5.2 Soldering and Contact Impedance

Soldering introduces flux materials that have the primary purpose of chemically removing oxides to ensure metallic surfaces are joined properly with minimal contact impedance. When soldering, there are basically two different types of flux material. There are water-soluble flux materials, which must be cleaned off after the soldering process by appropriate cleaning processes, and there are low-solid fluxes (LSF) or "no clean" flux materials on the market. For specific cleaning procedures, refer to the solder-paste manufacturer's recommendation for the specific soldering paste and flux. The main advantages of water-soluble fluxes are that they are typically the most highly active and more effective than "no-clean" products. Being more highly active, the water-soluble fluxes support a much better wetting process which leads to less soldering dwell time and consequently less thermal shock. The main disadvantage is the cost associated with the cleaning in the assembly process. The principle advantage of a "no-clean" product is lower cost, while the disadvantage revolves around post-soldering reliability.

Even when using the "no clean" products in ultra-low-power applications, PCB cleaning is recommended to achieve maximum performance by removing flux residuals from the board after assembly. The residuals left behind are typically weak organic acids and oils. The weak organic acids dissolve in water making them sources of leakage currents and failures in humid environments. Additionally the ester oil can act as an entrapment for dust which can also act as a humidity absorber. It is important to work with your manufacturer to identify sensitive or precision circuits that can be impacted by flux residuals and select a cleaning process to remove them. The flux residuals on the board can cause leakage current paths, especially in humid environments. In general, reduction of losses in the oscillator circuit leads to a better safety margin and, thus, also increases performance and reliability.

The MSP430 MCU package is qualified against JEDEC Std 020 to withstand the specified maximum peak reflow temperature allowed at certain moisture sensitivity level (MSL). This is the maximum allowed reflow profile. The solder-paste supplier usually supplies a suggested reflow profile that is within the JEDEC Std 020 maximum range. Thus, the JEDEC recommendations for the soldering profile for devices and the recommendations of the soldering materials supplier should be carefully followed, to achieve high reliability and quality solder joints.

If the MSP430 MCU package is to be exposed to any reflow temperatures after the liquid cleaning process, the board with the mounted MSP430 MCU package should be baked, to dry out the part before the following reflow process. In this case, the devices must be baked according to the JEDEC Std 020 (24 hours at 125°C) before processing through an additional solder-reflow step or performing a rework soldering.

Another important topic in the category of soldering is how to properly solder the metal housing or 'can' to the PCB. Without proper consideration of the manufacturer's guidelines, it is possible to damage the crystal from thermal stress. Adhering to specific guidelines can be hindered because of the large footprint that can remove heat from the area requiring higher temperatures or longer dwell times. Again, it is critical to adhere to the manufacturer's specification for soldering the crystal to prevent overheating and damaging the part.