Understanding Soldering – Part 3: Solder Paste Application

I once heard comedian Rita Rudner ask the question during a performance; “How come when you mix water and flour together you get paste...and then you add eggs and sugar and you get cake? Where does the paste go?” My knowledge of the chemistry of cooking isn’t advanced enough to answer her question, but the basic form of paste is exactly as she described it and can still do quite a lot. For years it was the go-to solution for children’s art projects in grade school, and even today people will still use this simple paste to create amazing paper-mâché crafts like piñatas and masks.

You know what else paste is good for? Soldering circuit boards. OK, you’re right. The paste used for soldering components onto printed circuit boards is a much more complicated recipe then just water and flour, but it is still a paste. Solder paste is actually an inventive combination of powdered solder and thick paste-like flux used in the solder reflow process. How this is used in different solder paste applications for the assembly of printed circuit boards is something that PCB designers should know about, and which we will take a closer look at here.

What is the Paste Used for Solder Paste Application?

Electronic components were first hand soldered onto circuit boards until the wave soldering process was perfected for the mass production of PCBs. Next came the development of surface mount parts which were smaller than their thru-hole counterparts due to their lack of long leads, and eventually they became the dominant component package type used on circuit boards. The lack of thru-hole pins in surface mount parts, however, forced the development of a new method of soldering in order to hold them in place on the board until the soldering was complete. This new process is known as solder reflow, and it doesn’t use the standard pool of molten solder that the wave process uses. Instead, a sticky solder paste is used to hold the parts in place and solder them as well.

Solder paste is a combination of a powder made up of metal solder particles and sticky flux that has the consistency of putty. The flux not only does its usual job of cleaning the soldering surfaces of impurities and oxidation, but it also provides a temporary adhesive that holds the surface mount components in place. The solder powder used in the paste can vary in its chemical composition with different material types and percentages used depending on the needs of the board being soldered. For instance, solder paste is available in both lead and lead-free versions to satisfy the restriction of hazardous substances (RoHS) directive. Solder paste is also classified by the size of the metal particles that make up the solder powder. These particles must be spherical in shape and can vary in size according to the type standards specified in IPC J-STD 005.

Solder Paste Grades According to Particle Size

Type 1 Type 2 Type 3 Type 4 Type 5 Type 6 Type 7 Type 8
150-75um 75-45um 45-25um 38-20um 25-10um 15-5um 11-2um 8-2um

Once the solder paste has been decided on, the next step is getting it applied to the circuit board.

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How Solder Paste is Applied to Printed Circuit Boards

Solder paste can be applied individually to pins with a syringe, but as you can imagine, this is a very slow manual process and would never work for regular production work. Instead, circuit board assemblers will use either a stencil or jet printing to apply the solder paste.

Stencil: Using the CAD data from your PCB design tools, the manufacturer will have a stencil made with openings or apertures the size and shape of the surface mount pads on your design. These stencils are often laser cut for precision and can be built from a variety of materials at different widths depending on the components to be soldered and the amount of paste required. The stencils can be plated and coated with materials that will make the aperture openings smooth and help with the even application of the paste. A squeegee is used to spread solder over the stencil and fill all of the apertures evenly in an operation that typically takes 15 to 45 seconds per board. Once the printing process is complete, the board is inspected with optical scanning equipment to verify the quality of the applied solder paste. Next, the stencil is cleaned and the printing process is repeated for the next board.

Jet Printing: Although stencil printing does a great job for large production runs, it takes time to get a stencil built and there can also be difficulties controlling the correct amount of solder for each component. Jet printing solder paste, on the other hand, is made from a different formula so that the paste is more liquid, resolves these issues. Using the same PCB design data that a stencil is manufactured from, the jet printer deposits the correct amount of solder on each surface mount pad on the board. The printer ejects tiny droplets of solder paste from a cartridge which can be easily switched in and out depending on the type of solder that is needed for each application. Jet printing gives PCB assemblers a great amount of flexibility as well as the ability to precisely control the amount of solder being applied.

Once the solder paste is applied, the next step is to run the circuit board through the solder reflow system to complete the process.

Successful Solder Paste Applications Result in Good Solder Joints

Solder paste and the solder reflow process is used primarily for surface mount components. There is also the option however of using solder paste instead of wave soldering for thru-hole parts as well. Intrusive reflow solder, known also as pin in paste (PIP) or pin in hole (PIH), saves time and money because there is only one process, the reflow process. As long as the thru-hole components themselves can withstand the reflow oven temperatures, and there is enough clearance around the leads in the holes for the solder paste to be distributed, the reflow process can produce good solid solder joints on plated thru-holes.

Once the board has been prepared with solder paste and the components have been installed, it is ready to be reflowed. The board will travel through a series of industrial convection ovens that regulate different temperature levels to complete the entire reflow process. The board will first be heated to a specific temperature, and then held at that temperature to activate the flux in the solder paste in order to remove the oxides on the soldering surface. Then the board will be heated again for the solder to reflow, after which the board will go through a cooling process to solidify the newly formed solder joints.

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At Tempo Automation, we have many different solder processes available, and we are ready to work together with you on the process that will best fit the needs of your prototype and/or high-volume PCB manufacturing. We have the experience and the resources that you need to make sure that your design is manufactured to the highest levels of quality.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the different solder paste applications that we can provide for you, contact us.

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