Soldering techniques for LCDs | Focus LCDs
This article about soldering techniques is the second on a series written by Barbara Dutra, an exchange engineering student from Brazil, currently attending Arizona State University, college of electrical engineering. She is currently an intern at Focus Display Solutions.
The majority of Liquid Crystal Displays require electronic components to be attached to the LCD Glass via a printed circuit board. These components are permanently attached using solder via LCD soldering techniques.
There are several types of solder available including both restriction (reduction) of Hazardous Substances Directive (ROHS) and non-ROHS.
Although most LCDs are now manufactured according to ROHS requirements, it is possible to order a LCD module to be built to non-ROHS standards.
A non-ROHS LCD is not a custom LCD and therefore does not require a tooling or a non-recurring engineering (NRE) fee.
Solder’s job function
The soldering of electronic components is the act of joining two pieces mechanically by melting a combination of metals that becomes a permanent adhesive once cooled.
There are many types of soldering tools and methods used depending on the application. For example, there is the soldering of mechanical parts with special materials in large equipment such as aviation, or solders used in very small scale applications such as in Surface Mount Devices (SMD) components, and finally there is one extremely delicate type of soldering processes used in the case of the amendment in optical fibers.
In this article I will talk about the solder used in electronic components.
Solder used for electronic circuits
All electrical appliances have electronic circuits and these circuits are made up of electronic components attached to a printed circuit board (PCB) using solder. The solder’s primary function is to create a good electrical contact and good mechanical rigidity.
A poor solder connection can seriously reduce or cut the flow of current causing the circuit to fail in its operation. ( Note: A poor LCD solder connection increases the amount of resistance. The higher the resistance the more heat that is generated and the more power required to operate the device. This is a critical concern in battery powered applications that have a LCD and a Liquid crystal module with a backlight.)
LCD Soldering devices
There are several types of devices used in the soldering of electronic components:
- soldering irons
- soldering guns
- soldering stations
Soldering irons are used for light and medium work, and soldering guns are used for heavier work.
The soldering station is used for SMD components in the industrial manufacturing of some PCB assemblies used in such products as display modules, cell phones and computer boards.
Soldering irons have a hollow base with a heating element located inside. Its tip, which is typically copper covered by another material that allows the transfer of heat, becomes heated when the iron is turned on.
The soldering gun contains an iron tip which heats almost instantaneously when the trigger is pressed. It also contains a small lamp to illuminate the place where soldering is taking place. This iron is suitable for heavier soldering applications or large components with thicker terminals that require more heat.
LCD Heat restrictions
Electronic components such as IC’s, micro-processors, diodes and inductors are very heat sensitive. They must be protected from excessive heating. When these components are attached to a PCB, heat is applied for a very short time (seconds), and in many applications heat is diverted from the component by a heat shunt.
See our article on delaminating LCD polarizers talking about the damage to PCB, components and LCD polarizers due to excessive heat.
A heat shunt is a product that draws the heat away from the body of the components and dissipates the heat into the environment.
Traditional solder material
Traditional solder is composed of two low melting point materials, tin and lead. Sometimes, these solders may contain small amounts of other materials for special applications.
This combination of metal alloys create a melting point between 680F to 698F, making it ideal for making joints (permeant bonds) between two metals. The solder is identified from the ratio of tin and lead with the numbers representing the Tin/Lead ratio.
Some of the most common ratios include 40/60, 37/63,50/50 and 60/40.
Many countries have banned the use of lead in solder due to pollution potential and risks for human health. Most countries have a dead line of when they plan to remove lead from electronics.
There are exceptions to this ban. Solder used in critical applications such as aviation, military and some medical devices are allowed to use lead. It is believed that bonds using a lead based solder are strong and do not crack over time or in ultra-harsh environments.
Non-Traditional solder material (ROHS)
The majority of industrial countries now require ROHS approved solder and soldering techniques. The use of ROHS solder in Liquid crystal displays has not increased its cost or lead-time.
FocusLCDs (aka: Focus Display Solutions) will use ROHS solder if the customer does not specific a specific type of solder.
There is a belief by some companies that ROHS solder has not been in use long enough to know if it is reliable over several years of use. Their concern is that when the solder is exposed to extreme heat and extreme cold over a long time period, that the solder will crack and allow for open circuits.
Traditional solder (through-hole)
PCBs contain copper that is used to create circuit paths, the components are attached to the circuit board with leads that are inserted into holes located on the board. The holes contain copper around the edges that receive the solder.
The solder flows through the holes and holds the component on to the PCB from the top and bottom.
Before starting the soldering process, a bit of solder is applied to the tip of the soldering iron, this allows heat to be passed from the heating element to the copper contact.
