Welding is one of the most important aspects of the manufacture and assembly of electronic products. Various electrical connections of parts are made by soldering concrete. The adaptation of automatic soldering has resulted in manufacturer benefits including reliability, cost effectiveness and increased productivity. Two leading technologies associated with automatic assembly include solder wire and beam solder. Both processes can provide quality work, but they are different.
The wave soldering machine is used for the manufacture and assembly of printed circuit boards or printed circuit boards and is a massive process. The molten tin (produced by the exciter) is used in liquid form and passes through this liquid “wave” caused by the excitation of the solder material and its components. PCB components are attached to the solder by contact with the waveform.
Wave soldering can be of two types. These include solder wire for PCB assembly and soldering for surface mount. Surface soldering mounts components to the board surface before contacting with the solder.
Wave Soldering: The four main steps in the wave soldering process are described below. Flux Spray: The solder stream is used to remove oxides from the board. The application of flux provides lower surface tension, thermal conductivity and smoother soldering process.
Preheating: PCBs are passed through the tunnel and heat is applied to them. This process activates the flow and the chain used is like a conveyor belt.
Wave Solder: Due to the high temperature, the solder paste becomes liquid. The wave is formed and the components are connected to the boards.
Cooling down: Once the temperature reaches its highest or highest level, it will be lowered later. Once the board and solder components have cooled down, the collection process begins. Factors such as temperature, contact wavelength, bandwidth, etc. are important for wave soldering.
Brazing Unlike welding, ray brazing involves temporary adhesion of printed circuit boards before components are sealed and exposed to heat. Created using temporary thread compound and solder powder. When heated, the solder dissolves and forms a permanent bond. Recirculation brazing is carried out in a machine called Reflow Brazing Furnace.
Soldering process: The solder material must be accurately positioned on the boards using a solenoid stencil. Then you use a Select and Paste pencil to attach the components to the pillow. Next steps
Preheating: Preheating ensures that thermal conditions are maintained and also helps remove volatile solvent that may contain solder paste.
Thermal wetting: Solder paste contains a flux that must reach the temperature at which it is actively activated. The stream must be activated to obtain benefits such as removal from oxidation.
Solder reflux: In this step, the solder paste is melted and illuminated. It is important that the correct temperature is reached. Extreme high temperatures can damage PCB components, and low temperatures can prevent soldering.
Cooling: Cooling is the curing of the soldered solder to ensure that parts and components are permanently installed on the printed circuit boards.
Using a soldering machine is essential to achieve maximum productivity. Wave soldering has been found to be best for DIP or THT assemblies, whereas light soldering for SMT assembly is good.
Distinguishing Lead-free Technology from Standard Process
If we talk about the fundamental points, then lead-free soldering practically does not differ from the traditional Sn / Pb-technology except for temperature. However, some changes may be required for certain operations of the workflow. So, for example, new types of solders and fluxes can affect the performance of the solder paste. Paste properties such as shelf life, shelf life, fluidity may change, which will require changes in the squeegee design and reflow modes.
When exposed to elevated soldering temperatures, swelling of IC packages, cracking of crystals, and malfunctioning of circuits can occur. Similar effects occur in printed circuit boards. Under the influence of temperature, the base stratification occurs, the flatness deteriorates, which negatively affects the accuracy of the IC installation, especially in large cases.
To assess the effect of elevated temperatures and longer soldering times, re-certification of the existing soldering technology is required. Such studies are currently being carried out by SEMI and JEDEC.
With regard to reflow, the effect of lead-free soldering is not the same at different stages of the process. All major changes are associated, first of all, with a higher soldering temperature. More careful selection of components and base materials is required.
Other issues are related to device cooling and board support. Multicomponent alloys containing more than two metals are especially sensitive to the cooling rate. These solders can form various intermetallic compounds depending on the cooling rate.
