27703.1-AN, Rev. K
Product Information
Guidelines for Designing Subassemblies Using Hall-Effect Devices
Introduction
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) technol-
ogy, 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 antici-
pated 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, 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 pro-
vides 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 Prod-
ucts (SMD and Through-Hole), AN26009, on the Allegro
website.
Stress-Sensitive Locations
There are several locations on a package which are vulnera-
ble 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.
Failure Modes
The locations shown in figure 1 are associated with the fol-
lowing 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.
By John Sauber and Bradley Smith
Allegro MicroSystems, Inc.
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.
Wedge bond
Ball bond
Die Face
Leads
Wires
C
B
A
27703.1-AN, Rev. K
2Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
(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 com-
pound 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.
Inside of the package, only a small portion of the leads is embed-
ded into the molding compound. In the case of the K package,
shown in figure 4, only 0.8 mm of the leads, which are 15.6 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.
Forming Leads
Lead-forming operations at the customer facility are often a
necessary part of preparing Hall-effect devices for use in appli-
cations. 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 precau-
tions 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 Exceptions for Certain ATS Packages following.
Standard Forming Procedures
A few simple precautions will ensure that lead-forming does
not induce damaging stress to the leads, the epoxy case, or the
internal IC.
• Leads should not be formed or clipped closer than 0.76 mm to
the package case, and they must be supported from above and
below, so that no movement or stress can occur in this area dur-
ing the lead-forming operation (see figure 5).
Figure 2. Gold bond wire has approximately one-ninth the cross-
sectional area of a human hair and is very fragile.
Figure 3. The “neck” thickness of the wedge bond is approximately
one-fourth of the wire, and is the most likely point of failure.
Figure 4. 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.
Ø0.025 mm
Au wire
Wedge bond
Ø0.076 mm
Human hair
Die
0.81 mm
15.6 mm
Molding compound
edges act as fulcrums
K Package
Force during
handling
Multiplied force
on wedge bond
27703.1-AN, Rev. K
3Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
• When lead-forming close to the package, it is wise to avoid
having protrusions fall into the area of the bend radius (see fig-
ure 6). Due to the stiffness of the material in this area, a smooth
radius will be difficult to achieve.
• The lead-former should clamp the leads sufficiently (top and
bottom) so that there is no force trying to pull the leads from the
epoxy package case during the forming process.
• All bends must be made over a smooth anvil with a radius of at
least one-half the lead thickness.
• Leads should not be deformed in the bend area by squeezing the
leads between the former and the anvil. Spring-back must be
eliminated by over-bending, not by deformation.
• Less stress is transferred to the package if a roller forming tool
is used rather than a push-rod.
• Lead-forming may result in tooling marks on the lead surface.
These are acceptable as long as the marks are not severe enough
to penetrate the plating and expose the leadframe base metal.
Figure 5. Setup for leadforming operations.
Figure 6. Protrusions from removal of dambars. Avoid forming bends in
this area.
27703.1-AN, Rev. K
4Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Exceptions for Certain ATS Packages
Certain Allegro gear tooth sensor IC (ATS) packages are designed
so that they can incorporate the Hall sensor IC with other com-
ponents, such as a pole piece or back-biasing rare-earth pellet, as
an optimized device. Some of these devices can accommodate
simplified bending procedures because of the additional lead sup-
port structures used.
For lead-forming of SA and SB packages, Allegro recommends
that all of the recommendations in the Standard Forming Proce-
dures section be followed.
The SE, SG, SH, and SJ packages have a molded lead bar (fig-
ure 7) that holds the leads coplanar and in position during ship-
ping and handling.
These packages can be lead-formed against the case body without
internal damage if the following recommendations are followed:
• If no clamping of the leads is possible, hold the package (any-
where but over the face of the die) and form the leads using a
“paddle” form mechanism (figure 8). This type of form allows
bending of the external leads next to the package with mini-
mal disturbance to internal components. Any other subsequent
forming of the leads to fit custom applications should have lead
clamping between the case and the formed leads to prevent
damage to the plastic case. These packages should not have
excessive force exerted against the die surface (branded face)
when holding the plastic case for lead-forming.
