1996 Nov 15 1
Philips Components
Chip resistors Mounting
MOUNTING
Due to their rectangular shape and small dimensional
tolerances, Surface Mounted Resistors are suitable for
handling by automatic placement systems. Chip
placement can be on ceramic substrates and
printed-circuit boards (PCBs). Electrical connection to the
circuit is by wave, vapour phase or infrared soldering. The
end terminations guarantee a reliable contact and the
protective coating enables ‘face down’ mounting.
The temperature rise in a resistor due to power dissipation,
is determined by the laws of heat - conduction, convection
and radiation. The maximum body temperature usually
occurs in the middle of the resistor and is called the
hot-spot temperature.
The hot-spot temperature depends on the ambient
temperature and the dissipated power . This is described in
the datasheets under the chapter heading “Functional
description”.
The hot-spot temperature is important for mounting
because the connections to the chip resistors will reach a
temperature close to the hot-spot temperature. Heat
conducted by the connections must not reach the melting
point of the solder at the joints. Therefore a maximum
solder joint temperature of 110 °C is advised.
The ambient temperature on large or very dense
printed-circuit boards (PCBs) is influenced by the
dissipated power. The ambient temperature will again
influence the hot-spot temperature. Therefore, the packing
density that is allowed on the PCB is influenced by the
dissipated power.
Example of mounting effects
Assume that the maximum temperature of a PCB is 95 °C
and the ambient temperature is 50 °C. In this case the
maximum temperature rise that may be allowed is 45 °C.
In the graph (see Fig.1), this point is found by drawing the
line from point A (PCB = 95 °C) to point B (Tamb =50°C)
and from here to the left axis.
To find the maximum packing density, this horizontal line
is extended until it intersects with the curve,
0.125 W (point C). The maximum packing density,
19 units/50 ×50 mm2 (point D), is found on the horizontal
axis.
Fig.1 PCB temperature as a function of applied power, mounting density and ambient temperature.
h
andbook, full pagewidth
0
150
102
1
MBC739
100
∆T
(K)
50
10 150
0
50
100
ambient temperature
50 100 150
(oC)
(oC)
PCB temperature (normally around 100 C)
o
mounting density (units/50x50 mm2)
C
D
B
A
0.25 W 0.125 W
0.0625 W
1996 Nov 15 2
Philips Components
Chip resistors Mounting
Thermal resistance (Rth)
Thermal resistance prohibits the release of heat generated
within the resistor to the surrounding environment. It is
expressed in K/W and defines the surface temperature
(THS) of the resistor in relation to the ambient temperature
(Tamb) and the load (P) of the resistor, as follows:
THS =T
amb +P×R
th
Due to their direct contact with the solder spot, chip
resistors dissipate over 85% of their heat via conduction to
the solder spot and hence to the PCB. Thus the PCB on
which the chip resistor is mounted functions as a heat sink.
Different PCBs have different heat conductance. Figure 2
shows the different values of heat resistance per material
type. Substrates with a higher heat conductance give
lower thermal resistance figures; substrates with a lower
heat conductance give higher thermal resistance figures.
It should be noted that the temperature of the terminations
of the chip resistor is virtually the same as the hot-spot
temperature. Therefore the power that may be dissipated
by the resistor is dependent on:
Tamb (which is also dependent on the packing density)
Rth of the PCB
maximum solder spot temperature (generally 110 °C).
handbook, halfpage
10 5 resistor 510
d (mm)
Rth
(K/W) 200
50
phenolic
paper
epoxy
DIN44050
96% alumina
MGA205
100
150
Fig.2 Heat resistance for 1206 sized resistors
as a function of distance and material.
Fig.3 Hot-spot temperature rise (∆T) as a
function of dissipated power.
handbook, halfpage
0 0.1 0.150.05 0.3
60
∆ T
(K)
0
20
10
40
30
50
MGC999
0.2 0.25P (W)
(1) (2) (3)
(1) For size 0603: Rth = 400 K/W.
(2) For size 0805: Rth = 250 K/W.
(3) For size 1206: Rth = 200 K/W.
