Part 1: THT Assembly

PCB assembly for through-hole technology is different than the Surface mount technology as THD

Can not be mounted on top of the PCB so there are two procedures to follow for THT assembly. One of them is the traditional Manual method and second one is automatic method. Some steps are discussed here in detail.

Step1: Component’s placement

Themanual methodof components placement is slow and of course precise method to achieve the THT assembly. This will need complete attention of the engineer or a worker who is placing components on the board. The things engineer need to verify during components assembly are the components polarity, components placement and carefully placing components on Serial no mentioned on the silk screen. The regulation and operation standards must be maintained during the manual assembly.

Automatic assemblyof Through hole assembly is different than SMT because in THT it is not as simple as to pick and place the components on the PCB. THD have leads that must pass through the PCB during the assembly, so THT automatic assembly procedure pushes the through-hole component in the PCB. One of the automatic THT assembly machine is shown in the image below.

Step 2: PCB Inspection

To confirm that the components placement procedure is going on according to the plan, A PCB inspection automated machine is used which compares the THD assembled PCB with the original layout of components provided by the user or company. This procedure helps to rectify the damaged or unaligned through-hole components. The damaged pieces are then manually checked by engineers and proper modifications are done according to the Problem.

Step 3: THT Soldering

There are different types of soldering methods used for THT and SMT assembly. Let’s discuss them one by one.

Wave soldering

Wave soldering is a bulk soldering process used in the manufacture of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. when the circuit board contacts this wave, the components become soldered to the board. Wave soldering is used for both through-hole printed circuit assemblies and surface mount. In the latter case, the components are glued onto the surface of a printed circuit board PCB by placement equipment before being run through the molten solder wave. The through-hole components have been largely replaced by surface mount components. Wave soldering has been replaced by reflow soldering methods in many large-scale electronics applications however there is still significant wave soldering where surface mount technology summit is not suitable for example large power devices and high pin count connectors or where simple through-hole technology prevails certain major appliances.

There are many types of wave solder machines however the basic components and principles of these machines are the same. The basic equipment used during the process is the conveyor that moves the PCB through the different zones. A pan of flutter used in the soldering process, a pump that produces the actual wave, the sprayer for the flux and the preheating pad. The solder is usually a mixture of metals. A typical slaughter has the chemical makeup of 50% tin, 49.5% lead and 0.5% antimony. Flux in the wave soldering process has a primary and a secondary objective. The primary objective is to clean the components that are to be soldered principally in the oxide layers that may have formed. There are two types of flux corrosive and non-corrosive. Noncorrosive flux requires proclaiming and is used when low acidity is required. Corrosive flux is quick and requires little proclaiming but has a higher acidity. Preheating helps to accelerate the soldering process and to prevent thermal shock. Some types of flux called no-clean fluxes do not require cleaning, their residues are denied after the soldering process typically. No-clean fluxes are especially sensitive to process conditions which may make them undesirable. In some applications, other kinds of flux however require a cleaning stage in which the PCB is washed with solvents and or deionized water, to remove flecks residue. Different combinations of tin lead and other metals are used to create solder. The combinations used depend on the desired properties. The most popular combinations are sack tin SN / silver AG / copper Cu alloys, SN 63 PP 37 s and 63 a which is 63 % ,10% and 37% lead. The latter combination is strong, has a low melting range and melts and sets quickly. High arching compositions give the solder higher corrosion resistance but raise the melting point. Another common composition is 11 percent tin, 37 percent lead, 42 percent bismuth and 10 percent cadmium. This combination has a low melting point and is useful for soldering components that are sensitive to heat, environmental and performance requirements, also factored into alloy selection. Common restrictions include restrictions on lead PB when rose compliance is required and restrictions on pure tin SN, when long term reliability is a concern. It is important that the PCBs be allowed to cool at a reasonable rate. If they are cold too fast, then the PCB can become warped and this order can be compromised on the other and if the PCB is allowed to cool too slowly, then the PCB can become brittle, and some components may be damaged by heat. The PCB should be cooled by either a fine water spray or air cooled to decrease the amount of damage to the board. Thermal profiling is the act of measuring several points on a circuit board to determine the thermal excursion it takes through the soldering process. In the electronics manufacturing industry space, statistical process control helps determine if the process is in control, measured against the reflow parameters defined by the soldering technologies and component requirements. Products like the solder star wavy shuttle and the optimator have been developed special fixtures which are passed through the process and can measure the temperature profile along with contact times, wave parallelism and wave heights. These fixtures combined with analysis software allows the production engineer to establish and then control the wave solder process.

