In this article, I will discuss choosing the right solder paste among the best available products for soldering SMDs (surface mount devices) to PCBs (printed circuit boards) especially if you are a novice in soldering components on PCBs by hand without using advanced techniques, such as a reflow oven.
Solder paste is a mixture of small solder spheres held within a specialized form of solder flux. Due to the structure of a solder paste, it can be easily applied to the board by hand during the PCBA (PCB assembly). In that regard, you can decide the solder paste you want to utilize for your board even after designing and manufacturing the PCB. However, I highly recommend learning the basic classification methods in solder pastes before scrutinizing product descriptions for choosing the right solder paste for your board. So, in the following sections, I will focus on the discrepancy between various solder pastes and the classification of them.
Firstly, solder pastes are classified by size because the size and shape of the metal particles in the solder paste determine how well the adhesive will print on the board. A solder ball in the paste has a spherical shape; this helps in reducing surface oxidation and ensures impeccable joint formation with the adjacent particles. Irregular particle sizes not acceptable, as they tend to clog the stencil, causing printing defects. To produce a high-quality solder joint, the spheres of metal need to be very regular in size and have a low level of oxidation. Based on the particle size by IPC standard J-STD 005, there are eight types of solder pastes categorized by particle size. The specifications of each type - 1, 2, 3, 4, 5, 6, 7, 8 - according to the solder ball size, are depicted in the table below.
Also, solder pastes can be categorized according to the type of flux used in the mixture of the adhesive. And, each type has benefits and limitations depending on the requirements of soldering for a PCB.
Rosin based solder pastes: Rosin based pastes consist of rosin, a natural extract from pine trees. These fluxes let designers clean the board if needed after the soldering process using a solvent - potentially including chlorofluorocarbons. In that regard, mending inaccurate solder joints is simple with it. However, rosin-based pastes are lead-free, so they have a high melting point as compared to other lead-included adhesives.
Water-soluble flux-based solder pastes: Water-soluble fluxes are composed of organic materials and glycol bases. There is a wide variety of cleaning agents and solvents for these adherents. They contain high molecular weight compounds, such as polymers, which are not as efficient as rosins/resins at preventing re-oxidation. However, water-soluble pastes give boards a cleaner look after the board passes through the reflow stage, the flux residue is burned off and easily washed off in a board washer. In other words, the residue does not remain on the board after soldering, and this type is usually selected for improving the final product aesthetics.
No clean solder paste: A no-clean flux consists of resins and various levels of solid residues. No-clean pastes save not only cleaning costs but also capital expenditures and floor space on the board. Although the no-clean flux-based solder pastes sound charming, they require a speckless assembly environment and may need an inert reflow environment.
Aside from the flux types, for advanced soldering processes, you need to consider the solder paste alloy ratio: lead content, melting temperature, alloy powder particle size, and tensile strength. Most of the time, the solder paste alloy ratio is a decision best left to your manufacturing engineer, as they are the experts regarding the reflow ovens as well as which alloy ratios approvable. Nevertheless, if you need to choose a specific alloy ratio for your board, take these steps below before the selection process:
As you can see, depending on the requirements of a project, the best solder paste for SMDs differs. Hence, you need to choose the best option for your board according to the requirements of your project. There is no perfect product working miraculously for every possible board requirement :) But, as a starting point, I leave here three leading products in the market:
1) SRA Soldering Products Rosin Paste Flux
2) MG Chemicals No Clean, Leaded Solder Paste
3) Solder Paste Sn42/Bi57.6/Ag0.4 No-Clean Lead-Free