By John Sauber and Bradley Smith, Allegro MicroSystems, LLC
The Hall effect, discovered by E. H. Hall in 1879, is the basis for all Hall-effect devices. When this physical effect is combined with modern integrated circuit (IC) technology, many useful magnetic sensing products are possible. The Hall element, when properly biased, produces an output voltage that is proportional to a magnetic field. This small voltage is processed through a high-quality amplifier, which produces an analog signal that is proportional to the applied flux density. In Allegro® Hall-effect devices, the signal is conditioned and optimized for various types of magnetic inputs to produce a suitable electrical output.
Hall-effect elements respond to stress by modifying the output voltage versus the magnetic flux-density curve. For this reason, it is important that designers, from chip to final customer, understand that environmental stress from thermal or mechanical sources can affect the output of a Hall-effect element. The chip designer anticipates the end use, builds compensation circuits, and connects multiple Hall elements in such a manner as to minimize the effects of the anticipated environment. When the proper IC design is matched with the proper package design, environmental effects are minimized.
Although robust design techniques greatly reduce the effects that package stresses may place on the operation of the Hall-effect IC, it is important that assembly manufacturers take precautions to avoid unnecessary external stresses such as those caused by overmolding, gluing, welding, lead bending or forming,lead clipping or trimming, or clamping.
In addition to avoiding stresses which affect the electrical parameters, it is also essential to avoid stresses which could introduce any reliability risks. This application note provides design guidelines for subassemblies to avoid both of these problems.
While this document covers most of the assembly methods used for mounting Hall-effect devices, it does not cover soldering to conventional circuit boards. For information on that subject, refer to Soldering Methods for Allegro Products (SMD and Through-Hole) on the Allegro website.
There are several locations on a package which are vulnerable to stress, as shown in figure 1. Regardless of the method used for building a subassembly, it is important to minimize the stress in these areas.
Figure 1. Stress-sensitive locations. (A) Force over die face can cause die cracking and parameter shift. (B) Force over wires can cause damage to wedge or ball bonds. (C) Force or bending applied to leads can damage wedge bonds and cause package cracking.
The locations shown in figure 1 are associated with the following failure modes:
(A) Forces over the die face can cause cracking of the die. The die may fail immediately, or it may have a crack which is a latent defect. See the Design Validation Testing section for information on finding latent defects. Forces over the die can also cause electrical parameter shift. If force must be applied to the die face, it should be distributed evenly over the entire top surface.
(B) Forces over the gold bond wires can damage the ball bond (on the die-end of the wire) or break the wedge bond (on the leadframe end of the wire). These wires are extremely small, having a cross-sectional area that is approximately one-ninth that of a human hair (see figure 2). The "neck" of the wedge bond is even smaller, being about one-fourth of the cross-sectional area of the wire. Any deformation or movement of the molding compound relative to the wire can cause damage, as shown in figure 2 (right panel). Again, it may cause an immediate failure or a latent defect.
(C) Forces or bending moments applied to the leads can cause damage to the wedge bonds (possibly a latent defect), or package cracking.
Figure 2. (Left) Gold bond wire (Ø0.025 mm) has approximately one-ninth the cross-sectional area of a human hair (Ø0.076 mm) and is very fragile. (Right) The "neck" thickness of the wedge bond is approximately one-fourth that of the bond wire, and is the most likely point of failure.
Inside of the package, only a small portion of the leads is embedded into the molding compound. In the case of the K package, shown in figure 3, only 0.8 mm of the leads, which are 15.5 mm long, is inside of the molding compound. The resulting lever arm amplifies the force on the lead by a factor of nineteen so that even a small force can damage the wedge bonds. Because of this, it is important to follow the lead clamping guidelines during lead forming, and to avoid forces on the leads during other processing steps.
Figure 3. It is important to clamp the leads before any lead-forming operations. Because of the leverage effect, even a small load applied to the end of the lead is multiplied (in this package, by 19 times), and produces a large load at the wedge bond.
Lead-forming operations at the customer facility are often a necessary part of preparing Hall-effect devices for use in applications. For most Allegro devices, the few simple precautions described in the next section, Standard Forming Procedures, will ensure that lead-forming does not induce damaging stress to the leads, the epoxy case, or the internal IC. While these precautions should always be taken into consideration, exceptions exist for certain Allegro gear tooth sensor IC (ATS) packages with enhanced lead support. The exceptions are described in section titled Considerations for ATS Packages following.
