SMT patch red plastic performance analysis and instructions for use
1.1 Common plastic patch coating method
Coated plastic patch refers to evenly distribute the patch glue from a storage container (tube packaging, plastic tank) to a specified location on the PCB. Common methods of needle transfer, screen / stencil printing and injection method.
1. Needle transfer
Needle transfer method is to install a metal plate several needles, each needle alignment position to put a plastic patch before applying the needle bed immersed in a bath containing adhesive patch, and its depth of about 1.2-2mm and then the needle bed over PCB, gently press down firmly, when the needle bed is raised again, the glue will be due to capillary action and surface tension effects transferred to the PCB, how much glue and stickers from the needle diameter sheet of plastic viscosity to decide. You can manually control the needle bed can also be controlled automatically. This is one of the early application of the method, shown in Figure 16.
Advantages: All plastic point can finish first point, speed, suitable for mass production; less investment in equipment.
Disadvantages: When the PCB design needs to be changed, the needle position changes difficult; lack of precision plastic volume control does not apply to high precision occasions; plastic tank for open systems, easily mixed with miscellaneous hold, affecting the quality glue; environmental requirements, such as temperature, humidity and so on.
Assessment: There have been rare use of this method is generally used for pilot production, transfer method when using the stylus, and its viscosity adhesive patch requirements for the 70-90 • s.
2. Screen / stencil printing
Screen / stencil printing method to apply a patch of plastic, its principles, processes and equipment similar to those solder paste printing. It is through the openwork pattern screen / template, assign the plastic patch to the PCB, controlled by the viscosity and the coating thickness of the stencil glue. This method is simple and fast, high-precision than the needle plate shift, compared with early application of the text (see Figure 17), due to the high degree of gum drops after printing is not ideal and could not widely used. In recent years, Loctite Corporation launched Varidot squeegee printing technology, using a special plastic template, printable adhesive patch of different heights. Further washing the template is also relatively simple, and can significantly increase the productivity and utilization of existing equipment (printer) of the.
Advantages: first printing, complete the allocation of all glue points suitable for mass production; screen / stencil replacement, relatively inexpensive compared needle bed; press utilization increase, no purchase dispenser.
Cons: poor adaptability to change the PCB; glue exposed to the air, the external environment require high; only for flat printing.
Assessment: With the promotion of the new template technology, using the occasion will increase.
Patch of plastic viscosity requirements: viscosity values at 300-200 Pa • s.
3. Pressure injection
Pressure injection method is the most commonly used method of applying adhesive patch, dispenser and dispensing nozzle shown in Figure 19.
Its principle is the patch with plastic needle tube, head tube attached to the needle plastic mouth, and then the needle is mounted on the dispenser, the dispenser controlled by a computer program automatically assigns the glue to the PCB specified location. This method is flexible, easy to adjust, no templates, product replacement is very convenient, because of the emergence of high-speed dispensing, both suitable for mass production, but also many varieties suitable for the whole production. In addition, plastic patch being installed in the needle sealing, easy to pollution, high dot quality.
Its downside is that, dispenser price expensive, large investment costs.
Patch of plastic viscosity requirements: viscosity values at 100-150 Pa • s.
As the pressure dispensing process and wide application, advantages and more, will be discussed further below.
2.2 Factors affecting the quality of the gel-point
Excellent glue point should be bright surface, there is a suitable shape and geometry, without drawing and smearing. Research shows that many companies at home and abroad, many factors affect the quality of the glue point depends not only on plastic patch quality, but closely related settings to optimize dispensing parameters and process parameters, respectively, are described as follows:
1. plastic patch quality
Plastic patch should have excellent thixotropic, suitable viscosity, satisfactory early Sticky and initial strength. About the physical meaning of these concepts and their impact on the dispensing process were discussed previously. Usually high viscosity adhesive patch than low viscosity adhesive patch prone to smearing and drawing other illnesses, poor early Sticky adhesive patch trailing prone Sticky higher than the beginning of the adhesive patch. Of course, low viscosity adhesive patch its initial strength is poor, prone elements shift, prone to collapse and other defects after high temperature.
2. Dispenser parameter setting
Dispenser operating parameters have a great impact on the quality of the gel-point. Factors dispenser for dispensing quality performance impact mainly related to two systems, one pressure-regulating system, the second is related parameter settings, including the distance the plastic nozzle needle size, glue between the mouth and the PCB, and the pressure P and time t is set.
