A METHOD OF SOLDERING LEADED AND SURFACE-MOUNTABLE COMPONENTS TO A PRINTED CIRCUIT BOARD

Abstract: Leaded and surface-mountable components are soldered to a printed circuit board having lands and plated through holes, by the successive steps of applying solder paste to one side of the printed circuit board to cover the lands on said one side of the board and to fill the plated through holes, placing components on the board, and reflowing the solder.

Description: This invention relates to a method of soldering leaded (PIH) and surface-mountable components to a printed circuit board. Conventional techniques for populating printed circuit boards (PCB) with a variety of component types, such as leadless, leaded through hole and surface-mountable, require several process steps. Traditionally, different attachment techniques are used for each type of component. Leaded components are frequently inserted into holes through the PCB. The assembly is then passed over a solder wave. Surface mount attachment requires depositing solder loaded flux on the PCB component site, placing components on the PCB into the wet solder paste, or using an adhesive to hold the component on the PCB drying the solder paste, reflowing the solder paste, using vapour phase, infrared radiation (IR), or convection heat. The final step is cleaning using an appropriate solvent to remove flux and ionic residues. When it is desired to have these two and/or additional types of components on the same circuit board the difficulty of reliably attaching each type in a sequence determined to avoid adverse effects on the joints for the other types becomes very difficult. U.S. -A- 4,515,304 discloses a method of mounting leaded and surface-mountable components on the same printed circuit board. However, this reference teaches mounting the leaded components on one side by inserting their leads into through holes, crimping the lead ends, turning the board over, applying solder paste using individual applicator nozzles to the lead ends and to surface mount solder pads on the opposite side. The surface-mountable components are then positioned and the whole board is reflow soldered. It is indicated that the process may be repeated if it is desired to have surface-mountable components on the same side as the leaded components. EP-A-0194152 discloses a method of mounting leaded and surface-mountable compounds on both sides of a printed circuit board, where they may be kept in place by the soldering material, e.g. a cream or paste, deposited on the surface or previously inserted into the metallised holes of the board. The binding properties of the soldering material are subsequently developed by heating. The present invention seeks to provide a simple technique for populating a printed circuit board (PCB) with a variety of component types. The invention provides a method of soldering leaded and surface-mountable components to a printed circuit board, the board having lands on both sides of the board; and plated through holes (PTHs); comprising the steps of: screening solder paste on lands located on a first, uppermost, side of the board; depositing adhesive on component sites on the first side of the board; placing surface mountable components on said component sites on the first side of the board: curing the adhesive; inverting the board; depositing solder paste onto the second side of the board to cover the lands on the second side of the board and to fill the plated through holes; placing surface mountable and leaded components on the second side of the board; and reflowing the solder. How the invention can be carried out will now be described by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic plan of a printed circuit board; Fig. 2 is a cross-section on the line 2-2 in Fig. 1; Fig. 3 represents a partial cross-section of a circuit board with a leaded pinned grid array (GPA), placed into the wet solder paste; Fig. 4 represents a partial cross-section of a circuit board with a leaded dual inline package (DIP) placed in set solder paste; Fig. 5 represents a partial cross-section of a circuit board with a surface mount component placed in wet solder paste; Fig. 6 is a side view of a single sided PCB assembly with a variety of component types; Fig. 7 is a side view of a double sided PCB with its back side assembled; Fig. 8 is a side view of the double sided PCB with solder paste applied to the top side; Fig. 9 is a side view of a double sided PCB assembly with a variety of component types. In Fig. 1, a PCB 10 is shown schematically viewed from above. A pattern of PTHs 12 is shown, PTHs 12 are provided, for example, for accepting a Pin Grid Array (PGA) component type. A pattern of PTHs 14 is provided for accepting a dual inline p ackage (DIP). Lands 16 are illustrated for a small outline (SO) package, and lands 18 are shown in a pattern corresponding to that of a plastic leaded chip carrier (PLCC). Fig. 2 shows in cross-section the printed circuit board 10 after a predetermined amount of solder paste 20 has been applied to fill PTHs 12 and 14 and to cover surface mount lands 16 and 18. Solder may be controllably applied as disclosed in co-pending European Patent Application Number EP-A-0263944 (our Docket Number AT9-84-077). Fig. 3 is a cross-sectional view of the printed circuit board 10 with PTHs 12 having solder 20 partially pushed through by the pins 30 of the PGA module 34. In Fig. 4 PTHS 14 are similarly filled with pins 36 from DIP 38 and solder 20. A surface mount component 40 with a J lead 42 is shown in Fig. 5 with solder 20 on surface mount land 18. If the solder paste is a solder loaded flux then a curing step is required at this point in the process. The assembled card is baked to remove the volatiles from the solder paste. The drying is a critical process. Typically drying for surface mounted components on PCBs ranges from no drying to a maximum of 30 minutes at 80°c. For this example employing surface mounted and PIH components in solder paste a drying time of 45 minutes at 80°c appears to be best suited for achieving reliable reflowed solder joints for both surface mounted and PIH components on PCBs. This step also facilitates handling of the populated board because the baked solder paste retains the electronic components in position. After the cure step, solder is reflowed. A variety