Devices can be recovered from scrapped assemblies for reuse in the same way. Imperfect connections of devices which are not themselves faulty, which work for a time and then fail, often triggered by thermal expansion and contraction at operating temperature, are not infrequent.Īssemblies which fail because of bad BGA connections can be repaired either by reflowing, or by removing the device and cleaning it of solder, reballing, and replacing. Bad connections produced during assembly can be detected and the assembly reworked (or scrapped). Connecting pins are not accessible from the top for testing, and cannot be desoldered without heating the whole device to the melting point of solder.Īfter fabrication of the BGA package, tiny balls of solder are glued to the pads on its underside during assembly the balled package is placed on the PCB and heated to melt the solder and, all being well, to connect each pad on the device to its mate on the PCB without any extraneous solder bridging between adjacent pads. The surface tension of the molten solder, which is on the board's solder pads, tends to pull the device into precise alignment with the pads if not initially positioned totally correctly.īall grid arrays (BGA) and chip scale packages (CSA) present special difficulties for testing and rework, as they have many small, closely spaced pads on their underside which are connected to matching pads on the PCB. High quality demands or specific designs of SMDs require the precise application of solder paste before positioning and soldering the unit. Reliable solder joints are facilitated by use of a solder profile which preheats the board, heats all the connections between the unit and the PCB to the melting temperature of the solder used, then properly cools them. The smaller the pitch and size of the components, the more precise working must be.įinally the newly placed SMD is soldered onto the board. The precise placement of the new unit onto the prepared pad array requires skillful use of a highly accurate vision-alignment system with high resolution and magnification. A soldering iron or hot air gun can be used with desoldering braid. It is quite easy to remove these residues by heating them to melting temperature. The pad array on the conductor board should then be cleaned of old solder. Reflow profiles or processes that are not well optimised can lead to poor or non-wetting, damaged components or cold soldering.Īfter soldering the boards are inspected in detail.Heating a single SMD with a hot-air gun to melt all solder joints between it and the PCB is usually the first step, followed by removing the SMD while the solder is molten. The processed board is cooling down in the last zones of the reflow oven, the solder alloy solidifies and makes the solder joints. The component that has the lowest maximum temperature allowance will define the maximum temperature. This maximum temperature must remain under the maximum allowed temperature of the components. In the third section the solder paste is melted and the process reaches its maximum temperature. The Temperature gradient is important because too quick changes of temperature can cause damage to components.ĭuring the thermal soak the flux is activated to reduce oxidation of pads and leads of components and improve wetting. The aim of the preheating stage is to accumulate heat smoothly in the board and the components.

Reflow solder profiles usually have four stages, preheat, thermal soak, reflow and cooling. The solder profile is influenced by the number of layers in the PCB, the copper distribution on the board, the number and size of components. Solder profiles need to be optimised for each individual assembly job.

Multiple segments help to get an optimised heat transfer and minimum temperature differences between different points on the PCBA. These are long convection ovens with many heating zones that together create a soldering cycle.Įach heating zone of the oven has a regulated temperature set to the solder profiles for the assembly process. The solder paste melts during this process and cools down again to create a good solder joint. During the Pick & Place operation, components are glued to the board by the solder paste.Ī reliable connection is made during the reflow soldering process.

Solder reflow station