GB2319202A - Hot gas soldering - Google Patents

Abstract

A soldering workstation for printed circuit boards has a hot gas nozzle or nozzles arranged to pass hot gas (eg nitrogen) over selected areas to effect soldering while avoiding heat sensitive elements on the board. A preheating step may be provided before the soldering step and the hot gas may be projected from both sides of the board.

Description

AUTOMATED SOLDERING TECHNICAL FIELD This invention relates to automated soldering and addresses the need for an automated soldering system which provides for the generation of reliable solder joints while preventing damage to heat sensitive components.

BACKGROUND ART Conventional soldering techniques can be categorized into three groups (mass soldering, manual soldering with soldering iron, point soldering). Conventional techniques for automatically soldering electronic components to printed circuit boards (lPCBTS), such as vapour phase reflow or infra-red reflow, cannot be used when an electronic component to be located on the board is very sensitive to heat.

Mass soldering Conventional mass soldering techniques involves the exposure of the entire PCB assembly to the heat energy required to solder the electronic component to the PCB. This heat energy damages heat sensitive components making such a process unsuitable where such components are involved. In vapour phase reflow soldering, damage to the electronic component body can be caused when solder vapour, at a temperature of approximately 217"C, condenses on the body of the component.

Other conventional mass soldering techniques such as infra-red reflow require that heat sensitive components be shielded from the heat energy thus creating an additional step in the soldering process.

Manual soldering Heat sensitive components are usually soldered manually, using a soldering iron and are therefore subject to the inconsistencies associated with this process. Manual soldering with a soldering iron has several problems in that it yields inconsistent solder joint quality. The solder joint quality created by this process is dependent on the operators skill level, which has a tendency to vary between operators. This process can also be potentially damaging to the electronic components or PCB through physical contact with the soldering iron. Another disadvantage is that this manual process has inherently low rates of productivity.

Point soldering There are several known point soldering systems available which are capable of soldering heat sensitive components to PCB's. Some of these systems use laser energy to heat the solder joint to the required temperature. However these known systems can cause damage to the PCB by the burning of impurities on the surface of the PCB during the soldering process. Special care must be taken when using these systems to focus laser energy on the application and prevent it from reflecting to a location outside the machine. In any event point soldering system costs tend to be high which is a drawback limiting the applications of existing point soldering systems.

DISCLOSURE OF INVENTION According to a first aspect of the present invention there is provided a method of soldering at a work station comprising the steps of: locating at a soldering stage at the work station an electronic unit such as a printed circuit board bearing a heat sensitive element; directing a nozzle or nozzles at the unit at the soldering stage so that a jet of heated gas from the or each nozzle can be caused to pass over the unit for a predetermined period in a direction that does not impinge on the heat sensitive element.

According to a first preferred version of the first aspect of the present invention the step of causing a jet of heated gas to issue from the or each nozzle is regulated so as to flow for a predetermined period.

According to a second preferred version of the first aspect of the present invention or the first preferred version thereof the step of regulating the flow of gas acts to regulate and control the flow of gas which is then heated to the required temperature for soldering.

According to a third preferred version of the first aspect of the present invention or any preceding preferred version thereof wherein the predetermined period is at least in part controlled by way of the speed at which the electronic unit is conveyed past the nozzles.

According to a fourth preferred version of the first aspect of the present invention or any preceding preferred version thereof the critical process parameters such as gas temperature, gas flow rate and conveyor speed are monitored and controlled in specified ranges to yield the desired process results.

According to a fifth preferred version of the first aspect of the present invention or any preceding preferred version thereof the heated gas is nitrogen According to a sixth preferred version of the first aspect of the present invention or any preceding version thereof there is provided at least one control feedback stage serving to confirm that a process parameter falls into a predetermined specified ranges.

According to a seventh preferred version of the first aspect of the present invention or any preceding preferred version thereof there is provided a visual monitoring step for the soldering process.

According to a second aspect of the present invention there is provided a work station providing for an automated soldering step for an electronic unit such as of soldering a component to a printed circuit board including: a location adapted to receive on a pre-determined alignment a unit for soldering: at least one gas nozzle directed towards the location at a unit aligned thereat so as to provide for the or each nozzle to direct a heated flow of gas towards a first pre-determined region on the unit and to direct the flow away from a second pre-determined region of the board; allowing the heated gas flow into the first region for an operating period to provide for a solder joint in the first region to be exposed to a sufficient amount of heat to achieve the desired soldering performance.

