Undissolved flux can leave a mess under low-standoff parts.

Figure 1 shows a side view of solder joints on an LGA/QFN package. Between the terminations, evidence of flux residues is visible and would be common on a standard assembly process. Flux residues are also visible under the package.

(The typical standoff height of these packages is below 0.004˝. The example here has a standoff height of 0.008˝.)

Looking at the flux residues, it is suggested the cleaning process has not been correctly or completely reviewed. The residues look to have been through a cleaning process and are not being dissolved, or are insoluble due to poor process compatibility. A proper evaluation of the process, paste, cleaning system and chemistry needs to be conducted to confirm the residues are compatible and the cleaning system is capable of cleaning the component standoff height on this LGA/QFN package.

Dr. Davide Di Maio is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); defectsdatabase@npl.co.uk. His column appears monthly.