(1) pressure-regulating system
Pressure directly affects the volume of the plastic, and the magnitude of the pressure is set and kept constant by adjusting the quality of the machine and the sensitivity of the switching signal changes in air pressure in the syringe and other factors to determine at high speed dispensing of very fast, time in milliseconds magnitude. Therefore, machines and valves require high sensitivity, and features a special valve in the injection pipe is usually a machine dispensing pressure 5bar regulation within the scope of the production is to set a minimum threshold pressure to achieve good dispensing quality, lower than this threshold pressure, it can not guarantee the glue points out.
(2) the plastic nozzle size and related parameters
Figure 18 is a plastic nozzle needle dispensing location map, IDN represents the plastic nozzle needle mouth diameter, W is gum drops diameter at the bottom (see Figure 19); ND means that when positioning the needle touches the PCB, PCB plastic nozzle needle from a distance, and said the move until the height; H is the effective height of glue (see Figure 19).
A large number of experiments show, drawing smearing when dispensing, shall comply with the following two rules of thumb to avoid:
One rule of thumb is: the diameter of the gel-point (W) and the inner diameter of the nozzle (IDN) ratio of 2: 1.
However, in the production of plastic the size of the dot diameter W again be determined by the size of the mount components, in other words by the size of mount components to determine the diameter of the gel-point W, then glue dot diameter W chosen by needle mouth.
Another rule of thumb is: When the inner diameter of the plastic nozzle needle finalized, should adjust the distance the plastic nozzle opening to the PCB ND, which is an important parameter, ND, also known as the stopper height. When the machine is working, thimble touch receiving PCB, the machine sends a signal via a pneumatic mechanism so that the valve is opened, the air pressure is applied to begin booster hose inside, let the pressure is P, and forced out of the glue while pressing time is set to T, when after the time is in place, the pressure valve is closed, stop dispensing. Then the dispensing head to the next dispensing position. Obviously, when the start of the dispensing ND, P, T of the three values is adjusted to an appropriate parameter, ND place when, P, T set is the best, the glue points on the drop fully PCB (see Fig. 19).
When ND is too small, P, T set again too large, too little space between the plastic nozzle with PCB, glue overflow to the surrounding pressure and will even flow near the locating pin, and easy to contaminated needles (Fig. 20). Conversely, ND is too large, P, T set also too small, then glue dot diameter W will be small, and the gel-point height H will increase, so that when the dispensing head movement at the moment, there will be drawing and smearing (see Figure 21). It should be carefully adjusted P, T value when the ND value is determined.
Now glue the inner diameter of the nozzle needle stopper height ND experience supporting parameters are listed in Table 5.
Table 5 plastic nozzle inner diameter is highly correlated with the stop watch
Plastic nozzle inner diameter of the IDN
0.4
0.5
0.6
Fixed height only
Stopper height NA
0.1-0.2
0.15-0.25
0.2-0.3
Half of the value of the inner diameter of the needle tip
Actual production, because of different dispenser device, the dispensing nozzle inside diameter of the preparation is not the same, the corresponding height of the stopper will be different, it should be used to set the parameters of the machine according to the principles described above. Moreover, in addition to the production of the correlation coefficient should be carefully adjusted, it should also regularly check the dispensing nozzle wear dressing thimble and glue the ends of the mouth, wear too much should be replaced, and keep the jack, plastic mouth clean to prevent viscose agents and dust adhered to the locating pin.
(3) Z-axis height of the return
In actual production, there is an important machine parameters, i.e., the head (Z axis) distance of the rise backhaul (sometimes referred to as latency), this is because, when the stopper lever into contact with PCB, the machine can work instructions issued this signal is no "time difference", but signaled the real glue is squeezed out of a moment of apparent "time difference", also known as "delayed" effect, just as to charge the capacitor, the need for a time, which because the gas has "compressibility" features. In addition, a combination of factors and the impact on the sensitivity of the machine, hose diameter, plastic nozzle head length signal related. Related to the operation principle shown in Figure 22.
Therefore, if the Z-axis return distance is too small, then glue the plastic nozzle will be dragged from one dot to another dot, easy to produce tail, leaving only when the glue is completely plastic mouth the moment, the dispensing head to leave for the best. Some dispensers using long dispensing, dispensing is to reduce speed, ensure that the glue is completely out of the plastic nozzle exit. Taking into account the reduced speed glue, it will affect the yield, so take the bull dispensing, dispensing increase the average speed, to achieve both eliminate smearing and improve production purposes.
