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Adhesives/Epoxies & Dispensing
December 31, 1969 |Estimated reading time: 13 minutes
Step 5
Chris Marinelli
Dr. Barry Burns
Surface mount adhesives (SMA) are used in both wave and reflow soldering to maintain component positioning on the printed circuit board (PCB), and to ensure that parts are not displaced in transit through the assembly line (see sidebar).
Most SMAs used in PCB assembly are epoxies, although acrylics are still applied for specific applications. Epoxies became the more prevalent adhesive technology worldwide after high-speed dispensing systems were introduced and the industry learned to work with products with relatively short shelf lives. Epoxies generally provide good adhesion to a wide range of substrates while delivering excellent electrical properties.
Desired Attributes
Epoxy adhesives are formulated to provide a range of benefits to the user, including good dispensability; consistent dot profile and size; high wet and cured strength; and rapid cure, flexibility and resistance to thermal shock. Epoxies allow for high-speed dispensing of very small dots, offer excellent on-board cured electrical properties, and are non-stringing and non-slumping during the heat-cure cycle. (Because epoxies are heat sensitive, they must be stored under refrigerated conditions [5°C] to ensure maximum shelf life.)
Using visual detection or automated equipment, SMAs must contrast with the typical green or brown substrates. With automated vision-control systems used to assist this process, red and yellow have emerged as the two basic adhesive colors. However, the ideal color for an application depends on the visual contrast between board and adhesive.
Typically, heat curing of epoxies occurs in-line, in an infrared (IR) tunnel oven. While the minimum temperature to initiate cure is 100°C, in practice, cure temperatures range from 110° to 160°C. Temperatures above 160°C will yield faster process times but are more likely to result in brittle joints.
Bond strength is critical to the performance of an adhesive. It is determined by several factors, such as degree of adhesion to the component and PCB, adhesive dot configuration and size, and level of cure. The three most common causes of poor bond strength are inadequate cure, insufficient amount applied and poor adhesion.
Dot Profile
An adhesive`s flow behavior, or rheology, affects the way an epoxy dot is formed as well as its shape and size. SMAs are formulated to permit rapid and controlled dispensing and to form adhesive dots of a defined shape (Figure 1). To ensure well-defined and maintained dot profiles, adhesives are engineered to be thixotropic (i.e., they thin when agitated and thicken at rest). In this process, the viscosity of SMAs decreases when shearing stresses are applied during dispensing, permitting easy flow. When the adhesive hits the PCB surface, it quickly restructures and recovers its original viscosity.
Dot profile also is influenced by thixotropic recovery rate, viscosity at zero shear rates and other factors. The actual dot shape may be "peaky"/conical or rounded hemispherical. However, the dot profile is defined by non-adhesive parameters such as dot volume, dispensing needle diameter and standoff height. That is, for a given adhesive grade, it is possible to produce either exaggeratedly high, narrow dots or low, wide dots by adjusting their parameters.
After chip placement, the dispensed dots have two requirements: They must have a diameter smaller than the space between the solder pads, and be high enough to bridge the gap between the PCB surface and the body of the device - while not interfering with the placement head. The adhesive gap is defined by solder pad height above the PCB soldermask surface and the difference in end metallization and body thickness of the device. This can vary from less than 0.05 mm for flat chips to more than 0.3 mm for the larger small-outline packages (SOP) and quad flat packs (QFP).
Dispensing tall dots ensures good adhesive coverage on components with high standoffs. Tall dots also permit adhesives to be squeezed out between low-standoff components without risking pad contamination. Often, two sets of dispensing parameters are used side by side for a single adhesive grade: one producing a high, large-volume dot for high standoff components, the other yielding moderate height and volume dots for flat chips and metal electrode face (MELF) parts.
The adhesive dot size is also controlled by the internal diameter/standoff height ratios of the selected nozzles. Typically, dot width-to-height ratio will range from 1.5:1 to 5:1 (h/w = 0.2 to 0.6), depending on dispensing system parameters and adhesive grade. These ratios can be optimised for any component by adjusting machine settings.
Avoiding Voids
Moisture in an adhesive dot can boil during curing and cause voids that weaken the joint and open paths for solder to penetrate under the device, possibly causing circuit shorting because of solder bridging. In a syringe, adhesives have an insignificant level of moisture; however, if left in an uncured state and exposed to ambient conditions, the adhesive may absorb moisture, particularly in humid environments. For example, moisture absorption frequently is a problem with pin-transfer dispensing because of the large exposed surface area presented by open baths of adhesive. It can also occur in syringe dispensing applications where long delays between dispensing and curing exist or where ambient conditions are very humid. In response, most SMAs are formulated with raw materials that have low moisture pickup properties to minimize this effect.
Using a lower temperature slow cure with a longer warm-up time helps moisture escape before curing occurs and may solve the problem of void formation. Similarly, moisture can be eliminated from the substrates by storing components in a cool, dry location or by conditioning the materials before use in a moderate temperature drying oven. Avoiding process stoppages before the adhesive cures and using a special-grade adhesive that absorbs minimal moisture helps reduce voiding problems.