Reflow Soldering

There are about three different forms of solder and three forms of flux. Flux is a material that removes any oxidation and prepares the metal areas to be bonded. So, the second soldering technique is called reflow. This is used for surface mount technology (or SMT) components. For this we use solder paste which is a paste of flux with tiny balls of solder suspended in it. The paste is applied to the board with a stencil and the components are stuck into the paste using a robotic pick-and-place system. Then the board goes into the reflow oven. This is a convection oven with multiple different temperature zones inside. In the oven the board is slowly heated, and the flux goes to work until the solder beads melt and flow into place, then cool and solidify. To do a double-sided SMT board you do one side and then flip it over and repeat the process. On the second pass we don't let the bottom side get quite as hot, but the solder will flow again so experienced board designers know to put all the heavier components on the "top" side as a matter of design for manufacturing.

For double-sided boards to be wave soldered you place the SMT components first, on one side, using glue to hold them in place and then place the through-hole components on the other side. The SMT components go through on the underside and get soldered in the wave along with the through-hole pins. For both reflow and wave solder we'll use a thermal profiler to program the different temperature zones. We need to ensure that all areas of the board reach the proper temperature for the right amount of time for optimal soldering without overheating. At Z-AXIS we create and store a unique profile for each board design we run, showing temperature versus time for multiple areas on the board as it travels through the oven or wave solder.

Manual soldering

Finally, some components are best to be hand soldered. The flux is usually in the core of the solder wire. We hand solder components like lithium batteries that can't go in the ovens or tall, tippy parts. Everyone involved in soldering should be trained to IPC standards. There is an initial 40-hour classroom training course with re-certification every two years. It includes an online exam through IPC and a hands-on exam in which participants assemble a test board and have their work assessed by a certified trainer. At Z-AXIS we have a certified trainer on staff for both IPC-A-610 and IPC J-STD training. There's a lot more to soldering than we can discuss here - for example, leaded versus lead-free solder, water-soluble versus no-clean flux, how to do thermal profiling, and many other considerations.

Part 2: Why and when to Use THT

Before the invention of the SMT or surface mount technology all components were assembled to the PCB with leads going through component holes into the board. This technology dominated the industry until the end of the 80s. Today most components placed are SMT components.

THT advantages

One could think that THT or through-hole technology is now outdated and avoided where possible as SMT is more efficient cost-effective and adapted to dents aboard designs however THT offers specific benefits that keep it relevant. The bonds created between THT components, and the board are far stronger than SMT bonds making THT the ideal choice for components that will undergo mechanical and environmental stress or high heat in case of prototyping. THT components are also easy to change making them perfect for testing and hobby users.

THT Disadvantages

There are disadvantages too. Through whole components are much bigger and the component themselves are on one side but the solder joint is on the other side so using space on both sides of the board. The assembly process is not automated, depends upon the skill of the operators and is therefore less reliable and more expensive than SMT. Before placing the through-hole components we first cut and bend the leads to the required size some components are not just fixed on top of the board but are mounted on the side. These are called edge mounted connectors. To combine the advantages of surface mount device and through hole technology some applications require mixed technology components most common are USB connectors that need to be mounted on dense boards with little free space, so these are optimal for SMT technology but need a good mechanical stability only possible with through-hole leads to mount such components we use a pin in pace technology.

Part 3: SMT and THT Differences

Most contract manufacturers concentrate on their surface mount technology capabilities for PCB assembly. The SMT lines can be almost completely automated making it possible to run a high volume of boards with little human intervention and a low cost per board. Through whole assembly whether automated or manual is a slower process, it takes resources away from the high-volume business that many manufacturers prefer.

Only about 10 percent of the parts we buy are through-hole. Nearly 75 percent of the boards we run are mixed technology, which is a combination of through-hole and SMT parts one reason for this is that nearly half of our board assemblies are for high powered boards which use capacitors inductors and transformers to transfer energy. The size of these components is proportional to the energy they can store so for a high-power board we need to use some fairly large and heavy components and that means through-hole technology, in addition things like customer accessible connectors, that have to withstand stress as cables are connected and removed are likely to be through-hole components. Through-hole components are held in place by the pins as well as the solder so you can get a much stronger and more reliable mechanical bond than with surface mount components that have only solder to hold them in place. If a contract manufacturer is focused on higher volume SMT business, it's important to understand how they will handle through-hole components when you need them at z-axis. We run mixed technology boards every day we have a modern stuff line with a paperless process for assembly instructions and assemble in a single piece flow from the stuff line to a wave solder to inline inspection, d-paneling, qc testing