEMI
by iNEMI on November 27, 2019 12:01:47 PM EST
The iNEMI PCBA Cleanliness Project recently completed a study on the impact of cleanliness on PCBAs with bottom-terminated components (BTCs). Focusing specifically on QFNs, the team evaluated different materials and test methods to identify key factors affecting reliability and evaluate trade-offs between cleanliness and performance. 
 

Cleanliness Challenges with Bottom-Terminated Components  

With higher electronic manufacturing demands, contract manufacturers are expanding their offerings to include services in the broader range of a product’s life cycle. Shrinking product form factors, along with higher component I/O densities, willFig1 WITH caption continue to drive higher placement densities. One component notable for trapping harmful residues is the family of bottom-terminated leadless components. When soldering BTCs, blocked outgassing channels can fill the underside of the component with flux residues. No-clean flux systems should leave benign surface contaminants; however, the low standoff gap, tight pitch and mass of solder can increase the levels of flux residue and create a reliability concern. 
 
There are multiple factors that can change the properties of surface contamination present on the electronic assembly, such as:                                      
  • Materials selection — chemistries of materials within the manufacturing process (solder pastes, flux chemistry, wash solution, etc.)
  • Processing parameters — settings within the manufacturing process (stencil thicknesses, nozzle pressures, factory environmental conditions, etc.)
  • Hardware selection — geometric properties of hardware used within the design (component standoff height, termination size, shape and spacing, PCB conductor thickness, etc.)
 
Materials characterization followed by methods for controlling the process are vital for reducing variation.  The following table highlights factors that can influence the harmful nature of residues left on the printed circuit board after assembly. 
 
 
 
 

Project Focus

The iNEMI project team, drawn from across the supply chain, studied the impact of cleanliness on PCBAs with BTCs. This consisted of fabricating a test board and developing a DOE to study the key issues that may contribute to residues under the BTCs. Using single-row QFN component packages (as a representative example of a BTC) on a multi-quadrant test board, the team utilized clean and no-clean solder pastes and conducted various levels of cleaning (no-clean, water clean and solvent clean). The team then conducted surface insulation resistance (SIR), ion chromatography (IC) and other testing to determine the trade-offs between electrical performance, ionic contamination levels, and cleanliness.
 

Project Conclusions

Key findings from the project were that: 
  1. Ionic contamination trapped under the component termination can cause leakage currents 
  2. Highly active water-soluble fluxes fail quickly if not cleaned
  3. No-clean flux trapped under the component termination lowers SIR values
  4. Site-specific extractions under the QFN found high levels of activators which indicates that the low standoff height and high solder mass compromise flux outgassing
 
The team determined that a more reliable product can be produced by focusing on “insulation” properties and that assemblers can use fine pitch components such as the QFN so long as material properties are well understood. Furthermore, parts can be adequately cleaned so long as impingement energy, cleaning fluid and wash time factors are understood.
 
The PCBA Cleanliness project team is recommending that more work be done to prove out test methods that can be used at the point of the manufacturing process, just prior to the application of conformal coating. Additional process control methods should also be investigated. Interested in future project work in this area? If so, please contact Mark Schaffer (iNEMI) at marks@inemi.org.   
 

For Additional Information 

The team presented a summary paper at SMTA International on September 26, held end-of-project webinars October 7 & 8, and published a project report.  These documents are all available on the project web page.   
 
 
 
 
 
 
 
iNEMI
Written by iNEMI

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