During an adhesive dispensing process, adhesive is transferred onto the PCB solder mask in a position appropriate for holding the later placed components until the PCB solder mask is wave soldered.
In certain cases, the adhesive dispensing process is also employed to retain heavy components in the second pass of a double-sided reflow process.
Many factors have to be taken into account when dispensing adhesive onto a PCB. The most essential factors are mentioned in the following list.
- X,Y,Z gantries
- Head type
- Nozzle type
- Nozzle temp. control
- PCB handling & PCB Support
- Vision system
- Dispensing Repeatability
- Auger rotation
- Droplet number
- Thixotropic behavior
- PCB flatness
- Solder mask
- Flatness / finish
- Solder pad
- Flatness / finish
- Height in relation to solder mask
- Dust & dirt
- Air circulation
- Air humidity
- Static electricity
While all the factors in the five groups have varying grades of importance, all play a major role in the end result. Therefore, all aspects should be taken into consideration in order to achieve high quality goals.
The equipment employed for adhesive dispensing can be divided into 2 main groups – off-line and in-line. Selecting an off-line or inline system relies on the job in hand. An off-line dispenser unit should be sufficient if the process is seldom used and only in small production batches, while an inline system will be required for high out-put placement lines, where the product cycle-time is short.
Three different dispensing methods are used for both off-line and in-line systems such as time/pressure, piston pump and auger pump. In all three methods, the similarity found is the dispensing head movement on X, Y and Z-axis gantries enabling it to move over the stationary PCB and transfer the adhesive dots.
To resolve bottlenecks, multi gantry dispensers can be used to achieve high speed dispensing. These machines enable speeds up to 120.000 dots per hour on a relatively small machine footprint. In order to increase the possible variation in height and dot diameter, two to four dispenser heads are usually fitted per gantry.
In order to make perfect adhesive dots, the PCB support must be able to hold the PCB in a locked position and be absolutely parallel to the X and Y-axis. The design of the PCB support should allow for both fast change over and flexibility. In order to ensure the required dispensing repeatability and accuracy an automatic vision alignment system using PCB fiducial marks together with a closed-loop servo system should be included. These systems align with the PCB by correcting the program coordinate data.
The latest dispensing unit can be provided with plenty of options such as automatic conveyor width adjustment, computer control, automatic needle calibration (X, Y, Z), adhesive temperature control and so on. In the case of really high output lines, another dispensing method could also be used – the pin transfer method (not covered in this article).
The dispensing system must transfer uniform and precise adhesive height and dot shape at different volumes onto the surface of PCB solder mask. The adhesive should possess adequate attachment capability, also called “green strength”, to hold the components in place until cured. This cured adhesive must then hold the SMD components in the right position until reflow or wave soldered (second pass in double-sided reflow process).
The deposit and shape of the adhesives are established by various factors, such as adhesive supply pressure, adhesive viscosity, auger rotation (auger system), dispense time (time/pressure system), dispense rate, number of piston strokes (piston system) and nozzle standoff height.
The SMT adhesive dispensing process has certain limitations. Smaller than 0603 (1608) chip components cannot be placed in SMT adhesive as this carries a high risk of failure in terms of solder skip. It would be difficult to solder when the adhesive protrudes onto the solder land.
Also, there is no guarantee that wave soldering of components this size will succeed. Another drawback is to dispense adhesive dots that are high enough to hold components with a high body standoff. This indeed depends on the height of the nozzle standoff foot and the nozzle diameter. When dispensing adhesive onto the PCBs, a number of parameters are essential to obtain a good result. The parameters for each type of system are described below.
Time / Pressure System
The oldest and most commonly used SMT adhesive dispensing system available is the time / pressure system. However, it is also the most sensitive of the three described systems.
Time and Pressure
With regards to a time / pressure system, a fairly high air pressure is applied to the top of adhesive cartridge for a controlled period of time (ms) so as to transfer the required adhesive volume to the PCB. It is important that the air pressure is accurately adjusted and periodically checked to sustain uniform dot sizes. The dot size is then determined by the selected dispense time, nozzle inner diameter and nozzle standoff height. The air pulse response time will differ if an adhesive cartridge is full or close to empty. This, along with air pressure variation, can cause the process to fluctuate and lead to dot size variation.
