What Do You Know About Solder & Flux

Every integrated circuit, passive component, wire, and connector is cleaned with an acid called flux and attached to printed circuit boards with a carefully engineered combination of metals called solder

As the temperature of a pure substance, such as water, increases, it will undergo phase transitions at specific temperatures and pressures. In alloys, the discrete transition temperatures that exist between the solid-phase α and liquid-phase L morph into transition temperature ranges as a new intermediate phase of matter appears that is part liquid and part solid α+L.  If you’ve ever seen dirty snow in the wintertime, you’ve likely seen that the intermediate phase appears as slush in a puddle.

Having an assortment of solder to choose from allows a variety of benefits, the first of which is that it allows multiple reflow cycles during the PCB assembly process. To form a proper bond, the metallic pad of an IC must form bonds with the solder, and the solder must then form bonds with the copper on the PCB pad.  This cannot happen if there is any oxidation on the surface of either the part or the pad of the PCB.  Flux is used to cleanse pads immediately before the solder covers the joint.

Flux is used wherever solder is used to both clean the base materials and to decrease the surface tension of the solder.  It is a collection of molecules that can strip oxygen atoms from the surface of a base material and provide an atmospherically impervious surface layer that prevents new oxygen molecules from bonding.  In electronics, flux is mixed with the solder. 

Base materials, such as copper, start to react with oxygen in the atmosphere the moment they from etching/electroplating tanks at the fabrication house.  The oxygen binds to the surface copper molecules on the outer-sides of the PCB and prevents the solder molecules from bonding with the copper. Even if the oxide layer is sanded from the surface of the copper, it will reform almost immediately upon exposure to room air.  A layer as thin as one or two molecules is sufficient to keep solder from properly bonding to copper.

The flux is designed to melt at a lower temperature than the solder so that it has time to react with the base metals.  It spreads along the surface of a material and then floats atop the solder metal when the solder begins to spread.

 

For more details on the phase changes of mixtures and alloys, different types of fluxes, melting temperatures, and wetting characteristics download this free whitepaper.

Recent Posts

Summit Interconnect Acquires Royal Circuit Solutions and Affiliates

The combination solidifies Summit’s position as one of the largest privately owned printed circuit board (PCB) manufacturers in North America with a  footprint that will now encompass eight manufacturing facilities. The acquisition significantly broadens the scope of Summit’s product offering while expanding the company’s business portfolio of key customers and end-markets. Summit Interconnect is pleased… View Article
Read More

Tech Chat with Thermal Expert, Mike Jouppi

Ever wondered how many vias to use in a thermal package? Or how to route high current traces in small spaces? Good news! Mike Jouppi is back! Check back for a recording of this important PCB resource. In the meantime, feel free to download the Thermal Resistance calculator discussed live.  Download Thermal Resistance Calculator
Read More

Flex PCBS: Design Tips and Material Selection Advice – On-Demand Webinar

In today’s high-tech world, electrical engineers have many choices when it comes to designing their PCBs, including what type of laminate to use. Laminates form the foundation for a high-functioning PCB. In this on-demand webinar, learn how to select the right laminate for your project based on material properties, suppliers, types and more.
Read More

Share

Share on twitter
Share on pinterest
Share on facebook
Or use this public share link