According to a first preferred version of the second aspect of the present invention there is provided control means for the optimization of the solder quality by monitoring and adjusting critical process parameters to provide the optimum thermal cycle.

According to a second preferred version of the second aspect of the present invention or the first preferred version thereof the electronic unit includes a laminar member such as a circuit board and the first region is defined on a first side of the member and a complementary first region is defined on the opposite side of the laminar member to the first side, the complementary first side being similar in form and extent to the first side and separate from it only by the thickness of the laminar member. Typically at least one further gas nozzle directed towards the complementary first region so as to provide for the or each further nozzle to direct a heated flow of gas towards the complementary first predetermined region on the unit so as to provide for the simultaneous heating of the laminar member in the first region on the first side and the complementary first region on the opposite side to the first side.

According to a third preferred version of the second aspect of the present invention or of the preceding preferred versions thereof incorporating one or more preheaters providing a clearly defined pre-heat zone for stimulating flux applied to a solder deposit in the first region and/or the first complementary region to provide for cleaning of deposited solder joint prior to a reflow stge. Typically in order to maximize heat transferred to a solder joint at a given temperature in the first region and the first complementary region two pre-heaters and two reflow heaters are used to heat both sides of the PCB assembly simultaneously.

According to a fourth preferred version of the second aspect of the present invention or of the preceding preferred versions thereof there is provided an insulating member disposed to limit heat transfer from heaters to components in the work station.

The present proposal is concerned with the provision of an automated method of soldering heat sensitive components by focusing heat flow on the area to be soldered while directing the heated flow away from the heat sensitive portion of the component. Simultaneously heating the PCB assembly (electronic component and PCB) from both sides provides for soldering at lower temperatures than with conventional systems.

This invention can be regarded as a point soldering device which uses hot nitrogen gas to reliably solder heat sensitive components to PCB's. The invention has been designed to provide for the heat energy in a flow of hot nitrogen gas to be directed away from a heat sensitive portion of an electronic component. The system regulates and controls the flow rate of nitrogen gas which is then heated to the required temperature for soldering.

Nozzles attached to the gas heaters are aligned so that hot nitrogen gas is directed away from the or each location on a PCB where a heat sensitive component is to be situated. A reliable solder joint is formed after the solder joint has been exposed to the hot nitrogen gas for a predetermined period of time. Typically the period is controlled by way of the speed at which the PCB assembly passes the heaters.

Critical process parameters such as gas temperature, gas flow rate and conveyor speed are monitored and controlled in specified ranges to yield the desired process results. The system features several feedback systems serving to confirm that the process parameters fall into the specified ranges. A video camera can be positioned to provide for visual monitoring of the soldering process.

This system presents several novel features to reliably solder heat sensitive components to PCB's. The or each nozzle is designed to direct heat away from the heat sensitive portion of the component and the operating period is selected to provide for the solder joint to be exposed to a sufficient amount of heat to achieve the desired soldering performance. The solder joint quality is optimized by monitoring and adjusting critical process parameters to provide the optimum thermal cycle. This is further enhanced by the presence of pre-heaters which create a clearly defined pre-heat zone, stimulating the flux applied to the solder deposit to clean the solder joint prior to reflow. The pre-heaters have the added function of increasing productivity by reducing the exposure time required at the reflow heaters. There is also the added benefit of using hot nitrogen gas to transfer the heat as it creates an inert soldering environment yielding a reliable, strong and defect free solder joint. In order to maximize the heat transferred to the solder joint at a given temperature two pre-heaters and two reflow heaters are used to heat both sides of the PCB assembly simultaneously. This double sided heating enables soldering to occur at much lower gas temperatures than single sided heating which thereby serves to avoid damage to the electronic component being soldered.

The machine construction has particularly been improved over earlier designs with the addition of a ceramic insulating plate to reduce conductive heat transfer in the machine. This insulating plate reduces the heat transfer from the heaters to the machine components, thereby improving the operation of the mechanism used to position the reflow heaters. The insulating plate is installed such that there are no direct fastener contacts between the machine components subject to conductive heat transfer from the heaters. This feature prevents heat from being conducted through the fasteners and enhances machine performance.

BRIEF DESCRIPTION OF DRAWINGS An exemplary embodiment of present invention show a nitrogen reflow system for an automated soldering flow line process: Figure 1 is a diagrammatic view with pictorial elements; Figure 2 is perspective and part section view of a work station shown in Figure 1; Figure 3 shows a component of Figure 2 in more detail; Figure 4 shows an exploded perspective view of components including the component of Figure 3; Figure 5 shows in section an assembled version of components shown in Figure 4; Figures 6 to 8 show graphs relating to operational features of the process; and Figure 9 shows an assembly used to obtain data shown in Figure 8.