• If possible, leave the molded lead bar attached during the
lead-forming operation. If the application does not lend itself
to leaving the lead bar attached, 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 spac-
ing.
Inspection criteria for sufficient clamping
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.
• The top side of the lead should have even clamping marks in the
plating. The marks should:
▫ show uniform clamping across most of the width of each indi-
vidual lead, and
▫ show the same shape and depth of clamping across all of the
leads.
Figure 7. Molded lead bar used to constrain leads on some packages for
handling.
Figure 8. Lead forming with a paddle form.
Paddle Form Blade
27703.1-AN, Rev. K
5Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
• The bottom side of the lead should also have uniform clamp-
ing marks. As an additional criterion, the die burr (an artifact of
stamping) on the bottom of the lead should be flattened out. If
the burr is not flattened, the clamping force was not sufficient.
• The clamping force should NOT be so great that it changes the
cross-sectional shape of the base metal, or leaves any visible
damage to the molding compound.
High Temperature Handling Precautions
Thermoset molding compounds have a glass transition tempera-
ture, 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.
General Precautions for Soldering and
Welding
When a process requires that a formed lead be soldered or
welded, there are three main rules to keep in mind:
• Not too short – If possible, it is best to avoid extremely short
leads. A longer lead is more easily bent without creating high
forces. This allows for alignment and forming tolerances, and
also reduces stresses from any expansion mismatch with the
leadframe that it is soldered to.
• Not too hot – As mentioned above, the strength of the molding
compound is greatly reduced at high temperatures. Soldering
and welding operations should be done at the lowest tempera-
ture and in the shortest time possible. Using a longer lead also
minimizes the amount of heat which reaches the device case.
• Not “spring-loaded” – Although forming and alignment toler-
ances mean that leads will usually need to be deformed slightly
during welding or soldering, the less deformation the better.
If a lead must be significantly bent when it is attached in the
application, spring energy will likely be stored in the lead. Any
subsequent high temperature processing (such as overmolding)
or prolonged exposure to high temperature operating environ-
ments may cause the lead to move within the molding com-
pound, resulting in wedge bond failure.
If there are concerns that a given process or design may be creat-
ing high stresses in the leads, which could be causing a reliability
risk, refer to the Design Validation Testing section for informa-
tion on methods for finding latent defects.
Soldering of Leads
In addition to the information in this application note, refer to
Soldering Methods for Allegro Products (SMD and Through-
Hole), AN26009, on the Allegro website. That includes guide-
lines on lead finishes, solders, fluxes, contaminants to avoid, and
general processing parameters.
Welding of Leads
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-fus-
ing (Sn-fusing). The choice of process may be determined by the
application and by production conditions.
Tin-fusing Versus Resistance Welding
• Resistance welding involves heating the parts to be joined until
the base metal is in a plastic state, and then forging the parts
together.
• Tin-fusing involves vaporizing a thin layer of tin plating, which
produces ultra-clean base metal surfaces. An application of
pressure then results in an extremely strong diffusion bond
between the two parts.
• Most types of conventional resistance welding equipment can
also be used for tin-fusing, the difference is that much lower
voltages and currents are used.
Advantages of Tin-Fusing
Tin-fusing may have advantages over resistance welding for
small electronic devices:
• It is very difficult to resistance-weld copper to copper without
destroying the part.
• Tin-fusing causes less deformation of the leads than welding
because it is quicker and because melting the base metal is not
required.
• Tin-fusing creates a diffusion bond of base metal to base metal,
which is often stronger than the bond created by welding (for
copper to copper).
Guidelines for Tin-Fusing
• Reducing the current level used is a process advantage. Tin-
fusing requires much less current than resistance welding. To
avoid overheating the device, the lowest current which creates
a reliable bond should be used. This also reduces the cost of the
electricity required. Figures 9 and 10 are examples for compari-
son.
