4.2 Solder Stencils (For More Detailed Information See Appendix A)
The formation of reliable solder joints is a necessity. The large numbers of pads on the μModule can present a challenge in producing an even solder line thickness. To this end, careful consideration must be applied to thestencil 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. Stencils are usually made of brass or stainless steel, with stainless steel being more durable. Apertures should be trapezoidal
to ensure uniform release of the solder paste and to reduce smearing. Hence dimension A < B. Refer to Figure 2.
The solder joint thickness for the μModule pad should be 50μm to 75μm after reflow. Thickness of the stencil (C) is usually in the 100μm to 150μm (.004" to .006") range. The
actual thickness of a stencil is dependent on other surface mount devices on the PCB. A squeegee durometer of 95 or harder should be used. The blade angle, pressure, and
speed must be fine-tuned to ensure even paste transfer. An inspection of the stenciled board is recommended before placing parts; as proper stencil application is the most important factor with regards to reflow yields later in the process. As a guide, it is recommended to use a stencil thickness of 125μm (.005") for the μModule.

4.3 Pad Stencil Design
The pad opening stencil dimensions will depend on the specific μModule pad dimensions and the necessary aspect ratio. The aspect ratio relates to the manufacture of stencils. Stencil manufacturers will typically require the aspect ratios to be greater than 1.5. Reference IPC-7527. ASPECT RATIO = Aperture Width/Stencil Thickness

5.0 PACKAGE TO BOARD ASSEMBLY PROCESS
5.1 Placement and Alignment
The pick and place accuracy governs the package placement and rotational alignment. This is equipment/process dependent. Slightly misaligned parts (less than 50 percent off the pad center) will automatically self-align during reflow (see Figure 3). Grossly misaligned packages (greater than 50 percent off pad center) should be removed prior to reflow as they may develop electrical shorts, as a result of solder bridges, if they are subjected to reflow. There are two popular methods for package alignment using machine vision:
Package silhouette The vision system locates the package outline. Terminal recognition Some vision systems can directly locate on the pad metallization pattern. Both methods are acceptable for μModule placement. The terminal recognition type alignment tends to be more accurate, but is also slower since more complex vision processing is required of the pick and place machine. The package silhouette method allows the pick and place system to run faster, but is generally less accurate. Both methods are acceptable, and have been successfully demonstrated by major pick and place equipment vendors and contract PCB assembly houses.

5.2 Solder Reflow
There are no special requirements necessary when reflowing μModule components. As with all SMT components, it is important that profiles be checked on all new board designs. In addition, if there are multiple packages on the board, the profile should be checked at different locations on the board. Component temperatures may vary because of surrounding components, location of the device on the board, and package densities. To maximize the self-alignment effect of a μModule (see Figure 3), it is recommended that the maximum reflow temperature specified for the solder paste not be exceeded. A good guide is to subject the PCB to a temperature ramp not exceeding 4°C per second. The reflow profile guidelines are based on the temperature at the actual solder pad to PCB land pad solder joint location. The actual temperature at the solder joint is often different than the temperature settings in the reflow/rework system due to the location of the system
thermocouple placement used to monitor the temperature. The furnace needs to be profiled using thermocouples at various locations on the PC board. A thermocouple should be placed on one of the largest and smallest components on the PCB. It is suggested that the peak temperature differential between the smallest and largest package be 10°C or less for average size PC boards. Linear Technology Corporation provides a reflow profile on its
web site www.linear.com. Reference Jedec/IPC Standard J-STD-20b for reflow recommendations. μModules are typically moisture sensitive and fall into level classifications
defined by JEDEC. Specific levels are stated on moisture sensitive labels shipped with LTC devices. Linear Technology has tested the μModules for a specific reflow profile (245°C peak body temperature)

5.3 PCB Cleaning
Residue from the surface mount process can create resistive connections between pads on LGA packages. If a low residue, no-clean solder paste is used, PCB cleaning is not required and has little effect on a μModule. With the elimination of materials containing CFCs, most companies have moved to a no-clean or aqueous flux-based system. “No clean” fluxes and solders simply mean that there are no harmful residues left on the board that could cause
corrosion or damage to the components if left on the board. Residues have sometimes been shown to be a collection point for outside contamination on the board surface. Because there are so many different types of noclean solder pastes available, application specific evaluations should be performed to identify if any remaining residue still needs to be removed from the boards in final production.

5.4 Inspection
Inspection of a μModule on a PCB is typically accomplished by using transmission type X-ray equipment. In most cases, 100 percent inspection is not performed. Typically X-ray inspection is used to establish process parameters, and then to monitor the production equipment and process. Transmission X-ray can detect bridging, shorts, opens and solder voids. There are many different types of X-ray inspection equipment available and functionality varies. X-ray inspection system features range from manual to automated optical inspection (AOI). Different systems also provide single or multiple dimensional
inspection capabilities. As explained in section 5.1 of this guide, a μModule will self align to the land pad using surface tension during the solder reflow process. As a result, it is unlikely that a μModule will be marginally misaligned. If misalignment does occur it is likely to be by an entire pad. This effect makes it possible to do a gross visual alignment check
after reflow. Visual checks can be aided by the use of PCB fiducial marks which also aid manual placement of units during any rework.

6.0 Rework
μModule rework procedures are an adaptation (and in some cases a simplification) of Ball Grid Array Package rework procedures. The basic elements of this procedure are as follows:
• PC board bake 125°C for 24 hours
• PC board preheat 125°C
• Reflow of component solder
• Vacuum removal of component
• Cleaning and prep of PCB lands
• Screening of solder paste
• Placement and reflow of new component
• Inspection of solder joints