POWER MANAGEMENT

SC2615

11

 2003 Semtech Corp.

www.semtech.com

Description

The MLP18 is a leadless package whose electrical

connections are made by lands on the bottom surface

of the component. These lands are soldered directly to

the PC board. The MLP has an exposed die attach pad,

which enhances the thermal and electrical characteristics

enabling high power applications. Power handling capability

of the MLP package is typically >2x the power of other

SMT packages. In order to take full advantage of this

feature the exposed pad must be physically connected

to the PCB substrate with solder.

Thermal Pad Via Design

Thermal data (

θja) for the MLP18 is based on a 4 layer

PCB incorporating vias which act as the thermal path to

other layers. (Ref: Jedec Specification JESD 51-5). Based

on thermal performance, four-layer PCB’s with vias are

recommended to effectively remove heat from the device.

Vias should be 0.3mm diameter on a 1.2mm pitch, and

should be plugged to prevent voids being formed between

the exposed pad and PCB thermal pad due to solder

escaping by capillary action. Plugging can be accomplished

by “tenting” the via during the solder mask process. The

via solder mask diameter should be 100µm larger than

the via diameter.

Two layer boards have no vias, thus any heat sinking must

be accomplished in the same plane as the metal traces.

This will typically require an increase in the PC board area.

Mounting Considerations

Solder Mask

Design the solder mask around all pads on each side,

i.e. there should be no solder mask between adjacent

terminal fingers.

Exposed Pad Stencil Design

It is good practice to minimize the presence of voids within

the exposed pad inter-connection. Total elimination is

difficult but the design of the exposed pad stencil is

important, a single slotted rectangular pattern is

recommended. (If large exposed pads are screened with

excessive solder, the device may “float”, thus causing a

gap between the MLP terminal and the PCB land

metalization.) The proposed stencil designs enables out-

gassing of the solder paste during reflow as well as

controlling the finished solder thickness.