The Importance of Using the Right Solder to Ensure Electrical Reliability

Surface mount electronics used in demanding environments increases the importance of the solder joints’electrical reliability. It is important to ensure that solder joints are reliable and perform well when exposed to thermal expansion incongruities. Such discrepancies can make solder joints less reliable. To prevent these problems, manufacturers and providers of SMT (Surface Mount Technology) assembly products offer higher-quality solder material that is non-corrosive to stop oxidation, grain structure stability and prevent the buildup of fatigue damage.

Lead-free, no-clean, zero-halogen solder paste is designed for assembly processes that require passing the JIS Copper Corrosion test and residue with good pin testing property. It enables electrical reliability by making sure that fine pitch printing is accurate, down to 180µm circle printed, with a stencil that is 100µm thick. Excellent print volume repeatability adds value by minimizing defects, which occur with variability in the print process. It should achieve the IPC7095 Class III voiding performance, too.

High-quality lead-free and halogen-free solder paste ensure long stencil life and consistent performance for a minimum of eight hours when printing continuously, without adding new paste. High and long tack force life ensures good self-alignment and high pick-and-place yields, and the wide reflow profile generates the highest-quality solderability for high density and complex assemblies in nitrogen and air reflow, with soak and ramp profile temperatures ranging from 175°C to 185°C. Apart from ensuring electrical reliability, the solder reduces random ball solder levels to minimize the need for rework and to increase first time yield.

Electrical reliability in ultra-fine applications require solder that can guarantee superb printing performance down to a pad size of 180μm. This type of solder comes in lead-free, zero-halogen, and no-clean formulations for ultra-fine feature printing and air reflow. Such solder pastes are also ideal for assemblies with components that are sensitive to warpage.

Types of Solder Pastes

Solder pastes have come a long way from formulations that contained lead to lead-free and halogen-free options. Solder paste with lead is still being used in certain applications that require a heavy-duty adhesive for circuitry, but some providers of SMT (Surface Mount Technology) assembly solutions already carry a wider range of solder pastes in different alloy offerings. An example would be the low-Ag solder paste, which is known for its good soldering performance at a lower cost. With the advancement in technology, the availability of solder pastes that fits your unique process and meets environmental regulations is becoming increasingly assessable.

Tin-lead (Sn/Pb) solder paste is formulated in different alloys to ensure good print volume repeatability while minimizing the variations in the print process. Some types of tin-lead soldering alloys reduce print cycle times as they speed up print speeds while extending the amount of prints between undercleaning stencils. These pastes can withstand lengthy and hot soak reflow profiles for excellent wetting of lead-free surfaces using a tin lead paste alloys.

Lead-free solder paste performs well in high throughput applications. It can be formulated for different performance indicators, such as low temp reflow, universal high print speed, high soak profile in through hole applications, and PoP (Package-on-Package) dip paste. Lead-free, halogen-free solder paste for low temperature reflow processes improves solder joint performance for thermal cycling and drop shock cycling.

Halogen-free solder paste is designed to eliminate finished products as possible sources of halogen, the by-product of incomplete combustion, which occurs in discarded electronic products and circuit boards. The hazardous material can contain furan-like compounds, corrosive and acidic gases, and dioxin, all of which are dangerous to human health and the environment.

Assembly of PCBs (Printed Circuit Boards) used in handheld devices like smartphones and tablets, as well as in military and aerospace equipment, medical devices, network servers, consumer electronic products, and automotive systems require advanced solder pastes specifically formulated for such applications.

Why Is There a Need for Lead-Free Solder Bar?

Solder is used to establish a permanent bond or connection between metal pieces on a PCB (Printed Circuit Board.) It is made of metal alloys, with tin-lead being the most common composition (i.e. Sn63Pb37 or Sn60Pb40). However, due to health and safety issues, an on-going shift to lead-free solder bars continues to be implemented following RoHS (Restriction of Hazardous Substances Directive) and WEEE (European Union Waste Electrical and Electronic Equipment Directive). Manufacturers of electronic and electrical equipment in the US may qualify for tax benefits by going lead-free or by minimizing the use of conventional lead-based solder.

Although a lead-free solder bar may help increase the safety of workers and the environment, it may not be the best solution for certain applications where medical projects and aerospace equipment are being manufactured. Providers of SMT (Surface Mount Technology) assembly solutions continue to come up with ways to create lead-free solders that can apply to those industries. A lead-free solder bar for commercial use may contain bismuth, silver, copper, tin, antimony, zinc, indium, and bits of other metals with melting point typically ranging from 5°C to 20°C, which is higher than lead-based solder bars. However, there are manufacturers that have formulated lead-free solder that can melt at much lower temperatures.

Lead-free solder bar is commonly used in the wave soldering process. Sn96Ag4 and Sn97Ag3 alloys are preferred when there is the need to reduce or stabilize the copper content in a wave solder bath, especially when process conditions require the removal of oxides and other impurities that can increase dross generation and defects, like solder bridging. Reputable manufacturers of lead-free solder bars carefully formulate the composition of their Sn-Ag variants to ensure best-in-class yield, fast wetting speed, excellent solderability, excellent performance across various flux technologies, and reliable drainage. They use a process that controls the amount of impurities, while making sure that it meets or exceeds acceptable standards like IPC J-STD-006C.

Selecting the Right Lead-Free Alloy for Your PCB Assembly Process

Wave soldering is the widely-used method in high volume production of PCBs (printed circuit boards.) Traditionally, a wave soldering process depended upon the tin/lead eutectic Sn63/Pb37 alloy, but the new environmental safety requirements and regulations have changed this norm resulting in the introduction of environmentally friendly, lead-free alloys to enable a lead-free wave soldering process.

There are various lead-free alloys available for the wave soldering process. To choose the right one, consider the following factors:

•    The criteria for selection – Examine the process yield DPMO (defects per million opportunities), board and component compatibility, total cost of ownership, in-service reliability, and process maintenance.
•    Initial screening – Analyze the selection of available alloys and create a short list of prospective products.
•    Process yield examination – Consider the optimized process settings and the DoE (design of experiment) setup.
•    Testing reliability – Test the lead-free alloy to see how it performs under thermal cycles and soldering performance.

Process yield can be examined using DoE, with the variables such as the pad finish, Sn/Cu/Ni and alloy type SACX, solder-pot temperature, conveyor speed, the preheat temperature, and the flux. Reliability for every alloy can be determined by making the boards go through thermal shock cycling in 300 cycles with temperatures ranging from -25°C to +85°C.

Choosing a lead-free soldering alloy may be challenging. Keep in mind that the lead-free wave solder alloys have different properties. Hence, conducting extensive in-house testing can help in making an informed decision. Some of the best lead-free alloys are suitable replacements for Sn63. They can stabilize and minimize copper content in the solder bath, if process conditions call for it.