Philips Components Chip resistors Mounting point of the solder at the joints. Therefore a maximum solder joint temperature of 110 C is advised. 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 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 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. 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. 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". 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 x 50 mm2 (point D), is found on the horizontal axis. 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 0.25 W 150 0.125 W MBC739 0 handbook, full pagewidth T (K) (oC) 0.0625 W 100 50 B C 0 1 10 D 10 2 mounting density (units/50x50 mm2) 50 100 150 150 (oC) PCB temperature (normally around 100 o C) A 100 Fig.1 PCB temperature as a function of applied power, mounting density and ambient temperature. 1996 Nov 15 1 ambient temperature 50 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: MGA205 handbook, halfpage R th (K/W) THS = Tamb + P x Rth 200 150 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. olic en ph aper p 100 xy epo 50 440 50 DIN ina 96% 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: alum 10 Tamb (which is also dependent on the packing density) Fig.2 Rth of the PCB maximum solder spot temperature (generally 110 C). 5 5 resistor 10 d (mm) Heat resistance for 1206 sized resistors as a function of distance and material. MGC999 60 handbook, halfpage T (K) 50 40 30 (1) (2) (3) 20 10 0 0 0.05 0.1 0.15 0.2 0.25 0.3 P (W) (1) For size 0603: Rth = 400 K/W. (2) For size 0805: Rth = 250 K/W. (3) For size 1206: Rth = 200 K/W. Fig.3 1996 Nov 15 2 Hot-spot temperature rise (T) as a function of dissipated power. Philips Components ,, , z| y |, , y { ,, , { z Chip resistors Mounting FOOTPRINT DIMENSIONS handbook, full pagewidth D solder resist pattern | E C G preferred direction during wave soldering solder land / solder paste pattern B A F occupied area tracks MBG628 E AVAILABLE AREA FOR TRACKS underneath the CHIP For dimensions see Tables 1 and 2. Fig.4 Recommended dimensions of footprints. Table 1 Reflow soldering; for dimensions see also Fig.4 FOOTPRINT DIMENSIONS (mm) SIZE CODE A B C D E PROCESSING REMARKS F G PLACEMENT ACCURACY (mm) 0402 1.50 0.50 0.50 0.60 0.10 1.75 0.95 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 0805 2.6 0.9 0.85 1.4 0.5 3.0 2.1 IR or hot plate soldering 0.25 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 Table 2 Wave soldering (no dummy tracks allowed for the high voltage series); for dimensions see also Fig.4 FOOTPRINT DIMENSIONS (mm) A B C D E F G PROPOSED NUMBER AND DIMENSIONS OF DUMMY TRACKS (mm) 0603 2.5 1.1 0.7 0.8 0.3 3.0 1.7 1 x (0.3 x 0.8) 0.15 0603 2.7 0.9 0.9 0.8 0.15 3.2 1.9 1 x (0.15 x 0.8) 0.25 SIZE CODE PLACEMENT ACCURACY (mm) 0805 3.3 1.3 1.0 1.3 0.7 3.9 2.4 1 x (0.3 x 1.3) 0.25 1206 4.5 2.5 1.0 1.7 1.25 5.0 2.8 3 x (0.25 x 1.7) 0.25 1218 4.5 2.5 1.0 4.8 1.25 4.6 5.9 - 0.25 1996 Nov 15 3 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. 300 MLA859 10 s T 260 C 245 C (C) 250 10 s 215 C 200 180 C 150 40 s 130 C 100 2 K/s 50 0 0 50 100 150 200 250 t (s) Typical values (solid line). Process limits (dotted lines). Fig.5 Infrared soldering. 1996 Nov 15 4 Philips Components Chip resistors Mounting 300 10 s T MLA861 (C) 235 C to 260 C 250 200 second wave 5 K/s first wave 2 K/s 200 K/s 150 100 C to 130 C forced cooling 100 2 K/s 50 0 0 50 100 150 200 t (s) 250 Typical values (solid line). Process limits (dotted lines). The resistors may be soldered twice in accordance with this method if desired. Fig.6 Double wave soldering. 300 MLA860 T (C) 250 20 to 40 s 215 C 200 180 C 150 forced cooling 130 C external preheating 100 C 100 internal preheating, e.g. by infrared, max. 2 K/s 50 0 0 50 100 150 200 250 t (s) Typical values (solid line). Process limits (dotted line). Fig.7 Vapour phase soldering. 1996 Nov 15 5