The tip of the soldering iron is briefly placed simultaneously against both the leads of the component and the copper of the trace. The solder flows into the hole and forms a strong electrical bond as the solder cools down to room temperature.
Warning: This operation must be quick so that the soldering iron heat does not damage the track or electronic component. If there is excess solder or join is faulty, the solder can be reheated and removed with the use of the nozzle.
As a general rule, it is not wise to reheat the pads on the PCB more than twice. Otherwise the pads may lift up damaging the PCB.
Solder in SMD components
The SMD components are fixed on the board to copper pads and without a plated through hole in the PCB.
The solder of the SMD components is responsible by the mechanical support and electrical connection between the component and the copper pad on the PCB.
The components are attached to the PCB via dipping (in a pool of liquid solder) or wave soldering machines.
Soldering of SMD’s is more complex for a variety of reasons such as exposure to high temperatures and soldering conditions. The SMD components have very small terminals for soldering, and they are on the copper side of the board that will be exposed to heat. The risk of heat damages to the components is much higher than in a conventional method of mounting components with through-hole.
There are two welding processes: wave soldering and reflux soldering. The majority of Chip on Board (COB) LCDs contain a combination of both SMD (sometimes called SMT for surface mount technology) and through-hole.
Wave soldering of components on a LCD
In the wave solder process, the board should be reversed to receive the wave soldering, so it is required that the components are pre-pasted with an epoxy adhesive. On a conveyor belt, the board passes through a tub with solder alloy in liquid state.
The machine produces waves of solder (similar to ocean waves) and this solder wave contacts the plate by welding all components SMD terminals mounted on the bottom face of the plate.
Reflux soldering of components on a LCD
In the case of reflux soldering, the components on the LCD are placed on the board that will be carried by a belt into an oven. A solder paste is applied to the plate and this paste is melted in the oven, soldering all components. This process will solder the components of the upper part of board. Then the plate goes to the soldering wave form, where the lower side components are soldered.
Note: the LCD soldering contacts the ends of the SMD device and not the component itself.
When a SMD component is defective or poorly welded, in some cases, the issue can be corrected using a soldering iron. However, an experienced person is required for this to be carried out due to the very precise work involved.
It may be more cost effective, to not repair the solder joint.
LCD Soldering problems
To avoid problems in solder quality, care must be taken.
The soldering iron should use only the required amount of solder and avoid unnecessary heating of the board and the components.
First remove the solder and then the soldering iron. Do not move the terminal until the solder cools and never “blow” on the solder. This may create air bubbles to form creating a poor solder joint and increasing resistance in the circuit or misalignment of the component.
Once the solder is cooled, tap the terminal with a wire cutter to make sure that soldering is secure and cut off the excess terminal with cutting pliers.
Note: “flow soldering” should not be used for soldering printed circuits due to their acidic nature which can corrode from their residues, thereby damaging the components.
When a solder has conductivity problems caused by improper soldering, the joint is said to be cold soldered.
Cold soldered joint
A good LCD soldering joint will be shiny and even, a poor LCD soldering joint will have a dull appearance and be filled with holes or gaps that allow for the component to break free. Also, a poor solder joint will create more resistance.
The LCD soldering may have a good bond with the lead of the component, but a bad contact with the trace on the PCB. This can happen by insufficient heating of the lead, or the printed circuit board is dirty or rusty. Below is an image of this type of solder failure.
This can be seen on Chip on Board (COB) character LCDs.
The opposite solder issue can occur when the weld has a strong adhesion to the lead of the component, but a bad contact with the component terminal. This problem can be quickly identified since the component will be wobbly and move easily. The cause is the same, insufficient heating of the terminal, terminal or dirty or rusty.
The image below shows a good LCD soldering contact with the trace, but little adhesion to the lead of the component. Note the lack of contact between the lead and the solder.
Another problem is the microspheres: these are isolated spheres that are on the printed circuit board trace when the weld does not bond correctly. The microspheres ( a fancy name for air bubbles) can be formed beneath the solder holding the components to the trace. This condition will create a short circuit.
This can be a major headache since the LCD soldering will look good and you will have a difficult time locating which solder joint is causing the broken circuit.
The image below shows a poor LCD soldering weld that can occur on a solder joint for a LCD Display. The component will behave as if there is no connection at all.
LCD Soldering concerns
When selecting a display for a new product design or for a replacement LCD, LCD soldering quality is critical to the success of your product.
Contact Focus Display Solutions for any questions or concerns you have regarding LCD soldering requirements for LCD displays. We are based in Chandler, Arizona and support customers in the US and Canada.
Call us at 480-503-4295 or you can find out more information on our web page at FocusLCDs.com.