Improper reflow profiles make cleaning harder, if not impossible.
FIGURE 1a and 1b show QFN packages that have been reflow-soldered to boards, then mechanically removed in order to examine the flux residues under the body of the component and terminations. Many debates have been held on effective cleaning under this low-standoff package. It is clear in these examples that flux residues remain.
Experience shows that, with the correct paste reflow profile, flux residues can be cleaned. Still, flux residues must also be cleaned with the correct chemistry and cleaning process, else the results shown in Figures 1a and 1b are to be expected. The images show a combination of unsuccessful cleaning, practical solubility and white residues were not soluble in the cleaning process and remain on the parts and the board.
Assessments are needed for new parts and alloys to ensure reliability.
Solder joint failure on QFNs may occur for several reasons. These include:
The rate at which solder joints have been found to fail is due to thermal expansion of the solder alloy, joint height, temperature range, size of package, and size of die in package. These reasons for failure also relate to the product design and substrate thickness. To confirm product reliability for a specific environment, engineers need to undertake reliability assessments on any new component types and alloy combinations. The SEM images (FIGURE 1) were taken after 1000 cycles between -55° and 125°C with no apparent visual damage. Microsections did detect some level of cracking in selected joints. It’s fair to say many of these packages are used today, but when the package size increases, often the basic reliability questions are not being asked.
Imaged solder mask is preferred to filled vias to reduce voids and volatiles.
The QFN examples incompare soldering with and without through vias in the center pads. The difference is the correct paste stencil design and the use of solder mask around the vias. This prevents solder lost to the vias and has been shown to reduce void formation during reflow with convection and vapor phase soldering without the need for a vacuum.
Too much heat during reflow can force solder out of a BTC.
FIGURE 1 shows examples of solder beads after reflowing bottom termination components (BTC). The beads are related to placement force prior to reflow, where the solder paste deposit is displaced away from the pads before reflow soldering. It is uncommon, but solder beads have also been seen coming from the package itself due to excessive heat during reflow. Solder beads or balls on the side of packages and close to the board surface are related to paste printing, paste volume, stencil design, PCB pad size, placement force or reflow, and can easily be demonstrated.
Is it the mask, or is it the gold-plating underneath?
Peelable masking has been used in the past to protect gold key pads during soldering or from solder spitting during reflow, which leads to solder wetting spots on some terminals. This, in turn, may be a cosmetic issue, but also may affect the operation of the contacts.
In FIGURE 1, the peelable coating reflects poor adhesion of the gold to the surface of the pads. This problem is related to the preparation of the contact pads prior to gold or nickel plating and was not related to the assembly process or mask. Testing for gold adhesion using IPC methods showed a total lack of adhesion of the plating.