Dispensing Methods
SMAs may be applied to a PCB using syringe dispense, pin transfer or stencil printing. Used in less than 10 percent of total applications, pin-transfer dispensing uses an array of pins dipped in a tray of adhesive. The suspended adhesive drops are then transferred as a unit to the board. These systems require a lower viscosity adhesive with good resistance to moisture absorption because of the exposure to ambient conditions in the bath. Factors critical to controlling pin-transfer dispensing include pin diameter and design, adhesive temperature, the depth the pins reach in the bath, and the length of the dispensing cycle (including delays before and during pin contact with the PCB). Bath temperature, which controls adhesive viscosity and the adhesive dot`s quantity and form, should be set between 25° and 30°C.
Widely accepted for solder paste application, stencil printing can be used for dispensing adhesives, as well. Although less than 2 percent of SMAs are currently stencil printed, interest in this method has increased and new equipment developments are overcoming earlier limitations. Correct stencilling parameters are critical for good results. For example, on-contact printing (zero snapoff) may require a delay cycle to permit good dot formation. Additionally, off-contact printing (approximately a 1 mm gap) with a polymer stencil requires optimum squeegee speed and pressure. Metal stencil thickness is typically 0.15 to 0.20 mm and should be slightly greater (+0.05 mm) than the maximum gap between the device and the PCB.
More than 90 percent of SMT adhesives are currently dispensed via syringe (Figure 2), which can be further divided into two subgroups: pressure time systems and volumetric systems. Pressure time syringe dispensing is the most common method and the remainder of this step will cover this technique. Syringes can dispense up to 50,000 dph and are highly adjustable to meet changing production needs.
Troubleshooting Dispensing Defects
There are several adhesive dispensing issues that, left unaddressed, may lead to final processing defects. These include stringing, inconsistent dot size, missed dots and satellite dots. Adhesive stringing can result in pad contamination and poor solder joints. The adhesive must break quickly and cleanly from the nozzle as it retracts (Figure 3). Even adhesive grades formulated specifically for high-speed dispensing can string if the parameters are incorrect. For example, the risk of stringing is high when adhesive volume is too small for the nozzle`s diameter and the required standoff height, the result is a very tall, thin dot. While a smaller needle diameter/standoff height combination can solve the problem, stringing also can be caused by several non-adhesive-related parameters, such as electrostatic charge on the board, incorrect Z-stroke adjustment height, and board warpage or insufficient board support.
In the case of missed adhesive dots, components will not be placed properly. Missing dots can occur if line pressure is insufficient for dispensing (i.e., pressure at the back of the syringe may be lacking so the adhesive is emitted inconsistently). Similarly, inconsistent dot sizes affect overall bond strength between the board and component. This can occur for several reasons:
- The nozzle`s standoff post is landing on the solder pads. A replacement nozzle with a different standoff post location will solve the problem.
- Insufficient time allotted for adhesive recovery. Increasing the delay sequence can solve recovery problems.
- If there is insufficient pressure time to complete the dispensing cycle (or as the adhesive level declines), increasing the pressure-on/cycle time, typically expressed as a percentage of maximum, will correct dot size inconsistency.
- Because satellite dots are imprecisely located, they can result in pad contamination or insufficient bond strength. When nozzle standoff is too high for the intended dispensed dot, height reduction will eliminate satellites. If dot volume is too great for the nozzle, reducing pressure or using a nozzle with a larger inner diameter (ID) will solve the problem.
Factors Affecting Dispensability
Good dispensing does not depend on adhesive qualities alone. For pressure time syringe dispensing, a number of machine related factors influence dispensability and dot formation. The needle`s ID is critical to dot formation and must be significantly smaller than the diameter of the dot on the board. As a rule, the ratio should be 2:1. Dot sizes of 0.7 to 0.9 mm require an ID of 0.4 mm; those of 0.5 to 0.6 mm demand an ID of 0.3 mm. Equipment manufacturers typically provide specifications and guidelines to produce the desired dot sizes and configurations.
PCB-to-needle standoff, or stopper height, controls the dot height (Figure 4). It must be appropriate for the quantity of adhesive dispensed and the needle`s ID. For a given adhesive volume, the dot`s height-to-width ratio will increase with stopper height. Typically, maximum stopper height is half the needle`s ID; beyond this point, inconsistent dispensing and stringing will occur.
Current high-speed equipment uses dispensing cycles in which the pressure can be timed to start before the nozzle is in position. Nozzle retraction speed, retraction height, and the delay between dispensing and nozzle retraction all affect dot shape and stringing.
Finally, temperature will affect viscosity and dot shape. Most modern dispensers rely on temperature-control devices either on the nozzle or chamber to maintain the adhesive`s temperature above ambient. However, dot profile can suffer if the PCB temperature is elevated from that of previous processing.