1996 Nov 15 3
Philips Components
Chip resistors Mounting
FOOTPRINT DIMENSIONS
Fig.4 Recommended dimensions of footprints.
handbook, full pagewidth
MBG628
E
B
A
F
C
DG
preferred direction during wave soldering
,,
,,
,
,
,
yz
{
{
|
|
,
y
{
{
z
|
z
|
solder land /
solder paste
pattern
solder resist
pattern
occupied area
tracks
E AVAILABLE AREA FOR TRACKS
underneath the CHIP
For dimensions see Tables 1 and 2.
Table 1 Reflow soldering; for dimensions see also Fig.4
Table 2 Wave soldering (no dummy tracks allowed for the high voltage series); for dimensions see also Fig.4
SIZE
CODE
FOOTPRINT DIMENSIONS
(mm) PROCESSING REMARKS PLACEMENT
ACCURACY
(mm)
ABCDEFG
0402 1.50 0.50 0.50 0.60 0.10 1.75 0.95
IR or hot plate soldering
±0.15
0603 2.1 0.7 0.7 0.9 0.26 2.5 1.5 ±0.15
0603 2.1 0.5 0.8 0.9 0.0 2.5 1.7 ±0.25
0805 2.6 0.9 0.85 1.4 0.5 3.0 2.1 ±0.25
1206 3.8 2.0 0.9 1.8 1.4 4.2 2.5 ±0.25
1218 3.8 2.0 0.9 4.8 1.4 4.2 5.5 ±0.25
SIZE
CODE
FOOTPRINT DIMENSIONS
(mm) PROPOSED NUMBER AND
DIMENSIONS OF DUMMY
TRACKS
(mm)
PLACEMENT
ACCURACY
(mm)
ABCDEFG
0603 2.5 1.1 0.7 0.8 0.3 3.0 1.7 1 ×(0.3 ×0.8) ±0.15
0603 2.7 0.9 0.9 0.8 0.15 3.2 1.9 1 ×(0.15 ×0.8) ±0.25
0805 3.3 1.3 1.0 1.3 0.7 3.9 2.4 1 ×(0.3 ×1.3) ±0.25
1206 4.5 2.5 1.0 1.7 1.25 5.0 2.8 3 ×(0.25 ×1.7) ±0.25
1218 4.5 2.5 1.0 4.8 1.25 4.6 5.9 −±0.25
1996 Nov 15 4
Philips Components
Chip resistors Mounting
SOLDERING CONDITIONS
The robust construction of chip resistors allows them to be completely immersed in a solder bath of 260 °C for one
minute. Therefore, it is possible to mount Surface Mounted Resistors on one side of a PCB and other discrete
components on the reverse (mixed PCBs).
Surface Mounted Resistors are tested for solderability at 235 °C during 2 seconds. The test condition for no leaching is
260 °C for 60 seconds. Typical examples of soldering processes that provide reliable joints without any damage, are
given in Figs 5, 6 and 7.
Fig.5 Infrared soldering.
Typical values (solid line).
Process limits (dotted lines).
0 50 100 150 200 250
300
250
200
150
100
50
0
10 s
260 °C
130 °C
(°C)
t (s)
MLA859
245 °C
215 °C
180 °C
10 s
40 s
2 K/s
T
1996 Nov 15 5
Philips Components
Chip resistors Mounting
Fig.6 Double wave soldering.
Typical values (solid line).
Process limits (dotted lines).
The resistors may be soldered twice in accordance with this method if desired.
0 50 100 150 200 250
300
250
200
150
100
50
0
10 s
235 °C to 260 °Csecond wave
5 K/s
2 K/s
first wave
200 K/s
100 °C to 130 °Cforced
cooling
2 K/s
(°C)
t (s)
T
MLA861
Fig.7 Vapour phase soldering.
Typical values (solid line).
Process limits (dotted line).
0 50 100 150 200 250
300
250
200
150
100
50
0
(°C)
t (s)
215 °C
180 °C
20 to 40 s
internal preheating,
e.g. by infrared,
max. 2 K/s
100 °C
130 °C
external preheating
forced
cooling
T
MLA860
