The formation of reliable solder joints in replacing a QFN package is critical. The contrast between large exposed pad and small lead fingers of the QFN can present a challenge in producing an even solder line thickness. To this end, careful consideration must be applied to the stencil design. The stencil thickness, as well as the etched pattern geometry, determines the precise volume of solder paste deposited onto the device land pattern. Stencil alignment accuracy and consistent solder volume transfer is critical for uniform reflow-solder processing.
QFN Stencil from Polyimide
Miniature Metal Rework Stencil
There are a variety of QFN stencil types that can be used to rework such a QFN location. These types include but are not limited to:
- A miniature metal stencil which will print solder paste onto the PCB
- A miniature plastic stencil with “repositionable” adhesive that allows for printing in tight spaces
- A QFN stencil placed on the bottom of a device which allows for the bumping of a part prior to placement
Miniature stainless steel QFN stencils are durable stencils which will mimic the initial manufacturing process and deposit a uniform volume of solder paste without smearing. Apertures in the center ground region are reduced in volume anywhere from 30-50% in order to avoid the lifting of the device in the center thereby causing the periphery IO pads from being no connects..
The optimum and reliable solder joints on the perimeter pads should have about 50 to 70 μm (2 to 3 mils) standoff height. Assembly studies on various QFN packages have shown that standoff height is primarily determined by thermal pad paste coverage. The floating effects of the perimeter pads were minor and not a significant factor in determining standoff. It was also determined that not enough paste on the thermal pads could lead to inner row bridging due to the reduced standoff height. Area ratios and aspect ratios of 0.66 and 1.5, respectively, should not be exceeded, to maintain proper stencil design.
- The land pattern on the PCB should be 1:1 to the land pads on QFN package.
- The thermal pad design on the PCB should have 50-70% paste coverage and use some type of reduction pattern
Maintaining a web thickness between openings of 0.200 mm will allow space for flux volatiles to escape, thus minimizing voids.
An offshoot of this paste print and remove stencil is the peal and release adhesive backed plastic stencil. This QFN stenciling technique overcomes the problems associated with the mini metal stencils including their lack of coplanirty, the lack of getting even pressure and the resulting lack of consistency in pack volume in to each of the apertures. This stencil design insure that the stencil is intimate contact with the PCB with the added advantage of being able to be fitted in small spaces in a PCB.
QFN Bumped Device
A new type of rework as outlined in IPC 7711 126.96.36.199 makes the job of reworking QFNs simpler by “bumping” the bottom of the device using a “bumping” stencil. After the leads on the bottom of the QFN are bumped the device is placed either using a rework system (just like you would place a reworked BGA) or a stencil placed on the board which “fits” the bumps onto the correct pad locations on the PCB. This results in less voiding on the center ground area of the part and lifts the part off higher from the PCB allowing for cleaning to more easily take place underneath the QFN.