A few simple precautions will ensure that lead-forming does not induce damaging stress to the leads, the epoxy case, or the internal IC.
Figure 4. Setup for leadforming operations.
Certain Allegro gear tooth sensor IC (ATS) packages are designed so that they can incorporate the Hall sensor IC with other components, such as a pole piece or back-biasing rare-earth pellet, as an optimized device.
For lead-forming of SA and SB packages, Allegro recommends that all of the recommendations in the Standard Forming Procedures section be followed.
The SE, SG, SH, and SJ packages have a molded lead bar (figure 6) that holds the leads coplanar and in position during shipping and handling. Leave the molded lead bar attached during the lead-forming operation. Do not remove the bar until all forming of the leads is complete. This will prevent the leads from spreading apart and will optimize lead planarity and spacing.
Figure 6. Molded lead bar used to constrain leads on some packages for handling.
As mentioned in the previous sections, the lead must be clamped sufficiently to prevent pulling on the leads during forming. Inspecting the "witness marks" left in the plating can show whether or not the clamping was adequate.
Thermoset molding compounds, which are used for the Allegro package body, have a glass transition temperature, Tg, which is typically between 140°C and 160°C. When the compound is heated above its Tg level, it experiences a very significant reduction in its strength. Because of this, when the temperature of any process exceeds Tg, care must be taken not to apply loads to any of the locations shown in figure 1.
In addition to low strength above Tg, the molding compound also experiences viscoplasticity (creep), which allows the compound to deform slowly over time. Care must be taken not to deform the leads so that they become "spring-loaded," because subsequent high-temperature processing can result in lead movement, which also can result in damage to the wedge bonds.
When a process requires that a formed lead be soldered or welded, there are three main rules to keep in mind:
If there are concerns that a given process or design may be creating high stresses in the leads, which could be causing a reliability risk, refer to the Design Validation Testing section for information on methods for finding latent defects.
In addition to the information in this application note, refer to Soldering Methods for Allegro Products (SMD and Through-Hole), on the Allegro website. That includes guidelines on lead finishes, solders, fluxes, contaminants to avoid, and general processing parameters.
As described in this section, welding should be approached with careful attention, planning, and process testing because of the small geometries of the device cases and plating. There are two welding methods that have been used with success, conventional resistance welding, and a type of welding process called tin-fusing (Sn-fusing). The choice of process may be determined by the application and by production conditions.
Tin-fusing may have advantages over resistance welding for small electronic devices:
Figure 7. Damage caused by excessive heat during welding, which includes deformation of the base metal and flow of the tin plating. It was welded at 1700 A, 1.0 V, for 11 ms.
Figure 8. Leads welded with a lower current level (1100 A, 0.8 V, for 10 ms). There is very little damage, and the pull strength of the joint is essentially the same as the overheated leads in figure 7.
Figure 9. Damage caused by lead flattening resulting from welding leads that are too short.
Welding tin-plated copper leads onto a copper leadframe is possible, using a laser. The considerations are similar to tin fusing; thus, avoiding excessive power is important. "Dry" welding can be used, but solder paste can also be used and can provide better solder fillets, to make a stronger joint. Because a laser spot is focused to a very small area, care must be taken to ensure that the resulting bond surface area is sufficient to make a strong bond.
Allegro has taken steps to provide a good tin plate for tin-fusing welding. The typical industry standard for plating thickness mean is 14 µm but Allegro has chosen a standard mean thickness of 11.5 µm. This reduced thickness allows better control of the plating bath parameters and gives a superior quality finish with excellent solderability. It is also better for tin-fusing because there is less tin to melt, so spattering is controlled.
Most gluing, coating, potting, or encapsulation methods add stress to the package, which can result in electrical parameter shift and scatter.
Gluing a device into a cavity in a manufactured subassembly is a common method of assembling a Hall effect interface. The basic rules are:
A conformal coating is often used to provide both protection from the environment, as well as some amount of mechanical protection. For keeping out contaminants, the key properties are water vapor transmission rates and oxygen permeability. Based on these criteria, the best choices are (in order):
Fully encapsulating Hall-effect devices by overmolding with either thermoset or thermoplastic materials can cause parametric shift. Hall-effect devices encapsulated in this way should be retested over the full range of temperatures dictated by the application. The temperatures required for molding thermoplastics are usually above the reflow temperature of the plating on the leads, so the mold design must be such that the plating is not melted (tin melts at 232°C). If plating is melted during the mold process, it may flow against the body of the device and electrically short-circuit adjacent leads.
