Method of positioning and soldering of SMD components

The invention relates to a method of positioning and soldering of electronic components on a printed circuit board. To this end, soldering material is first heated to a suitable molten temperature, after which the component is put in the liquid soldering material with its connections and the soldering material is then allowed to cool. Soldering is preferably carried out in an inert or slightly reducing atmosphere. Heating of the soldering material may take place in a single stage or in two stages.

BACKGROUND OF THE INVENTION 
The invention relates to a method of positioning and soldering of 
electronic components on a printed circuit board, in which method 
soldering material is locally applied on the printed circuit board and a 
positioned component is fastened to the printed circuit board by melting 
and subsequent cooling down of said soldering material. 
Such a method is disclosed in U.S. Pat. No. 4,312,692. As described in this 
Patent, SMD components (Surface Mounted Devices) are put on the printed 
circuit board in contact with soldering paste, upon which the assembly is 
heated to a high temperature to soften or melt the soldering paste to a 
sufficient degree for obtaining a good adhesion between the components and 
the soldering material. The assembly is subsequently allowed to cool down 
again. As this Patent discloses, however, a disadvantage of this method is 
that components can easily become displaced during soldering, so that the 
positioning accuracy cannot be guaranteed, while also the formation of an 
even solder layer is difficult. 
To solve these problems, the U.S. Pat. No. 4,312,692 proposes to fix the 
components first on the printed circuit board with an adhesive, which is 
cured by irradiation. The assembly thus obtained of printed circuit board 
and components glued to it may then be brought into contact with molten 
soldering material in order to achieve the soldered joint. A disadvantage 
of this method is that glueing of the components on the printed circuit 
board involves an additional process step, which considerably increases 
both the manufacturing cost and the time required for manufacture. 
SUMMARY OF THE INVENTION 
The invention has for its object to provide a method by whose application 
the disadvantages described above can be avoided. 
According to the invention, the soldering material is first heated to a 
suitable melting temperature, then the component is positioned in the 
liquid solder with its connections, and finally the soldering material is 
cooled. 
To carry out the method described above, soldering material is applied in 
the relevant spots of the printed circuit board. The soldering material 
may comprise a solder layer applied by thermal means, by electroplating, 
or by molten soldering paste. The printed circuit board provided with 
soldering material is heated to a temperature close to or above the 
melting temperature of the soldering material. Subsequently the components 
to be applied are pressed into the molten soldering material, which is 
provided on the printed circuit board in an adapted pattern, by means of 
suitable tools in accordance with the pattern in which they are to be 
provided on the printed circuit board, upon which the soldering material 
is allowed to cool. 
The reproducibility of the soldering process can be well controlled in a 
preferred embodiment of the method according to the invention in that the 
heating of the soldering material takes place in two stages. 
To heat the soldering material, the printed circuit board may be heated in 
its entirety, for example by means of infrared radiation or with a hot 
plate. 
In the alternative the soldering material on the printed circuit board is 
heated locally. Infrared irradiation and hot air are very suitable heating 
means for this. The required thermal energy may be supplied in a very 
short period, in a concentrated form and accurately aimed by the use of 
laser irradiation for heating of the soldering material. Another 
alternative is the use of heated grippers for positioning the components, 
in particular ceramic components such as resistors and capacitors. 
A combination of several of the above measures is employed in an additional 
alternative, for example, by heating the soldering material in two stages 
both by heating the entire printed circuit board and by local heating. 
Thus, for example, the printed circuit board may be heated from above by 
infrared irradiation up to a temperature well below the melting 
temperature of the soldering material. The soldering material is then 
locally heated further in the soldering spots to well above the melting 
temperature of the soldering material, upon which the components are 
positioned. The soldering material will harden when the heat sources used 
for the second heating step are removed, and the components will become 
fastened.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The equipment 1 comprises an x-y table 3, two infrared (IR) lamps 5 for 
region heating, a halogen lamp 7 used for local heating, an IR-thermometry 
system 9 for remote temperature sensing and a component placement system 
11, all controlled by a computer 13. A printed circuit board PCB 15 is 
applied with solder in a known way and is fed to the x-y table 3. The 
IR-lamps 5 are switched on and the PCB 15 is heated to a temperature of 
about 150.degree. C. The PCB 15 is moved by the x-y table 3 to a position 
whereby placement position 17 to be heated is under the halogen lamp 7. 