Studies of standard surface mounting and wave soldering technology have shown that both economic and technological factors influence the choice of alloy. So, for example, indium-based alloys are very expensive, it is irrational to use them for wave soldering, when it is necessary to load a large amount of solder into the bath. However, this material can be successfully applied to fabricate the leads of flip-chip crystals.
The technologies of all components of the production process are constantly being improved. Most of the issues related to the soldering process have already been resolved. Manufacturers provide sufficiently detailed information on the method of soldering their products on their websites in the relevant sections.
Lead Free Solders
In the past few years, the process of transition to a new type of solder – lead-free solder – has developed rapidly. The pioneers in this field are considered to be Japanese manufacturers who pay great attention to environmental protection and strive to obtain a new safe and promising PCB assembly technique.
The main reasons for the transition to a new type of solders (in addition to environmental safety) are the higher performance characteristics of such solders. However, there are a number of reasons why the industrial application of this type of solder is still limited. The fact is that the lead-free type of solders has a higher soldering temperature, which affects the complexity of the soldering equipment: it is necessary to maintain a narrower boundary of the thermal profile.
The equipment should have temperature sensors located over the entire heating area of the printed circuit board and monitor the thermal profile in real time.
Naturally, the re-equipment of the assembly shop for the use of lead-free type solders is economically unprofitable for manufacturers, however, according to experts, the struggle for a cleaner environment and the requirements for improving the quality of soldering with a constant tendency to reduce the size of devices will lead to a complete transition of the electronics industry to lead-free the end of 2005.
Selection of the Optimal Thermal Profile
When using lead-free solder pastes, the temperature difference between the areas of the boards with a greater mass and less should be minimal. This is achieved with the correct brazing temperature profile. The following methods can reduce the temperature difference:
Increased preheating time: This method can significantly reduce the temperature difference, however, as the preheating time increases, the flux evaporates, which leads to a deterioration in wettability due to oxidation of the surfaces to be brazed.
Increase the preheating temperature. Typically the preheat temperature is 140-160 ° C, however, for lead-free solder pastes, it can be increased to 170-190 ° C. Since the preheating temperature is increased, the temperature jump between the preheating and soldering stage will be less than in a conventional thermal profile; therefore there will not be such a noticeable temperature difference between different parts of the PCB caused by different heating rates. The disadvantage of this method, like the previous one, is the rapid evaporation of the flux (even at the preheating stage), which affects the reliability of the soldering.
Does your company use SMD components on PCBs, but do you have to solder the terminals and connectors by hand or wave solder at best? In modern electronics, there is a clear trend towards the transition from lead-out components to SMD-components – reducing the size and weight of the final product has long been a mandatory requirement of the market.
Output components are used when additional mechanical fastening of the element to the board is required with a large dead weight (in particular, transformers) or possible mechanical stress (for example, connectors).
There are Three Main Types of Automatic PCB Assembly:
- Soldering in a convection oven;
- Vapor phase soldering;
- Wave soldering.
Currently, the traditional PCB assembly process consists of the following steps:
- Installation of components
- Reflow soldering of solder paste;
- Glue application and installation of output components;
- Wave soldering.
As a rule, in most cases, when designing devices, it is possible to avoid using lead components, with the exception of terminals and connectors. In this regard, the technological chain of assembly of printed circuit boards is forcedly extended.
PCB Manufacturers have developed and manufactures a range of PCB terminal blocks and connectors that are compatible with reflow soldering – convection oven and vapor-phase soldering, which allows lead components to be used directly in the SMT process.
The emergence of such components was made possible by the use of a special insulator material by PCB Manufacturers, as well as a number of technical and design solutions.
- Additional operations associated with the outlet components are excluded: glue application, installation of outlet elements, drying, soldering with a wave of solder – reducing the installation time;
- Packaging corresponds to the technological process – reduction of installation time;
- No pre-drying required – reduction of installation time;
- Manual soldering is excluded – stability of the results of the technological process;
- Closed application of solder paste – economical use of materials;
- Flanges, soldered in one technological cycle with connector contacts – reducing assembly time.
