MOUNTING TIPS FOR LCC MAGNETIC SENSORS AN216

MOUNTING TIPS FOR LCC MAGNETIC SENSORS AN216
AN216
SENSOR PRODUCTS
APPLICATION NOTE
MOUNTING TIPS FOR LCC MAGNETIC SENSORS
ABSTRACT
LCC PACKAGE HANDLING
Some of Honeywell’s Anisotropic Magneto-Resistive
(AMR) sensors are packaged in Leadless Chip Carrier
(LCC) components for surface-mount attachment to
Printed Circuit Boards (PCBs). This application note
shall describe guidelines for approaching the correct
methods for designing the mating circuit board pads
for locating LCC components, and recommended
process steps for successful solder reflow to bond the
LCC to the PCB.
The epoxy encapsulated LCC packages for
Honeywell’s AMR sensors are non-hermetic and
absorb moisture from the environment. When stored, it
is recommend that standard “dry pack” procedures be
implemented. These could be sealed bag storage with
desiccant pouches and humidity indicator strips
enclosed with the package. An alternative would
storage in a nitrogen dry box awaiting imminent
transfer to the assembly line.
THE LCC ADVANTAGE
If several days of uncontrolled exposure of heat and
humidity extremes (>30°C, >60%RH) are likely, a
baking cycle prior to assembly should be initiated. This
“bake-out” should be from 1 to 24 hours at +125°C,
with 8-hours a recommended duration. With the LCC
packages “dry” from a bake-out, little probability for
excess internal vapor pressure and resultant
“popcorning” at reflow will occur.
The rationale for the popularity of Leadless Chip
Carrier packages comes from the size advantage and
cost-effective materials used. When compared to
packages with gull-wing leads, LCC packages offer
significant reductions in PCB real estate needed for
small, densly populated circuit boards. For instance
the basic HMC1052 MSSOP lead package has about
a 16mm2 foot print (~4mm by 4mm) compared to the
HMC1052L LCC package foot print of 9mm2 (3mm by
3mm). The difference could mean the difference
between design-in and not designing an electronic
compass feature in small watches and hand-held
wireless products.
Similar to low-cost leaded surface-mount packages,
LCC packages use copper paddle/leadframes and
epoxy over-mold processes. The assembly process is
done in a reel-to-reel method starting with die
placement/attachment, then following with ball bonding
gold wires between the leads and die pads, epoxy
molding, and stamping remaining leadframe supports
from the completed package. All this assembly is done
with a minimum of human intervention, and results in a
high-volume, low cost packaging process. Figure 1
shows a typical cross-section of a LCC package.
Figure 1
LCC Package Cutaway
PCB DESIGN GUIDELINES
Mounting Honeywell magnetic sensors with LCC
packages has a few extra guidelines beyond what
normally applies to through-hole or standard SMT PCB
rules for layout. The following paragraphs shall cover
PCB solder “Land” pad sizing, thermal pad
recommendations, solder mask usage, and magnetic
material content issues.
LAND PAD SIZING
With the ever-shrinking package and pad sizes for
LCC components, over-sizing the corresponding PCB
land pads becomes an important consideration for
reliable connections. This is even more so for some
magnetic sensor applications as it is sometimes critical
to locate the sensor with mechanical precision.
Figure 2 shows a typical bottom-side package layout
for a LCC and also shows the pins and dimension
conventions. Honeywell’s LCC devices have exposed
center pads, but do not have any electrical connection.
It is recommended that the PCB not have a center pad
feature and use the area for trace routing, and vias
with cover-lay insulation to prevent shorting. Overall
package dimensions “D” (side-to-side) and “E” (top-tobottom) are not denoted in Figure 2, but the exposed
pad contains the dimensions D2 and E2. Pad spacing
or “pitch” is denoted by dimension “e” and maybe
stated in English (mils) or Metric (milli-meters) values.
