LGA - Land Grid Array
07 July 2006
A new component package format which is being increasing used in the industry is an "Land Grid Array" LGA, also referred to as a QFN "Quad Flat No Lead" or MLFP "Micro Lead Frame Plastic" package. The package is generally smaller, has a low stand off height and less lead inductance than other area array devices. The package is covered by JEDEC document in MO-220 and some information on design, process and inspection criteria is available from IPC. The package size ranges between 3 - 9mm square and is generally very robust and less likely to be damaged like a normal BGA or CSP package. Most suppliers do however have Moisture Sensitive Device MSD ratings applied as they are predominantly plastic. The leads probably better referred to as pads, they are only a small section of the lead frame which is exposed from the plastic moulding. They are positioned around the perimeter of the component with the ends visible on the side of the package.
Although most packages have part of the termination on the side of the moulding it is often not solderable. This is often due to the style/method of part production. There are two techniques used, punch and wafer sawing. These lead to the terminations on the side of the package being exposed or slightly recessed, tinned or not.
The package is soldered to the printed board assembly using a standard reflow process. Typically solder paste is printed with a 0.005-0.006" stencil on to the surface of the pads. The component is placed on the surface of the paste prior to reflow. Reflow can be conducted by convection or vapour phase. Normally the pads on the surface of the printed board are under the package body corresponding with the exposed leadframe pads on the part. There is often a small pad extension away from the component body outline so satisfactory reflow can be observed. The use of a pad extension in the design also allows one or more pads to be reflowed again with a simple soldering iron.
In addition to the perimeter terminations on the device a centre thermal land may also be included; this is also soldered to the board. The package may in this case be referred to as a thermally enhanced design. The outer pads are printed with solder paste with a one to one or slight reduced apertures on the centre pad due to its size has a different stencil design. Due to the area of the pad a full area print may cause lifting during reflow, it would certainly lead to voiding under the package. Engineers have experimented with different stencil designs favouring either an array of small dots, four equally sized square apertures or triangular apertures. With this large area breaking up the stencil aperture also aids print quality and eliminates paste scooping. Voiding with this package can be exaggerated by the use of thermal vias in the printed board design, based on the thermal performance of the device vias may incorporated in to the land on the board directly below the package.
The images show a package, component after placement onto paste, selected lead-free joints after reflow and an x-ray image of selected joints.
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