Features of Terminals and Connectors Compatible with Reflow Soldering Technology
It is thanks to the use of a special material that it became possible to use terminals and connectors in the reflow soldering process.
LCP GF is a liquid crystal polymer + glass fiber. Its main features are: high volume resistivity, dielectric strength and upper limit of permissible temperature, as well as dimensional stability with temperature changes.
The heat-resistant, halogen-free insulation material LCP (Liquid Crystal Polymer) has a melting point of +335 ° C and is therefore well suited to soldering temperatures. According to the European RoHS directive, there is a limitation on the content of substances hazardous to human health (in particular, lead) in electronic equipment.
But, as you know, the soldering temperature with lead-free solders is higher than for solders containing lead. SL-SMT connectors made from LCP material are ready for use with lead-free solders.
Terminals and connectors with glass fiber reinforced liquid crystal LCP as insulating material are characterized by high dimensional and dimensional stability (also at high temperatures) and low TLEC. As a result, a high pitch accuracy is achieved even with a large number of pins, and the process of installing components on a PCB is facilitated. Due to the fact that the TCLE of the LCP GF material is close to that of the main PCB material – FR4, bending of the board after soldering is prevented.
Components made of LCP GF material have MSL moisture sensitivity level class 1: the safe storage time of components at +30 ° C and 85% relative humidity is unlimited. This means that such components can be used immediately in the THR reflow soldering process without an additional drying step, without the fear of blistering or cracking (microcracks are especially difficult to detect).
Ensuring the Penetration of the Heat Carrier from Both Sides to the Soldering Point
For reflow soldering technology, a comprehensive and uniform heating of the solder paste is essential. For this purpose, the design of THR connectors provides for the access of the coolant to the soldering point from the side of the connector body.
Free space for meniscus formation:In contrast to wave soldering, during reflow soldering, a meniscus is formed on the side of the element. Therefore, the connector design must provide free space for the formation of a quality meniscus.
The Length and Shape of the Sealed Contacts
Two lengths of soldered contacts are available – 1.5 and 3.2 mm (3.5 mm for terminals). Contacts with a length of 1.5 mm have the following advantages:
- Reducing the travel of the machine when installed on a board – increasing the speed of installation.
- Possibility of double-sided installation of output components.
- Smaller dimensions – more components per package.
In cross-section, the soldered contact is a quadrangle with chamfers, its end also has chamfers. This provides the following benefits:
- Reducing the diameter of the hole in the PCB and, as a result, economical use of solder paste.
- Less force required to insert component pins into holes filled with solder paste.
- Increased soldering strength.
- Better recognizability in tracking systems (automatic optical inspection) due to the larger reflective surface.
PCB Manufacturers supplies components in packaging that takes into account the peculiarities of automated processes: in a tape on a reel or in a tube (case) for the corresponding feeders, as well as in a tray (pallet) for large components. The tape on the reel is available in standard widths: 32, 44, 56, 72 and 88 mm. The components are positioned in belts of the appropriate width – this allows optimal use of the feeders and the precise positioning of the components on the belt for a secure grip.
The final stage of the soldering process is quality control. Quality control for lead reflow soldering is similar to quality control for wave soldering. Currently, there are several ways to do this: optical inspection, fluoroscopy or destructive testing. The optical inspection can evaluate the shape of the solder joint, the color and the reflectance of the surface. Fluoroscopy allows you to identify voids and cracks, and destructive testing to assess the resistance to pulling the lead out of the printed circuit board. Of course, destructive testing is carried out only selectively.
Basic Terms and Definitions
- PCB (printed circuit board) – printed circuit board.
- SMD (surface mounted device) – components for surface mounting.
- SMT (surface mount technology) is a surface mount technology.
- THR (through hole reflow) is a combination of through-hole components and reflow soldering technology.
- THT (through hole technology) is a through hole mounting technology.
- TCLE – temperature coefficient of linear expansion.