MODE FOR CARRYING OUT THE INVENTION Referring to Figure 1 the process of soldering heat sensitive electronic components to printed circuit boards (PCB's) is shown with a sequence of process steps 1 to 6 beneath each of which step is shown a pictorial representation of the step. The process begins at stage 1 with the deposition of solder paste P onto a PCB 11. At stage 2, the solder paste deposit is heated to the soldering temperature in a vapour phase reflow oven creating a solid solder deposit. At stage 3 heat sensitive components C are mounted onto the printed circuit board. At stage 4 flux is applied to ensure proper solder joint formation. The PCB assembly is subsequently conveyed past pre-heaters and reflow heaters in a nitrogen gas reflow system.

During stages 5 and 6 a nitrogen reflow process takes place and a solder joint is formed between a lead on the component and a pad on the PCB. Difficulties in past practice stem from the fact that heat sensitive components are usually manually soldered to PCB's using a soldering iron. In such a process the operator heats up the lead of the component with the soldering iron tip and adds the required amount of wire solder. The inherent difficulties with this process are inconsistency, potential damage to the electronic component and PCB laminate, varying solder volume and low productivity.

Lasers have been employed to solder heat sensitive components to PCB's by focusing the heat energy on the solder joint. Difficulties have been encountered as impurities on the surface of the PCB are burned and the PCB laminate is damaged.

When using such a system care has to be taken as laser energy is difficult to control.

The present invention serves to overcome problems arising with the previous methods. The present invention provides a non-contact method giving consistent results for soldering heat sensitive components to PCB's. The heat energy delivered to the PCB is easily controlled and is applied on as clearly defined, focused, area of the PCB. In this way the directed heat energy does not damage the component or the PCB laminate.

(1) Data Acquisition Nitrogen gas temperature is measured by a K-type thermocouple installed in the body of the heaters and displayed by temperature controllers (for example Omron Model ESCX). The nitrogen gas temperature may be controlled by adjusting the temperature controller ro a pre-determined set-point temperature. The measurement of solder joint temperature is accomplished by attaching T-type thermocouples at the junction of the lead and pad surface on a test PCB. This test PCB is then processed by the system and the temperature data is recorded on a thermal profiling device (for example Yokogawa Model 4153).

(2) Definition of Operating Range The operating range is defined as the range of set point temperatures which yield a reliable solder joint and avoid damage to the electronic components. A reliable solder joint is characterised by a continuous, smooth junction formed by solder between the lead from the component and the pad on the PCB. Experience and experimentation has shown that reliable solder joints are formed for solder joint temperatures greater than 196"C.

Damage to the electronic component may be observed under a microscope. The component damage threshold is determined by gradually increasing the set point temperature and monitoring the condition of the body of the electronic component.

The damage threshold is marked by an observable change in the material of the body of the component usually presenting itself by the appearance of fibres on the surface of the electronic component body.

Knowing the set point temperature at which a reliable solder joint is formed and the set point at which component damage occurs allows us to define the operating range.

(3) Machine Construction Figure 2 shows a nitrogen reflow system work station 20 according to the present invention showing the arrangement of the major sub-components of the system and the flow of compressed air, nitrogen and oxygen through the system. In this case the flow of nitrogen is shown through front heaters only.

Nitrogen is filtered from compressed air supply A by an internal nitrogen generator 21 and delivered to flow control devices 22. These flow control devices 22 measure the nitrogen flow rate and compare it with the flow rate set by digital flow metres 23. Any necessary adjustments to the flow rate are made automatically by adjusting flow control valve inside the flow control devices 22. A controlled flow of nitrogen gas is then distributed to both the pre-heaters 24 and reflow heaters 25. The nitrogen gas is heated to the specified temperatures by passing it over the heating coils inside the heaters 24, 25. Both the nitrogen temperature and flow rate are controlled independently for each pre-heater 24 and reflow heater 25.

Pre-heat nozzles 31 and reflow nozzles 32 direct the heated nitrogen at the solder joint thereby transferring heat to the solder joint.

Figure 3 shows in more detail the arrangement of a pre-heater 24 and a reflow heater 25 in the nitrogen reflow system. In general, the combination of the preheater 24 and reflow heater 25 provides the necessary thermal profile to provide a reliable solder joint.