27703.1-AN, Rev. K
6Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
• Avoid short leads because they can cause the following prob-
lems:
▫ Welding can flatten out leads, and if the leads are very short
then the flattened area can damage the molding compound.
Also, the lateral movement of the lead that occurs when it is
flattened close to the package case can damage the wedge bonds
or cause a short between the leads (see figure 11).
▫ Welding operations create high temperatures which can dam-
age the package body if the leads are too short.
• Ideally, tin-fusing should be used with approximately 100% tin
plating. The presence of lead (Pb) does not affect the quality
of the bond, but the processing of Pb does create a potential
environmental health risk. During tin-fusing, the plating in the
area of where the two pieces touch is vaporized. For a typical
weld with Allegro parts, this would release less than 2 μg of Pb.
The Pb vapor must be collected and disposed of in a safe and
environmentally responsible manner.
• Many types of copper alloys can be successfully welded. The
best choice should be determined by the supplier of the welding
equipment.
• In general, alloys containing iron, such as Kovar or Alloy 42,
should be avoided because they are difficult to weld to a copper
leadframe with tin plating.
Allegro Lead Plating
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.
Reducing the tin plating thickness below the values noted above
is not recommended. Though such action may allow reduction of
fusing current, the thinner plating is more prone to whiskers and
less robust for traditional soldering. Allegro does not offer any
further reduction in plating thickness.
Developing a Tin-Fusing Welding Process
For additional information and advice on equipment and param-
eters, contact:
JCM Technical Services, Inc.
133 Park Ave., Iselin, N.J., USA 08830
Phone: 1.732.381.6210
Fax: 1.732.381.6213
E-mail: jpmang@jcmserv.com
Web: www.jcmserv.com
Figure 9. 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 10. 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 9.
Figure 11. Damage caused by flattening while welding
leads that are too short
Tin plate dripping
off the lead Deformed base metal
Movement of leads during welding
may damage wedge bonds
Excessive heat can damage
molding compound
Short-circuited pins
27703.1-AN, Rev. K
7Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Attaching the Device to the Subassembly
Most gluing, coating, potting, or encapsulation methods add
stress to the package, which can result in electrical parameter
shift and scatter.
Gluing
Gluing a device into a cavity in a manufactured subassembly is a
common method of assembling a Hall effect interface. The basic
rules are:
• Match the expansion characteristics of the glue or molding
epoxy as closely as possible to the component epoxy, which has
an expansion rate of 12 to 30 ppm/°C. Most highly-filled (non-
conductive) epoxies fit into this category and usually are good
choices.
• Surface-mount component mounting epoxies can also be used
to attach molded components. These materials do not match the
Hall-device characteristics as well as filled epoxies, but have
the advantage of being effective in very tiny dot sizes and they
have a fast cure time.
• Cyanoacrylate ( “super glue” ) is not a good choice for gluing
Hall-effect devices because it has a high rate of shrinkage when
it cures. If the glue is applied to only one side of the device, this
shrinkage can bend the device and cause severe stress. These
glues also tend to be biodegradable and can dissipate i
本文档为【注塑弯折要点注意事项】,请使用软件OFFICE或WPS软件打开。作品中的文字与图均可以修改和编辑,
图片更改请在作品中右键图片并更换,文字修改请直接点击文字进行修改,也可以新增和删除文档中的内容。
该文档来自用户分享,如有侵权行为请发邮件ishare@vip.sina.com联系网站客服,我们会及时删除。
[版权声明] 本站所有资料为用户分享产生,若发现您的权利被侵害,请联系客服邮件isharekefu@iask.cn,我们尽快处理。
本作品所展示的图片、画像、字体、音乐的版权可能需版权方额外授权,请谨慎使用。
网站提供的党政主题相关内容(国旗、国徽、党徽..)目的在于配合国家政策宣传,仅限个人学习分享使用,禁止用于任何广告和商用目的。