Cavity pressures in thermoplastic molds are very high. In general, pure hydrostatic pressure does not usually damage the device, as long as it is entirely inside the mold cavity. The mold design must be such that there are no bending forces applied to the Hall device during the molding process. Bending stresses can alter the device parameters and, if high enough, crack the die inside the epoxy package.
In situations where the Hall device forms a plug in an injection mold cavity with the device being held by the leads, it is important that the end of the device be supported. If there is clearance between the device and the end of the mold cavity, then the device can become a piston that is pushed forward in the mold cavity, pulling and stretching the leads.
The safest way to fully enclose a molded Hall-effect device is to design a housing (cap or sleeve) into which the device can slip-fit. The device can then be overmolded, potted, or glued into place. There are several considerations to keep in mind:
Location of the overmold features can also be a concern:
Either thermoset or thermoplastic materials can be used. Selecting a material with the following attributes can minimize the stress and the risk of parameter shift or damage to the die:
Most overmold materials are NOT hermetic, and will not completely protect the device from infiltration of contaminants. This is of particular concern in automotive applications where the assembly may be exposed to harsh environments and to substances such as automatic transmission fluid (ATF), salt water, and brake fluid. The use of a conformal coating, prior to overmolding, can stop moisture ingress and greatly reduce risks, but not eliminate them.
Potting is one of the best ways to assemble without inducing stress. When selecting a potting compound, the material should have the same attributes as listed above for overmolding materials, namely: low CTE, low modulus, and low cure temperature.
Potting with resilient materials, such as RTV silicones or urethanes, can reduce stress. However, when the resilient materials are enclosed in a housing, it is still possible for stresses to result due to differences in the coefficient of thermal expansion. Resilient materials typically have high expansion rates. For this reason, it is important to either leave one end of the container used for potting open, or at least leave some air space inside to allow room for expansion.
Potting with resilient foams is an excellent way to control stress from thermal expansion and still enclose the component. If the foam used is open cell, a sealer will be required to prevent the foam from filling with moisture.
Any ultrasonic welding of plastics in close proximity to the Hall-effect device must be done with caution to avoid work-hardening of the copper base material in the leads and possible breakage of the internal leads. Direct contact between the package or leads and the ultrasonic welding "horn" should be avoided.
Also, as previously mentioned, it is important not to bend the leads during a soldering or welding operation so that they are "spring-loaded." If bending or tensile stresses are stored in the lead, and then ultrasonic energy is applied, it can result in damage to the leads or to the wedge bonds inside the package.
Note: Regardless of what assembly methods are used, it is essential to perform empirical testing to evaluate the effects of stress-induced parameter shift in the final subassembly over the full range of operating temperatures in order to ensure that the parameters remain within the allowable limits.
In the research and design stage of application development, the customer should review the initial intended approach with close regard to the precautions in this application note and all other notes at the www.allegromicro.com website.
Many of these precautions are related to mechanical or thermal conditions which can cause latent defects such as cracked die or damage to the bond wires. If there is a possibility that latent defects are being created, then Allegro recommends the following test plan, which is often able to precipitate latent defects into hard failures. To isolate the effects of forming it is recommended that assemblies be tested without final overmolding, potting or encapsulation.
Assembly of a Hall-effect device into a subassembly may produce some shift in the magnetic parameters. In many cases where this has become a field problem, the selection of device parameters and magnet strength did not allow for minor parameter shifts. Testing of the completed assembly over the full range of operating temperatures should be conducted to determine if the final assemblies are operating close to magnetic limits.
New designs need not have problems with magnetic parameters. Allegro has available a calibrated linear device (pseudo-gauss meter) that can be assembled into prototypes of the intended design. The output readings from this device will allow mapping of the magnetic field and the resultant data will indicate which Allegro device type is best suited for the magnetic circuit design.
Allegro has field applications engineers who can be contacted for issues concerning finished assemblies. Contact information for engineers in your area can be found on the Allegro contact page.