Maintenance
Bent or worn dispensing needles and stoppers can detrimentally affect dispensing. Excess adhesive on the needle`s exterior can interrupt the smooth and consistent dot formation. And in extreme cases, adhesives can bridge to the stopper pin, disrupting dispensing. The all-purpose solution is to keep nozzle exteriors as clean as possible.
Cleanliness of the nozzle`s inner surfaces is another common source of dispensing problems. Adhesive buildup can occur on the ID, restricting flow. Adhesives also may partially cure in the nozzle if left for long periods in a warm environment or incompatible solvent. Changing adhesive grades can cause cross-contamination and nozzle blockage. (Needle blockages caused by cured or partially cured adhesives should be cleared using a drill bit before solvent cleaning.) Needles should be inspected regularly, but cleaned only when a dispensing problem becomes apparent. Cleaning can increase problems experienced when attaching an empty nozzle to the syringe.
Leaving dirty needles to soak in baths of solvent is a common but inefficient cleaning method. When soaking needles, use a compatible solvent but do not rely on soaking alone to remove all uncured material. A jet spray using a compatible solvent can blast the uncured adhesive from the needle bore. The needle is then dried by blowing dry, compressed air through the bore.
An alternative cleaning approach involves an ultrasonic or static bath. Uncured adhesive should be mechanically removed using a blunt tool for any larger cavities and a drill bit or piano wire of appropriate diameter for the needle bore. Immerse items to be cleaned in a reservoir of fresh solvent. For an ultrasonic bath, set at maximum power for three minutes at 40°C. For a static bath, agitate the items in the bath until the solvent is stained from the adhesive. Rinse the parts in fresh solvent to confirm cleanliness. Use a jet spray for needles with very small bores or cavities, and dry parts by blowing dry, compressed air through the bores.
CHRIS MARINELLI and DR. BARRY BURNS may be contacted at Loctite Corp., 1001 Trout Brook Crossing, Rocky Hill, CT 06067; (860) 571-5100; Fax: (860) 571-5430; E-mail: electronics@loctite.com.
Figure 1. Dot profile of dispensed epoxy adhesive. Its viscosity affects the way it is formed as well as shape and size.
Figure 2. A syringe-type adhesive dispensing system is capable of depositing up to 50,000 dph; the pressure time method is the dominant technique used in SMT.
Figure 3. Adhesive stringing during dispensing can be caused by incorrect material volume at the nozzle, faulty Z-stroke, electrostatic discharge, board warpage and insufficient board support.
Figure 4. Board-to-needle standoff, or stopper height, controls the height of the dispensed dot. It must be suited for adhesive volume and the needle`s ID.
SMT Solder Processing
PCBs that include a mixture of surface mount devices (SMD) and through-hole components may be processed via wave soldering. As shown in (a), the adhesive is dispensed directly on the substrate between the solder pads. The component is placed on the adhesive dot, which is then heat cured. Finally, through-hole components are inserted, the assembly is flipped 180° and put through the solder wave.
In reflow soldering-used on single- and double-sided PCBs that include SMDs- solder paste is stencilled on the solder pads in the first step (b). Next, adhesive is dispensed on the substrate, and the component is placed and reflow soldered. The adhesive is heat cured by the latter step. For double-sided boards, the substrate is then flipped and the topside similarly processed.
When devices are placed, the dispensed adhesive must have sufficient wet or "green" strength to hold them in position until cured. The cured joint needs only enough strength to hold the parts to the underside of the PCB as it passes through the soldering station. (At this point, forces on the joint are quite low, typically less than 2N for a flat chip.) However, an adhesive`s greatest challenge is to survive the thermal shock imposed as the joint moves from preheating to soldering. This requires a combination of good adhesion, strength and flexibility.
Once soldering is completed and the solder joint is permanent, the adhesive no longer serves a useful purpose. However, because it remains on the PCB throughout the board`s working life, it is important that it not degrade and threaten the integrity of the working circuit, especially when exposed to extreme temperatures and humidity. Accordingly, the cured adhesive must offer good cured electrical characteristics and be tested to IPC specifications for corrosion and surface insulation resistance (SIR) under extreme environmental conditions.
(a) For wave soldering, adhesives hold SMDs in position. The board is then flipped for through-hole parts insertion before encountering the solder wave.
(b) In reflow soldering, topside processing does not include adhesive dispensing or the curing steps (2 and 4).
ABCs of SMT
Anisotropic adhesive: A material filled with particles that will conduct current in the Z-axis only.
Condensation inert curing: An adhesive curing method using an inert atmosphere as the heat-transfer medium.
Drawbridging: A soldering defect in which a chip is drawn into an upright position representing a nonconnection. Also called tombstoning.
Hardener: A chemical added to a thermosetting resin to assist its cure.
Rheology: A term describing the viscosity and surface tension properties of solder pastes or adhesives.
Single-center reflow soldering: A process in which both surface mount, held by adhesives, and through-hole components are reflow soldered.
SIR: A test for surface insulation resistance, a measure in ohms of the material`s electrical resistance between conductors.