Electronic devices are typically manufactured by mounting electronic components on a printed circuit board (PCB) using surface mount technology (SMT). The electronic components generally have leads, balls or conductive pads (i.e. component pads) that make electrical and mechanical contact with corresponding conductive pads (i.e. board pads) on a surface of a PCB. The process that is used for attaching the electronic components to the PCB includes the stages of solder paste deposition (i.e. printing) on the PCB through a stencil, component placement on the pasted PCB and reflow or heating the PCB. For a double-side PCB, the PCB is then turned upside down and these steps are performed again.
The PCB is manufactured by a circuit board manufacturer according to the specifications that are provided by an electronic device manufacturer. Likewise, the stencil is manufactured by a stencil manufacturer according to the specifications that are provided by the electronic device manufacturer.
Solder paste deposition involves the use of a screen printer for depositing solder paste on the board pads that are located on the surface of the PCB. In this stage, the stencil is positioned over the surface of the PCB that contains the board pads with the stencil being aligned with the PCB in a predetermined orientation. The stencil has a thickness and apertures with specific tolerances. A squeegee blade, or a Proflow™ or Rheometric™ pump, or a dispensing needle, as is commonly known to those skilled in the art, is then used to apply the solder paste to the PCB through the stencil.
Component placement includes placing electronic components, such as QFPs (quad flat package), SOPs (small outline package), chips, BGAs (Ball Grid Array), CSPs (Chip Scale Package), and the like, on the surface of the PCB such that the leads or component pads of each of the electronic components align with the corresponding board pads covered with solder paste or paste flux on the PCB surface. The reflow process consists of inserting the PCB into a reflow furnace and using a certain reflow profile for heating the PCB to cause the solder paste, and any solder that makes up part of the board pad and/or lead finish, to melt and then allowing the PCB to cool such that the solder solidifies and there is mechanical and electrical contact between component pads and the corresponding board pads. The amount of heat that is applied and the length of the heating and cooling period in the reflow profile depend on the type of solder paste that is used, the thermal mass of the product, component temperature limitations and line cycle time.
Manufacturing defects can occur during the solder paste deposition, component placement and reflow stages of the SMT process. However, typically 60 to 70% of the defects occur during the solder paste deposition stage. Accordingly, it is necessary to routinely inspect the deposited solder paste on the PCB to determine if there are any defects such as missing solder paste, improper solder paste coverage on a board pad and solder paste bridging. These defects may occur for a variety of reasons. For instance, the particular solder paste that is used may not be suitable for adhering to a board pad on the PCB given the operating conditions used in the solder paste deposition stage or the stencil may be clogged. Alternatively, there may be too much or too little solder paste that is deposited. Once again this depends on the type of solder paste used and/or the operating conditions of the solder paste deposition process. If too much solder paste is deposited, then the board pad may be in electrical contact with more than one component pad on the electronic component following the reflow process. Also, solder paste on adjacent board pads on the PCB may merge together thereby forming an electrical short circuit or bridge between the adjacent board pads. If too little solder paste is deposited, then poor mechanical and/or electrical contact between the board pad and the corresponding component pad may result. Other defects which may occur includes voids, which are bubbles that are suspended in solidified solder. A void is formed from entrapped air and/or outgasing of materials from at least one of the board pads, the component pads and solvents from the solder paste.
In addition, in surface mount technology, there is an increasing drive towards electronic components that are smaller, cheaper and provide more functionality. Accordingly, an increased number of smaller electronic components are incorporated onto one or both surfaces of a PCB. These smaller electronic components have smaller component pad sizes and smaller pad-to-pad spacing (i.e. the distance between adjacent component pads). This size reduction has stretched the capabilities of screen printing equipment and solder paste and increased the incidence of defects in the SMT manufacturing process. Accordingly, this size reduction has required changes in the various stages of the SMT process such as using a suitable solder paste and being able to deposit the solder paste on smaller board pads.