As seen in Figure 3 hot nitrogen is directed away from the body of one or more electronic component/s mounted on the PCB. This is represented in Figure 1 as stages 5 and 6. The temperature of nitrogen passing through pre-heater 24 is set so as to create an effective pre-heat zone which prepares a solder joint for reflow. The benefits of this pre-heat zone are discussed in a following section. Reflow nozzle 32 is positioned a specified distance away from the PCB so as to maximize heat transfer to the solder joint by a mechanism shown in exploded view in Figure 4 and in partly assembled view in Figure 5. This comprises a linear guide 44, guide rail 43 and air cylinder 45. The temperature of the nitrogen gas passing through reflow heaters 25 is set so as to transfer sufficient additional heat to the solder joint to cause the solder deposit to reflow.

Reverting to Figure 2 the PCB and electronic components are conveyed past the pre-heaters 24 and reflow heaters 25 at a speed specified by the motion controller 27 interfacing with a stepper motor 28 and ball screw conveying system 29. The system speed, which determines the time the PCB is exposed to the hot nitrogen gas, has been set at 1 mm/s.

As mentioned earlier in relation to Figures 4 and 5 a ceramic insulating plate 41 is installed between the heater clamp 40 and the adaptor plate 42. This insulating plate 41 reduces the heat transfer from the reflow heaters 25 to the mechanical components which position the reflow heaters 25. The thermal insulating effectiveness of this plate is discussed in a following section.

(4) Effect of Pre-heat By using pre-heaters 24 the system can reliably solder electronic components to PCB's at lower operating temperatures than heretofore thereby reducing the potential for damage to the body of the electronic component. As shown in Figure 6, the operating range begins at a lower set point temperature when both reflow heaters 25 and pre-heaters 24 are used (case [1]). The start of the operating range is achieved when the solder joint temperature reaches 196"C, corresponding to a set point temperature of 382"C. When the reflow heaters 25 are used by themselves (case [2]) the start of the operating range corresponds to a set point of 395"C. The component damage threshold determines the ending point of the operating range and has been found to be 415"C for case [1] and 425"C for case [2]. An additional benefit of the pre-heat zone is that it stimulates the flux applied to the solder deposit to clean the solder joint prior to reflow. When exposed to the pre-heaters 24 the solder joint temperature is raised from ambient temperature (say 25 to between 55"C and llO"C. The reflow heaters 25 raise the temperature further from llO"C to the soldering temperature and create the solder joint between the electronic component and the PCB.

(5) Effect of Simultaneous Heating of both sides of the PCB Another important innovation of this machine which enables soldering to occur at lower temperatures is the simultaneous heating of both sides of the PCB (double side heating). By this method, heat is applied to both sides of the PCB (front and back). A solder joint on the front side of the PCB receives heat directly from the nitrogen gas stream coming from the front heaters and heat from the back heaters conducted through the PCB laminate. This conducted heat significantly raises the solder joint temperature for a given set point temperature. Without this double side heating, soldering certain heat sensitive components would not be possible as the component damage threshold would be reached before the solder joint reached the soldering temperature. The operating range for double sided heating (case[3]) is shown in Figure 7. The component damage threshold (set point temp. > 425"C) was reached before the solder joint temperature reached 196"C (soldering temperature) for single sided heating (case[4]). The pre-heaters have been turned off for a direct comparison of the heating effectiveness between double sided and single sided heating.

(6) Thermal Insulating Effectiveness of Ceramic Insulating Plate The ceramic insulating plate 41, installed between the heater clamp 40 and the adaptor plate 42, has a significant effect in limiting the heat conduction to the mechanical parts which facilitate heater positioning ( linear guide -44, guide rail 43 and air cylinder 45). By limiting this heat transfer, the heater positioning repeatability may be improved and the usable life of the mechanical parts extended. There is no direct fastener 46, 47 contact between the heater clamp 40 and the adaptor plate 42 thereby eliminating direct conduction of heat between these parts along the fasteners 46, 47 themselves.

Figures 8 show the temperatures measured by thermocouples installed in the mechanical components (as shown in exploded view in Figure 9) to facilitate heater positioning during operation. There is a 27.5"C drop over the ceramic plate 41 which significantly reduces the operating temperature of the adaptor plate 42 plate, linear guide 44 and guide rail 43 and air cylinder -45. Lower operating temperatures prevent damage to the seals inside the air cylinder 45 and facilitate smooth movement of the linear guide 44 along the guide rail 43. At elevated temperatures the hardness of the guide rail 43 decreases thereby shortening the life of this mechanical component. Without the ceramic plate 41 the temperature of the adaptor plate 42 would approach the temperature of the heater clamp 40 (72.5"C).