The halogen lamp 7 is switched on to locally heat position 17 when the 
thermometry system 9 measures the desired local temperature (+250.degree. 
C.) at the position 17. At this time a relatively cool component 18, thus 
at ambient temperature, is placed by the component placement machine 11 in 
the liquid solder. The halogen lamp 7 is then switched off and the PCB 15 
is moved by the x-y table 3 to a next position. The IR-lamp 5 is a 1000 
Watt IR-radiator, fitted with a parabolic reflector to produce a good 
parallel beam. In the alternative, the PCB may be heated by a plate 6 or 
in conjunction with lamps 5. Output from the lamps 5,7 is controlled both 
by input voltage level and by a switch, controlled by the computer. The 
component placement can be done by an automated component placement 
system. 
It will be clear that, when the method according to the invention is 
applied, components to be applied on a printed circuit board are 
simultaneously positioned correctly relative to the circuit board in one 
operation and soldered to the board. Compared with prior methods, the 
number of operations to which the printed circuit board and the components 
are subjected for achieving the soldered joints is considerably reduced. 
In addition, the use of glue becomes redundant. 
It is also possible in the method according to the invention to position 
the components closer together than in the case of the known method since 
the components, once positioned, will not or only very slightly move 
relative to one another or relative to the substrate. This also has a 
favourable influence on the freedom of design of the circuit 
configuration. 
Positioning and soldering of the components can take place while the 
printed circuit board is securely supported and a thinner printed circuit 
board can be used. For example, a printed circuit board of 0.8 mm 
thickness may be used as compared with prior board thicknesses of 1.6 mm. 
The thinner boards can be utilized, because the printed circuit board need 
not be subjected to further treatments, such as, for example, dipping in a 
solder bath or the like, after positioning and soldering. An additional 
saving in material as well as a lighter design of the printed circuit 
board is thus possible. Moreover, a better heat supply through the bottom 
of the printed circuit board takes place. 
Care must be taken to avoid degradation, delamination or warping of the PCB 
due to excessive heating, especially by heating the entire PCB to the 
temperature at which the solder is molten. To heat the solder only locally 
by the halogen lamp has the disadvantage that the time to reach the right 
temperature is relatively long. Therefore, the best method is the 
combination of total heating of the PCB to a temperature below the 
temperature at which the solder is molten and local heating for heating 
the solder spots, where a component must be placed, to a temperature above 
the temperature at which the solder is molten. The local heating is 
performed by the halogen lamp 7 as mentioned above. Because solder 
oxidizes readily, fluxing determines whether or not wetting takes place. 
Fluxing before heating is the easiest method: the disadvantage is that the 
flux loses its activity during the process. This implies that the last 
solder surfaces and components will not be cleaned, and oxides present 
will prevent wetting. Fluxing at the instant of placing the component is 
similarly unfavorable. The surface is cleaned, but evaporation of flux 
compound removes a significant amount of heat, so wetting takes longer. 
Fluxing before local heating is preferred. No local heat is removed, the 
flux 13. 180 cleans while the surface is being heated and wetting proceeds 
rapidly. When the correct temperature and fluxing are chosen, soldering 
times of 1 second per component are achievable and proper joints are made. 
The time between wetting the printed circuit board (PCB) and the placement 
of the component on the board must be as short as possible to avoid 
causing the flux to lose its activity. The emissivity of the PCB and the 
solder spots where the component must be placed plays an important role in 
the measurement of a correct contactless temperature by system 9. For one 
embodiment the PCB has an emission factor of 0.9 whereas, the solder has 
an emission factor of 0.2. Soldering takes place preferably in an inert or 
slightly reducing atmosphere to prevent oxidation of the molten soldering 
material, of the components and of the connection surfaces of the printed 
circuit board.