3. The relationship between the inner diameter of the plastic nozzle orifice size and components
Since the chip components sample sizes, and is therefore required between PCB bond strength are not the same, namely the amount of plastic coating between the components and PCB are not the same, so the dispenser often configured with different inner diameter plastic nozzle . Such as Panasonic dispenser configuration 3L, S and VS three kinds of plastic nozzle inner diameter, the inner diameter thereof are 0.58mm, 0.41mm and 0.33mm.
What is the inner diameter of the needle between the plastic nozzle and component size matter? Typically dot size should be on the brink of the distance between the pads, while also taking into account the accuracy of the glue point position. In φ0805, φ1206 chip components, for example, the relationship between the inner diameter of the pad size and needle-nose element, based on experience Company:
Glue dot diameter (W) / the plastic nozzle inner diameter (IDN) = 2/1
Available, the plastic nozzle inner diameter of 0.4mm and are 0.6mm, see Table 6 and Figure 23.
Element cemented area (2.0-0.4 × 2) × 1.25 = 1.5mm2;
Bonding strength to take 150kg / cm2 that is 1.5N / mm2;
The bonding strength of each piece of the theoretical 1.5 × 15 = 22.5N / tablets.
And usually element bonding strength greater than 0805 5N sufficient. If dual little glue
Rule, the strength is also increased.
Table 6-elements pad size and the inner diameter of the needle tip Relations
Component size
Pad width X
(Mm)
A distance between the two pads
(Mm)
Glue point allows a maximum diameter
W (mm)
Recommended plastic nozzle inner diameter
(Mm)
[0805]
2.0mm × 1.25mm
21.4
1.0
0.8
0.4
[1206]
3.2mm × 1.6mm
1.6
1.8
1.2
0.4-0.6
2.3 Optimization of process parameters set
1. glue point height
In introducing the geometry of the front of high-quality glue dots, spoke about the shape factor W / H is 2.7-4.6 for the best; then glue point height H, how to determine it? Let us look at the size of elements corresponding stickers on the PCB, as shown in Figure 24.
As can be seen from the figure, A is the thickness of the PCB pad layer, typically 0.05mm, B is an element end of the soldering tip encapsulated metal thickness, typically 0.1mm, SO-23 is up to as much as 0.3mm. Therefore, to reach the bottom surface of the PCB components good bonding, plastic patch height H> A + B, taking into account the state of the gel-point is inverted triangle top in order to achieve inter-element has 80% of the area of PCB combined project H should be reached (1.5) times the (A + B), and therefore, in order to increase the height H, should be designed to have a dimension between the rubber pad and the selection of auxiliary points with the bottom surface of the pin member in the plane of the smaller elements to achieve good the bonding strength, as shown in Figure 25.
2. The amount of the gel-point set
Early dispensing process for small-size RC components such as 0805, set up a dot, and now the trend is that all components are recommended Shuangjiao points and disposed outside elements, which guaranteed the quality of bonding is, in other words, even if a glue dot quality problems, there is a gel-point play a role in the adhesion. Dot position is set outside the element also takes into account the relative position of the pads and has increased, it is possible to prevent too large of a bond to the maintenance difficult. In addition, the position of the heat curable adhesive can be the desired position and desired light-curing adhesive both together.
For SOIC, 3-4 points are generally installed, than using only two points better, not only to increase the strength of the earthquake can also play a role, as in the adhesive prior to curing, the adhesive force is always limited for large devices, because weight increases, when inertia will increase. IC on a large mass of plastic point, if only two glue points, giving a "floating" feeling, a bit of a shock, there will be "slip" to increase the number of points is increased bonding glue area, prevent large element of "slippage" can play a good defensive role.
Comprehensive regulation of dispensing parameters and components related to the size of the parameters in Table 7
Table 7:00 machines process parameters and component size relational tables
Component size
Machine parameters
C0603
C0805
C1206
SOT23
SOD80
SO16-28
Plastic nozzle diameter (mm)
0.3
0.4
0.4
0.4
0.6
0.6
The height of the movable stopper (mm)
0.1
0.1
0.15
0.15
0.3
0.3
Dot points
2
2
2
2
2
4
The distance between the corresponding pin
0.8
1
1.0-1.2
1.0-1.3
2
Glue dot diameter (W) (mm)
0.5
0.7 ± 0.1
0.9 ± 0.1
0.9 ± 0.1
1.35 ± 0.15
17.7 ± 0.3
Pressure (bar)
3
3
3
3
2.7
2.7
Dispensing time (ms)
50
50
80
80
100
120
Hose Temperature (℃)
24 ± 1
24 ± 1
24 ± 1
24 ± 1
24 ± 1
24 ± 1
Dispensing process in common defects
Drawing / tailing is dispensing the common pitfalls, drawing / trailing easily contaminated pads can cause Weld.