There are some dispensing units that feature two to four dispensing heads on the gantry. Each is equipped with a different nozzle diameter and standoff foot, allowing the machines to dispense adhesive dots in a wide range of sizes. For example: A nozzle with a short standoff foot and an extremely small inner diameter will not dispense an adhesive dot properly for large PLCC packages. The sizes of the nozzles should be selected to suit the job in hand. On the whole, the smallest possible dot diameter equals twice the inner diameter of the nozzle. A combination of nozzle inner diameter and the standoff foot height dictates the upper limit of the dot diameter.
A twisted or bent PCB can lead to a large variation in the adhesive dot size and thus a variation in the strength of the component attachment. The use of a standoff foot will not only guarantee a more uniform dot height but would also, to some extent, dictate the dot height. Based on the nozzle inner diameter, the stand off height is usually between 0.15 and 0.3 mm. As a part of controlling the dispensing process, it is recommended to make a periodic height measurement of the standoff foot.
The standoff foot is continuously being driven against the PCB surface and will ultimately wear down and lead to problems such as different volumes, dot height and stringing. Another major concern is the angle at which the standoff is placed on the nozzle. It must be ensured that the standoff foot never comes in contact with earlier placed dots. Some systems come with a fixed standoff angle, which can pose some limitations to how flexible the system can perform, while others give the user a choice of various standoff angles to improve performance and speed. Next, the system is optimized with the specified angles and must not be modified during production. It is important that the operator is aware of this during cleaning or replacement of nozzles.
The viscosity of the adhesive is of utmost importance. In order to lower adhesive dot volume and shape deviation, an adhesive temperature control system should be used. It is important that the temperature setting is optimized for the selected adhesive but this should at least be set above the maximum ambient air temperature by a few degrees Celsius.
Auger Pump System
The auger system is also known as Archimedes pump or rotary pump.
The top of the adhesive cartridge receives a constant low pressure, causing the adhesive to press into the top of the auger chamber. When the auger is turned the adhesive moves downwards and exits through the nozzle. The amount of SMT adhesive dispensed relies on ambient temperature, the amount of rotation (degree) of the auger, and the adhesive thixotropic behavior.
With regards to the time/ pressure system, a wide range of nozzle sizes are required on the gantry, allowing the machines to dispense adhesive dots in a wide range of sizes. The rules for selecting the nozzle sizes for the job in hand are also identical.
In the case of time / pressure system, a mechanical nozzle standoff foot is employed to resolve problems either with bent and twisted PCBs. For this system also, the standoff height should be between 0.15 and 0.3 mm based on the inner diameter of the nozzle but if a mechanical nozzle standoff foot is employed this can be prevented. A uniform dot height can also be ensured with the use of a standoff foot.
With regards to the other two dispensing systems, the viscosity of the adhesive and flow behavior are very crucial. In order to reduce adhesive dot volume and shape deviation, it is essential to use a nozzle temperature control system. It is also important to optimize the temperature setting for the selected adhesive but this should at least be set above the maximum ambient air temperature by a few degrees Celsius.
Piston Pump System
The latest of the three dispenser types is the positive displacement pump or piston pump system. The benefit of this system is a restricted number of critical process factors. A piston pump system equipped with a nozzle temperature control system is highly accurate in terms of the amount of adhesive transferred to the PCB surface. The difference is typically within + / - 1% by volume. With this type of system, stringing and tailing is less common but if the viscosity of the adhesive is incorrect it can still be observed.
The utility of one of the available systems is as follows. When the piston is raised by the solenoid valve, a constant low pressure is applied to the top of the adhesive cartridge which fills the piston chamber with adhesive. On releasing the spring-loaded piston, one droplet is pushed out of the nozzle hitting the PCB below. Although a wide range of piston pump designs are available, all of them apply small adhesive droplets one at a time. Multiple droplets are stacked on top of one another to apply a larger adhesive volume. This means that the process can be easily controlled.
Only one nozzle diameter is required in this system. As mentioned before, a large adhesive dot is merely dispensed by multiple small dots on top of one another.