of reflow processes are available such as vapour phase, convection heating or IR. If necessary, the populated, reflowed card is cleaned by means of solvents and procedures appropriate for the flux used. This, of course, is a conventional process step. Fig. 6 is a single sided PCB 60 assembly having PIN modules PGA 34 and DIP 38, leaded surface mount components PLCC 40 and SO 41, and leadless component 50. Table 1 describes in detail the process steps and parameters for a single sided PCB assembly having leaded PIH, leadless and leaded surface mount components as shown in Fig. 6. PROCESS DESCRIPTION PARAMETERS SCREENING Solder paste screened on surface mount pads on the PCB solder loaded flux 90% min metal (63Sn 37Pb) ADHESIVE DISPENSE (optional) curing adhesive used to hold components UV or thermal curing, having elasticity at elevated temp. ADHESIVE/PASTE CURING (optional) UV or thermal cure Paste drying vendor spec. & 7 min. minimum 80°c for drying COMPONENT PLACE LEADED, LEADLESS AND PIH place all components into wet solder paste on the PCB placement accuracy ± tol. determined by tool, component,pad size and PTH dia. SOLDER PASTE DRYING remove volatiles from solder loaded flux 80° ± 5°c for 45 ± 5 min. SOLDER PASTE REFLOW reflow solder using IR, Vapour Phase or convection heat 205° ± 10°c peak 30-90 sec. 183°c appropriate ramp-up and cool-down CLEANING vapour degreaser using appropriate solvent for flux and ionic removal time, temperature and spray determined by solvent used The process possesses many advantages, not the least of which is that only a single solder application step is required for populating a printed circuit board with different types of components such as leaded, leadless, and surface mount. No crimping of the leaded pin-through-hole devices is required to retain them in position before permanent attachment. The component density is increased with this process as a PIH component can be placed at any position on the PCB. Typically in the prior art, PIH components must be placed in a restricted area along the PCB edge for wave soldering. The attachment process described so far is for a single sided PCB assembly. This process may be used in a process for double sided attachment of a mixture of leaded surface mount, leadless, and pin in hole components. There will now be described a double sided process which allows surface mounted devices to be attached to the top and bottom of the PCB, and PIH components to be attached to one side of the PCB. No advantage is gained by having PIH components on both sides of the PCB as the wiring and the component shadow area are common for both sides of the PCB. Double sided PCB assembly uses all the process steps described for single sided assembly; however the back side of the PCB is processed first. Fig. 7 is a side view of a double sided PCB 70 in the first stages of assembly, showing a first top or front side 72 and a second or back side 74. Leadless component 50, PLCC 40 and SO 42 are attached in the solder paste 20 and conventional adhesive 76. At this point a curing process may be used to secure the components to the PCB and partially cure solder paste 20. After curing, the assembly is flipped over for top side processing. Fig. 8 shows PCB 70 after the top side screening operation. Solder paste 20 is shown filling the PTHs 12 and 14 and deposited on surface mount pads 16 and 18. Bottom side 74 of PCB 10 is shown with surface mount components 40, 42 and 50. At this point the single sided assembly process steps described in connection with Figs. 1-6 are repeated. Fig. 9 is a completed double sided assembled PCB 70 having top side 72 PTH components PGA 34 and DIP 42, surface mount components 40, 42 and 50 mounted to both top side 72 and bottom side 74. Table 2 describes in detail the process steps and parameters for the double sided PCB assembly shown in Fig. 9 having leadless and leaded surface mount components on both sides of the PCB and leaded PIH components on one side. PROCESS DESCRIPTION PARAMETERS SCREENING (SIDE 1) Solder paste screened on surface mount pads on the PCB solder loaded flux 90% min metal (63Sn 37Pb) ADHESIVE DISPENSE curing adhesive used to hold components UV or thermal curing, having elasticity at elevated temp. COMPONENT PLACE LEADED AND LEADLESS place surface mount components into wet paste & adhesive placement accuracy ± tol. determined by tool, component and pad size ADHESIVE/PASTE CURING UV or thermal cure Paste drying vendor spec. & 7 min. minimum 80°C for drying SCREENING (SIDE 2) Solder paste screened on surface mount pads and in PTH on the PCB same solder as side 1. COMPONENT PLACE LEADED, LEADLESS AND PIH place all components into wet solder paste on the PCB placement accuracy ± tol. determined by tool,component, pad size and PTH dia. SOLDER PASTE DRYING remove volatiles from solder loaded flux 80° ± 5°c for 45 ± 5 min. SOLDER PASTE REFLOW reflow solder using IR, Vapour Phase or convection heat 205° ± 10°c peak 30-90 sec. 183°C appropriate ramp-up and cool-down CLEANING vapour degreaser using appropriate solvent for flux and ionic removal. time, temperature & spray determined by solvent used.

Claims: A method of soldering leaded and surface-mountable components to a printed circuit board, the board having lands on both sides of the board; and plated through holes; comprising the steps of: screening solder paste on lands located on a first, uppermost, side of the board; depositing adhesive on component sites on the first side of the board; placing surface mountable components on said component sites on the first side of the board; curing the adhesive; inverting the board; depositing solder paste onto the second side of the board to cover the lands on the second side of the board and to fill the plated through holes; placing surface mountable and leaded components on the second side of the board; and reflowing the solder. A method as claimed in claim 1, further comprising the step of drying the solder paste after the step of placing the components on the second side of the board and before the step of reflowing the solder. A method as claimed in claim 2, in which convection heat is used for drying the solder paste.

Inventors: BORA, MUHAMMAD-YUSUF JAMALBHAI, HOEBENER, KARL GRANT
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: H05K3
IPC codes: H05K3, B23K1