Solid State Electronics Center • www.magneticsensors.com • (800) 323-8295 • Page 1
AN216
SENSOR PRODUCTS
Putting all this together with the lands and pins, is
shown in the example of Figure 4.
Figure 2
LCC Package Dimensions
The outside terminal pins are finger-shaped soldercoated with the outside edge flat and the inside
rounded. The pin width is denoted as “b” and pin
length is denoted as “L”. The clearance between the
pin length and the exposed pad edges is “k”. Note that
the terminal pins have no edge reference dimension
and are based from the center of the package knowing
the edge pin count, pin pitch, and pin width. Even pin
edge counts will have a pin on the center line, and odd
pin edge counts will have the center line between the
pins (e/2).
Typical LCC “fine” pitch packages will nominally have
0.50mm pin pitch, with pin width in the 0.18mm to
0.30mm tolerance. Terminal pin lengths will nominally
have 0.30mm to 0.50mm values. To design terminal
pin lands, it is recommended that 0.025mm be added
to each edge of the pin width pin land for a total width
increase of 0.05mm. The inside land increase should
be about 0.05mm, with the outside land increase about
0.20mm for both solder filleting and a location for
probing the pins. Figure 3 shows the nominal land and
pad orientation.
SIDE VIEW
PIN
PACKAGE
TOP VIEW
0.025mm
FILET
LAND
PCB
0.05mm
0.20mm
Figure 3
Land Pad Design
PIN
LAND
Figure 4
3x3mm LCC Example
Another important PCB layout item is that pin
numbering proceeds clockwise from the bottom view
of the LCC (counter-clockwise from the top). Either a
chamfered corner of the center pad or a corner contact
feature in lieu of the chamfer will always denote the pin
1 location. Figure 2 shows numbering method in the
upper left side of the figure.
SOLDERING GUIDELINES
Most LCC packages have no special requirements
beyond normal procedures for attaching SMT
components to printed circuit boards. The exception to
this process is Honeywell HMC products that have
ceramic or FR4 substrate packages with epoxy top
encapsulation. These package designs use two solder
types with differing reflow temperatures. Inside these
packages, a high-temp reflow solder is used that
reflows at 225°C and above to make internal circuit
connections. On the package outside, low-temp solder
is recommended with a reflow temp range from 180 to
210°C.
Three heating zones are defined in SMT reflow
soldering process; the preheating zone, the soaking
zone, and the reflow zone. The preheating zone
includes the soaking zone, and nominally ranges from
2 to 4 minutes depending on temperature rise to arrive
in the 160°C to 180°C soaking plateau to active the
flux and remove any remaining moisture in the
assembly. Preheat rise times must not exceed 3°C per
second to avoid moisture and mechanical stresses
that result in “popcorning” the package encapsulation.
Solid State Electronics Center • www.magneticsensors.com • (800) 323-8295 • Page 2
AN216
SENSOR PRODUCTS
The soaking zone is a one to two minute temperature
stabilization time to bring the all the PCB assembly to
an even temperature. Typically this zone has a 0.5 to
0.6°C rise in temperature heading towards the main
reflow heating elements. The reflow zone is 30 to 90
second bump in temperature over the 180°C point to
reflow the screened solder paste before a gradual
cooling. The peak temperature is typically in the 210°C
to 225°C range. In dual temp solder parts, it is
recommended that peak temperatures remain at least
5°C below the internal reflow solder temperature (i.e.
220°C). Figure 5 shows a typical reflow profile.
Figure 5
Typical Reflow Profile
It should be noted that lead-free solders tend to
require higher peak reflow temperatures and longer
reflow times. Cooling zone temperature fall should
decrease not more than 6°C per second to avoid
mechanical stresses in the PCB assembly.
SUMMARY
The above tips for handling and attaching Leadless
Chip Carrier packages should assure reliable
operation of all electrical components used on printed
circuit boards. For further questions, please call for
applications support at 800-323-8295.
900310 7-04 Rev –
Solid State Electronics Center • www.magneticsensors.com • (800) 323-8295 • Page 3
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