5 min read
Hand soldering is used in the manufacture of electronic assemblies where strong reliable interconnections are required, especially in military and defense manufacturing. Stringent military regulations demand high performance and long-term reliability of sophisticated electronics in extreme environments, so solder joints, the main interconnection for the performance of an electronic system, must be formed flawlessly.
TOPICS COVERED
Why Do Quality Solder Joints Require Consistent IMC Formation?
The Challenge of Hand Soldering Highly Metalized PCBs
Common Mistakes To Avoid When Soldering High Thermal Demand PCBs
Robust Solutions for Soldering High Thermal Demand PCBs
Why Do Quality Solder Joints Require Consistent IMC Formation?
A strong solder joint is the main interconnection for the performance and reliability of an electronic system. To form a strong solder joint, a proper intermetallic compound (IMC) layer between the solder and the substrate must be created. IMC formation takes place when heat is applied, causing a metallurgical reaction between molten solder and metallization on the substrate. If heating is insufficient there is no IMC formation and no solder joint. However, too much heat can cause IMC overgrowth which is also problematic. The heating profile of the solder must be carefully controlled to form a proper IMC layer and avoid “cold” or “overheated” solder joints which are both highly susceptible to cracking and failure.
The Challenges of Hand Soldering Highly Metalized PCBs
High Radio Frequency (RF) printed circuit board (PCB) assemblies, commonly used in military communication, radar, and LiDAR devices, carry high-speed, high-voltage signals and are grounded by thick metal cores and ground planes. These hefty metallic layers, while great for reducing RF noise, can be especially challenging to solder.
The soldering challenge is that the metalized components and metallic substrate layers “steal” heat from the solder joint as it is being soldered. This makes it far more difficult to maintain the optimal heating profile necessary to form a strong solder joint.
An example of a highly metalized PCB assembly with mixed surface mount technology (SMT) and through-hole technology (THT) is shown in Figure 1.
Thermal Imaging Illustrates Heat Spread
Thermal imaging helps us understand how heat is pulled away from the solder joint, flows along areas of high metallization, and is redistributed during the soldering process. Figure 2 clearly shows heat spreading in heavy metallic ground planes. Of particular interest is the yellow area in the lower-right corner of the image, indicating heat redistribution and build up.
To Understand Heat Spreaders, Imagine a Frying Pan
To help understand the heat spreading effect, think of a large copper frying pan – if heat were applied to one small point on the frying pan (equivalent to the tip of a soldering iron applied to a sheet of copper), the pan would distribute that heat until the entire pan came up to a high enough temperature to cook.
Common Mistakes To Avoid When Soldering High Thermal Demand PCBs
Hand soldering highly metalized PCBs like those used in military, defense, and aerospace applications, place high-thermal demands on the soldering system and present challenges to even the most proficient soldering experts.
Extending Dwell Time to Compensate for Heat Spreading Can Damage Boards And Their Components
Technicians might compensate for heat spread by extending dwell time, the time they hold the hot tip of the soldering iron to the solder joint, to melt the solder. This is not advisable. Extending the dwell time results in more heat spreading throughout the PCBA, potentially harming neighboring components.
Turning Up the Heat Leads to Even More Potential Damage and Failures
If using a variable heat soldering iron, operators may also attempt to turn the heat up to its maximum setting. This compensation method is also not recommended, since it will decrease the life of the soldering tip and cause damage to both components and PCBs, resulting in reliability issues.
Robust Solutions for Soldering High Thermal Demand PCBs
Select The Largest Tip For The Job
To help overcome soldering challenges inherent in highly metalized components and PCBs, soldering technicians should select the largest soldering tip possible for the application. The video below shows a high thermal demand application utilizing three different tip sizes. In this example, the small and medium tips do not transfer enough heat to melt the solder and overcome the heat sink effect. The large tip, however, with increased contact area between the tip and the pad, transfers sufficient heat for this application.
Inductive Soldering for Rapid Heat Transfer
When manufacturing devices where reliability is paramount, inductive soldering systems can be utilized to reduce dwell time and improve heat transfer. Based on the physics of ferromagnetism, inductive soldering systems quickly generate heat by flowing an alternating current through an induction coil wrapped around a ferromagnetic alloy. An important feature of induction heating is that heat is generated inside the ferromagnetic alloy itself, instead of conducting heat from an external heat source. Thus, heat is produced and transferred rapidly.
Inductive Soldering Systems for Control of Tip Temperature
In induction soldering, the induction coil and ferromagnetic alloy can be built within an inductive soldering cartridge such that the cartridge tip is also the heater. This innovative design eliminates thermal resistance between the heater and the soldering tip, allowing rapid heat transfer and temperature recovery while delivering power and heat on demand.
Fixed-Temperature SmartHeat® – Complete Process Control for Contract Manufacturers
For highly controlled temperatures, self-regulating fixed-temperature inductive soldering cartridges can be used. Utilizing specific ferromagnetic alloys, each cartridge can be designed to possess a specific Curie Temperature (Tc).
The Curie point phenomenon is responsible for the self-regulating fixed-temperature feature of fixed-temperature inductive soldering cartridges. Ferromagnetic materials lose their magnetic properties when heated above their Curie Temperature. As magnetic properties are lost, the ferromagnetic alloy stops heating, allowing for precise control of tip temperature.
By creating soldering cartridges with different Curry Temperatures, METCALTM delivers a variety of extremely accurate fixed-temperature SmartHeat® options. By choosing the right METCALTM SmartHeat® cartridge for the application, process engineers can rest assured that technicians will follow their defined soldering process without the ability to crank the temperature up beyond a set point.
Summary
Hand soldering highly metalized military-grade electronics places high thermal demands on the soldering system. These high thermal demands combined with heat-sensitive components present challenges for even the most proficient soldering experts. To meet these challenges, manufacturers need a high-performance soldering system that can generate and modulate heat quickly with precise temperature control.
METCALTM inductive soldering systems create heat on demand, quickly and efficiently to overcome the challenges of highly metalized PCBs, components, and substrates. METCALTM SmartHeat® technology provides self-regulating fixed-temperature cartridges that literally control tip temperature at the molecular level.
For information on Metcal MX Series and CV Series SmartHeat® inductive heating soldering systems, go to Metcal.com.
Metcal, an OK International brand, is a benchtop solutions innovator, leading the way in hand soldering, convection rework, fume extraction, and fluid dispensing. Metcal breakthroughs have empowered global OEM and electronics assembly customers in contract manufacturing, automotive, aerospace, medical devices, industrial, and military sectors since 1982.