This would create elevated temperatures in the air cylinder 45 approaching the maximum recommended cylinder operating temperature (60"C as specified by the cylinder manufacturer).

Terms and Definitions Reflow The condition at which the solid solder deposit becomes molten and forms a continuous, smooth junction between the lead and the pad.

Soldering temperature The temperature at which reflow occurs. Solder will become molten at temperatures exceeding 183"C, however, the soldering temperature required for creating a properly formed solder joint for this system has been determined to be 196"C.

Solder joint Junction of lead, pad and solder.

INDUSTRIAL APPLICABILITY OF INVENTION The invention is currently used for soldering heat sensitive surface mount connectors to PCB assemblies as described herein. This invention could have possible applications in the newly emerging area of PCMCIA (Personal Computer Memory Card International Association) manufacturing. PCMCIA assemblies requires the soldering of heat sensitive surface mount connectors to PC Cards.

Claims (1)

1 A method of soldering at a workstation comprising the steps of: locating at a soldering stage at the work station an electronic unit such as a printed circuit board bearing a heat sensitive element; directing a nozzle or nozzles at the unit at the soldering stage so that a jet of heated gas from the or each nozzle can be caused to pass over the unit for a predetermined period in a direction that does not impinge on the heat sensitive element.

2 A method as claimed in Claim 1 wherein the step of causing a jet of heated gas to issue from the or each nozzle is regulated so as to flow for a predetermined period.

4 A method as claimed in any preceding claim wherein the step of regulating the flow of gas acts to regulate and control the flow of gas which is then heated to the required temperature for soldering.

5 A method as claimed in any preceding claim wherein the predetermined period is at least in part controlled by way of the speed at which the electronic unit is conveyed past the nozzles.

6 A method as claimed in any preceding claim wherein critical process parameters such as gas temperature, gas flow rate and conveyor speed are monitored and controlled in specified ranges to yield the desired process results.

7 A method a claimed in any preceding claim wherein the heated gas is nitrogen 8 A method as claimed in any preceding claim incorporating at least one control feedback stage serving to confirm that a process parameter falls into a predetermined specified ranges.

9 A method as claimed in any preceding claim wherein there is provided a visual monitoring step for the soldering process.

10 A work station providing for an automated soldering step for an electronic unit such as of soldering a component to a printed circuit board including: a location adapted to receive on a pre-determined alignment a unit for soldering: at least one gas nozzle directed towards the location at a unit aligned thereat so as to provide for the or each nozzle to direct a heated flow of gas towards a first pre-determined region on the unit and to direct the flow away from a second pre-determined region of the board; allowing the heated gas flow into the first region for an operating period to provide for a solder joint in the first region to be exposed to a sufficient amount of heat to achieve the desired soldering performance.

11 A work station as claimed in Claim 10 including control means providing for the optimization of the solder quality by monitoring and adjusting critical process parameters to provide the optimum thermal cycle.

12 A work station as claimed in Claim 10 or Claim 11 wherein the electronic unit includes a laminar member such as a circuit board and the first region is defined on a first side of the member and a complementary first region is defined on the opposite side of the laminar member to the first side, the complementary first side being similar in form and extent to the first side and separate from it only by the thickness of the laminar member.

13 A work station as claimed in Claim 12 having at least one further gas nozzle directed towards the complementary first region so as to provide for the or each further nozzle to direct a heated flow of gas towards the complementary first pre-determined region on the unit so as to provide for the simultaneous heating of the laminar member in the first region on the first side and the complementary first region on the opposite side to the first side.

14 A work station as claimed in Claim 10, 11, 12 or 13 incorporating one or more pre-heaters providing a clearly defined pre-heat zone for stimulating flux applied to a solder deposit in the first region and/or the first complementary region to provide for cleaning of deposited solder joint prior to a reflow stge.

15 A work station as claimed in Claim 14 wherein to maximize heat transferred to a solder joint at a given temperature in the first region and the first complementary region two pre-heaters and two reflow heaters are used to heat both sides of the PCB assembly simultaneously.

16 A work station as claimed in any of preceding claims 10 to 15 including an insulating member disposed to limit heat transfer from heaters to components in the work station.

17 A method of soldering or a work station therefor as hereinbefore described with reference to the accompanying drawings.