Reading time ( words)
Electronics manufacturing has evolved from the early days of hand soldering and wave soldering to the more modern surface mount technology (SMT) process. Most modern electronics are produced using the SMT process, but there are still some mixed technology applications, such as assemblies that are produced using both the SMT process and the wave soldering process.
The number of mixed technology assemblies has decreased over time due to advances in the SMT process and the cost savings and increased efficiencies associated with eliminating the wave soldering process.
The recent electronics industry transition from SnPb-based alloys to lead-free alloys has provided electronics manufacturers with the incentive to try to eliminate the wave soldering process step due to the following:
- Need for equipment upgrades in order to process new lead-free alloys
- Higher energy costs due to the higher processing temperatures of the lead-free alloys
- Higher material costs associated with lead-free alloys
- Increased efficiency associated with eliminating an additional process step
- Ability to avoid exposing the assembly to the additional thermal stresses introduced by the wave soldering process
Advances in the SMT process such as the development of the pin-in-paste (PiP) process, has enabled the soldering of through-hole components during SMT processing, eliminating the need for the wave process. In the pin-in-paste process, the solder paste is first printed directly on top of the through-holes on the circuit board. The through-hole component pins are then inserted into the solder paste, and completely through the through-holes. Solder materials suppliers have been instrumental in making these process advances viable by developing solder pastes with rheological properties optimized for this type of processing, increasing process yields and reducing issues such as paste dripping from the pins as the assembly moves down the SMT line, particularly in the reflow oven.
Solder material suppliers have also been proactive in developing materials and process solutions to some of the challenges that are inherent to the PiP process. One such challenge is the need to provide enough solder volume to completely fill the through-hole providing the required mechanical and electrical reliability. In many cases, due to restricted real estate on the surface of the circuit board and the intricacies of component design, it is not possible to overprint with enough solder paste (which is only about 50% solder metal to begin with) to completely fill the through-hole, especially on the inner rows of a fine pitch connector.
As board thickness increases, it becomes even more challenging to provide enough solder to fill the through-hole. One solution that has been developed that eliminates the need for a step stencil, is the use of solder preforms to provide the additional solder to the joint. Solder preforms can be stamped in various shapes from solder ribbon and are pure metal alloy.
The volume of solder required to fill a through-hole can be calculated and the appropriate sized preform can be stamped to provide enough solder to fill the through-hole. Preforms packaged in tape and reel can then be fed into pick and place equipment to be automatically placed in the appropriate locations.
Flux-coated preforms are also increasingly being utilized as part of a voiding mitigation strategy for certain die attach and bottom termination component (BTC) soldering applications where thermal management is critical. Void reduction in these joints means increased thermal conduction away from the die resulting in better die performance and longer lifetimes. Recent studies have shown that flux coated preforms used in conjunction with solder paste can give voiding levels below 5% on BTC components.1 In some die attach applications, flux coated preforms can be used in conjunction with dispensed paste flux, totally eliminating the need to print solder paste. Pick and place equipment is also now available on the market where dispensing of solder paste or flux material and component mounting can be alternately performed. This means that no separate dispensing equipment would be needed for such a process.
In the case of die attach and BTC soldering applications, the drivers leading to process elimination are improved product reliability and improved assembly process yields, but these are only made possible due to the materials and process development work done by solder materials suppliers.
1. Lifton, Anna; Sidone, Jerry; Salerno, Paul; Khaselev, Oscar; Marzi, Mike;. s.l. Void Reduction Strategy for Bottom Termination Components (BTC) Using Flux Coated Preforms. SMTAi 2017.
This article was originally published in the August 2017 issue of SMT Magazine.