1.1 INTRODUCTION
The Linear Technology μModule™ solution combines integrated circuits and passive components in a single package. The μModule integrates several technologies to bring a cost effective, advanced solution which maximizes board space and improves electrical and thermal performance. The μModule is overmolded in a solid array and individual units are saw singulated. All μModule components are leadless with electrical connections being made through the land pad array.

2.0 MANUFACTURING CONSIDERATIONS
2.1 SMT Process
Many factors contribute to a high yielding PCB assembly process. A few of the key focus areas and their contributing factors are highlighted in Table 1.

3.0 PCB DESIGN GUIDELINES

One of the key efforts in implementing the μModule package on a PC board is the design of the land pattern. The μModule has square metallized pads exposed on the
bottom surface of the package body. Electrical and mechanical connection between the component and the PC board is made by screen printing solder paste on the PC board and reflowing the paste after placement. To guarantee reliable solder joints it is essential to design the land pattern to the μModule pad pattern.

3.1 Land Pad Styles/Solder Mask
The industry has debated the merits of solder mask defined (SMD) pads and non-solder mask defined (NSMD) pads (Copper defined), see Figure 1. Both styles are acceptable for use with the μModule package; the only limitation is the accuracy of the pads. The tolerance of the μModule defines the method best used for your assembly process (see the LTM data sheet for drawing). The land pattern design for the μModule is SMD. The SMD is used due to the large current capabilities of the μModule. For surface mounting of the μModule, NSMD pads are recommended over SMD pads due to the tighter tolerance on copper etching than on solder masking. NSMD by definition also provides a larger copper pad area and allows the solder to anchor to the edges of the copper pads thus providing improved solder joint reliability.

3.2 Land Pad Design
IPC is an industry organization with standard specifications for determining PCB land patterns. Since the μModule is a new package style, it is recommended that this application note be used in conjunction with evolving guidelines in IPC.

3.3 Design of PCB Land Pattern for Package Terminals
As a general rule, the PCB pad should be designed 0.00mm to 0.127mm larger than the package terminal pad. (Refer to the LTM data sheet drawing for recommended solder pad). Linear Technology recommends 0.20mm to 0.30mm extension of the solder pads as they exit away from the package body. This helps for solder inspection and also prevents solder balling.

3.3.1 Thermal Pad Via Design
Thermal data (θJA) for the μModule is based on a 4-layer PCB incorporating vias, which act as the thermal path between the layers. Based on thermal performance requirements it is recommended to use a 4-layer PCB with filled vias to effectively remove heat from the device.

3.4 Surface Finishes
There are a variety of surface finishes commonly available. The key factor in selecting an acceptable surface finish is to ensure that the land pads have a “uniform” surface. Irregular surface plating, uneven solder paste thickness or crowning of the solder plating can reduce overall surface mount yields. Bare Copper with an Organic Solderability Preservative (OSP) coating, electroless nickel/immersion gold or electroplated nickel/gold finishes have shown to provide an acceptable land pad surface. One type of surface finish that should be avoided is referred to as a dryfilm process. This is because the copper undercut can
cause sidewall dewetting during the reflow process.

4.0 SOLDER PASTE SCREEN PRINTING PROCESS
4.1 Solder Paste
The quality of the paste print is an important factor in producing high yield assemblies. A Type 3 or 4, low residue, no-clean solder paste (Sn63/Pb37) is commonly used in mounting LGA packages, however water soluble flux materials are also widely used. Solder paste composition is often a compromise given the variety of components which must be placed on a PCB, and special SMT specific solder pastes are being marketed by solder paste
vendors that minimize voiding in the solder joint (see section 5.2).