First Sticky difference will result when components under loads that cause or cure element displacement Weld.
Plastic patch is too small, when the component placement because the glue is too small, causing insufficient strength after curing can cause off-chip components, but will appear in the wave soldering.
2.5 patch adhesive curing
After the patch of plastic coating, stickers Bahrain components can into a curing oven curing, curing the adhesive patch - a key step in the wave soldering process, in many cases due to poor or non-curing adhesive patch completely cured (especially in the case of the uneven distribution of PCB components Kin most common), during transport, the welding process, there will be parts fall off.
It should work earnestly cured. The use of different types of plastic, various curing methods, the two methods commonly used in the curing, one is heat-cured, and the other one is light-cured, are discussed in turn below:
1. Thermal curing
Epoxy type adhesive patch using thermal curing, heat curing is effected early in an oven, and now more on the infrared reflow oven cured to achieve continuous production. Before the official production should first adjust the furnace, the furnace temperature curing profile and make the appropriate product, more attention to doing the curing curve is: different manufacturers, different batches of the patch adhesive curing curve is not exactly the same; even the same kind of patch plastic, with different products, due to the size of the board, the number of different elements, the set temperature will be different, which is often overlooked. Often this happens: When welding IC device, after curing, all the pins also falls on the pad, but after wave IC pin will appear shifted even leave the pad and weld defects. Therefore, to ensure the welding quality, should insist on the temperature profile of each product were to be done, but also to do a good job.
Two important parameters (1) curing epoxy adhesive
Heat curing epoxy adhesive patch, and its essence is catalyzed epoxy curing agents gene at high temperatures.
XXXXX open loop chemical reaction. Therefore, the curing process, there are two important parameters should attract attention on: First, the initial heating rate; the second is the peak temperature. Heating rate determines the surface quality of the cured, while the peak temperature determines bond strength after curing. These two parameters should be plastic patch on suppliers, which provide only vendor curing curve than more meaningful, it can make you patch of plastic used in the performance of an understanding. Figure 26 is a graph using a different temperature curing curing adhesive patch.
As can be seen from Figure 26 Influence of temperature on the bond strength of the bond is more important than the effects of time on the bond strength at a given curing temperature, curing time increases, the shear force small increase, but the curing temperature when raised, the same curing time, the shear strength was significantly increased, but the fast heating rates sometimes pinholes and bubbles. Therefore, in production, should hold the PCB after first light board with a dispensing device into an infrared oven curing, cooling carefully with a magnifying glass to see if the patch of plastic surface with bubbles and pinholes, if found to have pinholes, should be carefully analyze the causes and find troubleshooting. In doing furnace temperature curing curve, the combination of these two factors should be repeatedly adjusted to ensure getting a satisfactory temperature curve.
Test Method (2) a curing profile
SMD adhesive curing curve test method in the infrared reflow oven and the instruments used, with the same solder reflow soldering infrared temperature curve method, where no introduction. Its heating rate and curing temperature curve according to design parameters provided by the vendor. Apart from the face of controversial negotiations with suppliers, but also to the relevant departments for testing differential scanning calorimetry (DSC), identification of adhesive performance.
2. Light curing / heat curing
When using light-curing adhesive, is used reflow oven cured with UV light, the curing speed and quality but also high. Typically reflow oven lamp power for incidental addition 2-3kW, about 10cm height from the PCA, the 10-15s causes the exposed element in vitro rapid curing adhesive patch while continuing to maintain a temperature of 150-140 ℃ oven for about 1min , you can make the following elements through adhesive curing.
2.6 Use of glue Notes
(1) Storage
Repurchase of plastic patch should be low (0 ℃) storage, and do the registration work, pay attention to the date of manufacture and service life (bulk purchase should be inspected storage).
(2) using
Note that when using plastic models, viscosity, according to the requirements of the current products, and recovery at room temperature for about 1 hour (large packaging should be about 4 hours) before stopping use, pay attention to track the first product, when used, new for the actual observation on all aspects of performance adhesive patch.
