-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Selective Soldering in Lead-free Assemblies
December 31, 1969 |Estimated reading time: 7 minutes
Most people are familiar with selective soldering, but may not completely understand its benefits. This article discusses the advantages and disadvantages of the selective soldering process, and explains why many companies are making the move toward selective soldering in a lead-free electronics manufacturing world.
By Todd O’Neil, Juki Corporation
By now most people have heard about and know what selective soldering is. However, there are still people that do not understand the importance and advantages of it. Selective soldering is a process used to solder the leads of thru-hole components, connectors, and other odd-shaped devices to a circuit board. In the past, selective soldering may not have been as feasible due to the amount of thru-hole components causing long cycle times on PCBs. For this reason, traditional wave machines continued to be the preferred method.
Unfortunately, when soldering with a traditional wave, there still is a need for expensive fixturing or additional process steps, such as gluing and curing. Gluing and curing require additional machines, people, time, and costs. Today’s needs are quite different.
Figure 1. Selective soldering system with dual pots and four mini-waves.
For years, it has been said that thru-hole was going away; but now everyone surely realizes it has not, and probably will not. However, there is a substantial decrease in the amount of thru-hole components on most PCBs assembled today. As a result, it does not make sense to spend much money or resources on fixturing for each assembly, or the additional processes of gluing and curing. This adds up to a significant amount of money and time. After all, many boards may have only a few thru-hole connections on them. This means the board must run in a traditional wave or be manually soldered. Manually soldering can lead to inconsistencies in both visual and quality aspects.
Many companies are moving toward selective soldering, which allows users to do exactly as the name implies - “selectively solder” only areas of the board with thru-hole components - instead of the entire board. Because the machine is “selectively soldering” the components, it makes sense to “selectively flux” those components as well. That is why selective soldering machines also have various options for fluxing, including stationary drop-jet fluxers, stationary spray fluxers, and X/Y-controlled MD and spray fluxers. Because of the different alloys, multiple flux tanks and nozzles can be used at the same time so that the correct combination can be achieved without changeover.
Often, pre-heating is not needed for selective soldering, but most systems are equipped with a pre-heater option, or it can be added. Various types of pre-heaters can be used individually or in combination, for example IR, convection, and quartz. Top and bottom heaters are also available; final configurations depend on the customer’s needs. Often, however, the only pre-heat needed is nitrogen pre-heat coming from the tank and through the nozzle. All selective soldering systems offer this. One key and beneficial option is a nitrogen system that is programmable for use only in areas where needed so that excess nitrogen is not wasted.
This system also has its own heating element to allow additional forced-air nitrogen to be adjusted up to 250°C, and blow on the board and the wave continuously. With higher eutectic temperatures and different flow characteristics for lead-free solder alloys, the ability to apply forced-hot-air nitrogen on the PCB and wave during the process is critical.
Using nitrogen while soldering helps in many ways. It allows users to lower the solder pot temperature when using lead-free. This is important to reduce thermal shock to the PCB and components. It also reduces dross amounts substantially and, if using SAC 305 alloys, this can equal a significant amount of money. More importantly, the final results matter - there will be less delaminating, less contamination, and less bridging and rework.
With the RoHS Directive in effect, companies are forced to buy a new wave, convert an old wave, or possibly get a replacement solder pot for their existing wave system. However, the most advantageous solution may be to choose a selective soldering machine. On a single selective soldering machine, soldering can be done with up to six individual mini-waves. The system will also offer up to an 18" traditional full wave, have miniature stamping nozzles, and have dual pots for both tin/lead and lead-free solders. While the original decision to purchase a selective soldering machine may be based on the need to meet new lead-free standards, it is easy to configure selective soldering machines to incorporate the full wave process as well - improving process flexibility while reducing equipment footprint and maintenance costs. The advantage is that while a user still has the original wave machine that can be used for tin/lead soldering, there also is the ability to solder both alloys in the selective machine without a changeover.
Tin/lead assemblies can run, and lead-free product can follow immediately by loading the program. Many manufacturers opt to continue running tin/lead on a wave machine, and then run certain products on the selective solder machine over the mini-wave using the tin/lead pot. They then may run new products requiring lead-free over the lead-free mini-wave or the full wave (up to 18"). With lead-free, there are demands for changes in the machines’ physical characteristics - the materials from which solder pots, pumps, impellers, and nozzles are made. Each supplier uses what he believes is best. For example, some companies use a process called “tenifer nitrate,” which is the same process implemented on GLOCK handguns used by military and police worldwide. This process is embedded into the steel, and hardens it to a level on the Rockwell scale of hardness just below a diamond. Testing on the GLOCK handgun requires firing a minimum of three million rounds to ensure that there is no damage to the barrel. Some companies also use a repetitive testing process on nozzles and pumps within selective soldering systems.
Figure 2. Examples of a mini-wave and a full wave.
If high-volume production and cycle time is important, there are several options for high-speed selective stamp soldering machines with cycle times as low as 25 seconds.
When using selective soldering, there is more control over the process compared to a traditional wave machine. There are different speeds that can be switched during the process, different Z heights, slow dips, peel-a-ways, etc. It is easy to fine-tune the process; however, there are additional tools to aid this. For example, a live endoscope camera can be used to view the soldering as it happens. When using live view of the soldering, there is no guesswork during fine-tuning. If there is an open or a bridge, it is simple to see the reason, such as the Z is too low or the process line speed is too fast.
Easier programming on all industry machines is a common demand. As a result, it is important to have an interface that is simple enough not to require an engineer to program or operate the machine. With an hour or two of training on the interface, it should be simple and straightforward enough to create new programs.
Programming can be done on the machine or at an off-line location. Both methods use identical software. The programming starts by importing an image file, such as .jpg or .bmp, or Gerber files. Once the file is imported, the existing nozzle library can be programmed.
Conclusion
The transition to lead-free is well underway, and in some cases, already apparent. However, many companies are not prepared to operate in a lead-free environment. In addition to lead-free considerations, the increase in double-sided PCBs, coupled with the decrease in thru-hole components, requires a different way of thinking about the soldering process. One solution is to convert existing wave equipment or purchase new wave equipment to handle lead-free applications. While it is possible to convert existing products for lead-free compatibility, it would require retrofitting the equipment with solder pots, solder pumps, impellers, and nozzles that would require lengthy downtimes and be a large investment in existing equipment. Purchasing a new wave soldering machine for lead-free would require additional equipment, floor space, increased nitrogen consumption, and still require gluing and costly fixturing.
The optimal solution may be to purchase selective soldering equipment. This solution would require less floor space, and, therefore, would require less maintenance. It would give the added flexibility to combine both a selective soldering process with a traditional full wave process in one machine. A selective soldering solution could also be configured with dual pots to allow for both lead and lead-free soldering in one machine without a lengthy changeover. A selective soldering solution would ultimately use less nitrogen and energy, would not require expensive fixturing, and would eliminate additional timely process steps. The advantage of selective soldering equipment is that it bridges the current processes used in electronic manufacturing seamlessly.
Todd O’Neil, selective solder product manager, Juki Corporation, may be contacted at (919) 460-0111; e-mail: toneil@jas-smt.com.