With most piston pump systems, the adhesive dispensing is a non-contact process allowing certain components to be in place already. A PCB height measurement unit is integrated in some systems to guarantee a uniform nozzle height, which can be adjusted around 4 to 6 mm.
The main critical factor of the piston pump system is securing a uniform adhesive viscosity. A nozzle temperature control system should therefore be used to reduce adhesive dot volume and shape deviation. It is important to optimize the temperature setting for the selected adhesive but this should at least be set above the maximum ambient air temperature by a few degrees Celsius.
The function of the adhesives is basically to keep the components on the PCB from placement until the components have been reflow (bottom side) or wave soldered. The adhesive pot life, the dispensing method, the dispensing speed, the curing process and the working environment are the main criteria that should be taken into consideration when selecting an SMT adhesive.
The flatness of the PCBs is integral to the dispensing quality. Bent or twisted PCBs will lead to large variation in the adhesive dot size and ultimately cause a variation in the strength of the component attachment. However, if a mechanical nozzle standoff foot is used it will be less critical, provided the PCB surface is not lower than the maximum stroke depth of the nozzles. It is important that the PCBs are clean and do not have fingerprints as these would result in poor adhesive attachment and finally make the components fall off the PCB during the wave soldering process or handling.
Another critical factor is the height of the solder pad with respect to solder mask and this should be considered when choosing the adhesive dot sizes and standoff foot height for each type of component. Actually, the most common problems happen when dispensing on poorly leveled HASL PCBs.
Dirt and dust particles from the air end up on the PCBs which can lead to poor adhesive attachment on the PCBs and eventually result in missing components on the PCBs. Based on the type of adhesive employed, air draught in the production area, high humidity, high temperatures and light can degrade or cure the adhesive qualities. For most adhesives, the viscosity changes with the temperature and hence it is critical to maintain a stable process temperature. The adhesive supplier’s data for the temperature window should be checked.
In some cases, static electricity can be a major problem. This occurs when the nozzle becomes charged. The adhesive will probably remain fixed to the nozzle and cause plenty of stringing. This problem can be prevented by placing an ion blower in the dispensing unit. Since the adhesive used is often an epoxy, extreme care must be taken during handling. Gloves will be required to prevent contact with the skin, particularly during the cleaning process following end dispensing. The time / pressure system is the easiest system to clean because it has only a few parts.
Adhesive dispensing is a delicate and highly sensitive process and therefore calls for trained and experienced operators. The operators should be able to predict problems and adjust the process accordingly to guarantee a good dispensing quality. Things like: shape of dots, position of adhesive dots and volume should be controlled regularly.
List of Failures Related to SMT Adhesive Dispensing
Missing components following the reflow (bottom side) or wave soldering process can be caused by inadequate, misaligned or missing adhesive dots. Missing adhesive can be caused by a damaged or too high standoff foot, clogged or partly clogged nozzle and entrapped air in the adhesive cartridge.
A contaminated surface on the solder mask can lead to poor adhesion to the mask. The mask often appears too shiny or oily. A usual sign indicating this problem is that the adhesive is never attached to the PCB mask, but only to the removed component. One must be aware that a poor curing process can also result in missing components following the wave soldering process.
Open joints can be caused by misaligned adhesive dots, tailing or stringing. Wave soldering cannot be carried out if the adhesive is dispensed onto or partly onto the solder pads and further pressed out by the placed component. Adhesive dispensed onto the solder pads could be a machine accuracy problem or the wrong setting leaving too much adhesive on the PCB surface.
Adhesive tailing or stringing can be caused by a number of factors such as poor PCB surface condition, poor adhesive thixotropic behavior, static electricity, wrong machine set-up parameters and incorrect dispense nozzle.
For auger and time / pressure systems, an excess amount of adhesive dispensed as opposed to the nozzle inner diameter and standoff height will lead to stringing and a huge variation of transferred dot volume. The access adhesive simply sticks to the tip of the nozzle due to a larger surface tension and then leaves a tail when retracted.
This information has been sourced, reviewed and adapted from materials provided by Epoxy Technology, Inc.
For more information on this source, please visit Epoxy Technology, Inc.