Applications need to clean dispensing syringe; filling not too full (2/3 by volume) and bubble removal treatment.
Do not use different models and different manufacturers glue mix with each other, when the replacement of species, all tools should be in contact with the glue thoroughly cleaned.
Note that the diameter of the gel-point check should be used, generally at the edge of the PCB in the process of high-test rubber 1-2 points, you can enter 0805 elements when necessary, is often observed before and after curing adhesive dot diameter changes, the patch use plastic quality really be aware of.
Good point PCB should sticky piece of plastic and cured, special circumstances should be suspended for dispensing glue points on the PCB to prevent the air to absorb water vapor and dust, resulting in placement quality.
(3) Cleaning
In production, especially the replacement of plastic types, or time with a long time, a syringe and other tools should be cleaned, especially needle-nose, mouth, etc. are usually small items should be classified needle treatment, metal needle tip should be soaked in a jar, bottle discharge cleaning fluid (by the supplier) or acetone, toluene, and the compound continuously swing, have a good cleaning capability. A syringe can also be cleaned with a brush soaked, with compressed air, lint-free cloth to clean the paper. Ethanol for uncured rubber have good cleaning ability, and environmental pollution.
(4) Return
The need for repair of components (cured) can be uniformly hot air gun heating element, such element has been soldered, but also to increase the temperature of the welding point can also be melted and timely tweezers to remove the element, a large heated IC needs repair station after removing the element should still be in coordination with the heat gun slowly eradicate residual glue with a knife, do not destroy the PCB silver bullion, you need to re-dispensing, local cured with a hot air gun (should ensure that the heating temperature and time).
3. Dispensing - wave soldering processes common pitfalls and solutions
1. Drawing / trailing
Phenomenon: Drawing / tailing is dispensing common defects.
The reason: the plastic nozzle inner diameter is too small, dispensing pressure is too high, the plastic nozzle distance from the PCB time too, viscose agents expired or bad quality, high viscosity adhesive patch, remove from the refrigerator fails to return to room temperature, too dispensing volume.
Solution: Change the inner diameter of the larger plastic nozzle, reduce dispensing pressure, adjust the "stop" high, for plastic, select the appropriate types of plastic viscosity, remove from the refrigerator to return to room temperature (about 4 hours), adjust the dispensing the amount.
2. Plastic nozzle clogging
Phenomenon: the amount of plastic mouth too few or no glue points out.
Reason: not completely clean pinhole clean, patch glue mixed with impurities, there is clogging phenomenon, incompatible glue mixed.
Workaround: for clean needles, for good quality plastic patch, patch glue grades should not be mistaken.
3. Runaway
Phenomenon: Only dispensing action, but no amount of glue.
Reason: mixed with air bubbles, plastic nozzle clogging.
Solution: a syringe in a bubble gum removal should be treated (especially their own equipment of plastic), according to the plastic nozzle clogging manner.
4. Component shift
Description: After curing components shift, serious component pin is not on the pad.
Reason: the uneven patch of plastic glue (glue two chip components such as a little more than one), while SMD components shift, early patch of plastic low adhesive power, PCB place after dispensing time is too long, semi-cured glue.
Solution: Check whether the plastic nozzle clogging, remove the plastic uneven, adjust the placement machine working condition, for glue, PCB placement should not be too long after the dispense time (less than 4 hours).
5. After curing, the Component bond strength is not enough, after the wave will be out pieces
Description: After curing, the Component bond strength is not enough, less than a predetermined value, and sometimes appear out of touch tablets.
Cause: The curing process parameters are not in place, particularly temperature is not enough, the element size is too large, heat capacity, light curing lamp aging, the glue is not enough, the component / PCB contamination.
Solution: Adjust the curing curve, especially to improve the curing temperature, the peak heat curing adhesive curing temperature is usually very critical, reached the peak temperature can lead to off-chip, light glue, it should be observed curing light is aging, whether the lamp there are black phenomenon; whether the amount of glue, components / PCB contamination is a problem that should be considered.
6. After curing component pin floating / shift
After curing the correct morphology shown in Figure 27.
Description: After curing component lead float or shift after wave soldering material will enter the pad, and in severe short-circuit and open-circuit, shown in Figure 28.
Reason: plastic patch is uneven, too much glue patch, patch into the element offset.
Solution: Adjust the dispensing process parameters to control the dispensing volume, adjust the placement process parameters.
