CA1167327A - Hot air rework tool - Google Patents

Abstract

ABSTRACT

HOT AIR REWORK TOOL

A method and apparatus for directing a stream of hot air to localized areas of a printed circuit assembly until a temperature sufficient to melt solder is achieved, thereby facilitating the soldering/desoldering of circuit components mounted thereon. The apparatus essentially com-prises a work station having upper and lower heat tubes through which a stream of turbulent hot air is directed, an adjustable support grid for supporting the pc assembly and a control means for controlling the temperature and flow of the air in the heat tubes. The method being dependent on the use of a heat sensitive lacquer for detecting when the solder holding the component to the pc assembly has achieved a desired temperature and at which temperature the component can be removed or replaced.

Description

1:L673Z7 HOT AIR REWOR~ TOOL
BACKGROUND OF THE INVENTION

The present invention relates to apparatus and a method for soldering/desoldering circuit components from a pc assembly. While the hot air rework tool (HART) has general applicability to any soldering/desoldering situation, its great~s~ dvantage to date has been in "field rewolk"
situations requiring the removal/installation of individual components (i.e., DIP's flatpacks, SSI, MSI and LSI inte-grated circuits) from a multilayer pc assembly at a customersite. For situations having multilayer pc assemblies of relatively few layers and/or components with relatively few pins, standard direct contact, resistance heating techniques can be used; but as the number of layers and pin counts in-crease so does khe difficulty of the removal/installationoperation. Primarily troublesome is the attandant heat sinking or spreading effect of the pc assembly, which efiect requires higher heats. ~lhe higher heats in turn can damage the pc assembly by causing "measling" or delamination of th~
layers and/or the components.

While many devices exist that facilitate the soldering/desoldering operation, from wave soldering to '~

~1673~7 direct contact heat transfer devices, some of which further have suction means for removing molten solder and extractor means for holding a component and facilitating its removal or insertion, none of these devices have proven to be cost effective or amenable to the "field rework" situation, es-pecially for relatively thick multilayer pc assemblies~
Prior to the development of the HART, components were gen-erally removed using soldering irons and vacuum solder "gob-blers. Il The process was relatively tedious though and re-quired a variety of attachments to accomodate the varioussize packages and pin configurations encountered on the high density, ~ultilayer pc assemblies.

The HART and method described herein however uses hot air as the primary heat transfer media and is adjustable to accomodate various size pc assemblies and components. The HART can heat either or both of the localizéd upper and lower surfaces of the pc assembly in the proximity of the desired component in either a concentrated or diffused manner, there-by creating a more uniform heat exchange environment which overcomes the heat spreading effects of the pc assembly. The HART also does away with the necessity of a solder suction device in that the air flow can more uniforml~v maintain the temperature of the solder in all the vias associated with a component so that all the pins can be inserted or removed without encountering any cold vias, which vias will not permit the component pin to be inserted or removed.

116~32~

The present soldering/desoldering method is further improved by the application of a heat sensitive lacquer to the component or localized area during the solder/desoldering operation, which lacquer will melt at a specific temperature and thus indicate that the component can be removed or a new component inserted.
SUMMARY OF INVENTION
In accordance with an aspect of the invention there is provided a soldering apparatus using hot air as a primary heat transfer media which controllably localizes the area of heating upon a workpiece, said soldering apparatus comprising plenum means for confining and directing alr flow; blower means attached to said plenum means for producing said air flow~ heater means within ~aid plenum means for heating said air flow in said plenum means to a temperature sufficient to cause solder to melt;
~electable size orifice means occluding the hot air flow output end of said plenum means for selectably concentrating the hot air flow to a selectably sized localized substrate area upon said workpiece; and means for ~upporting a workpiece substrate with respect to said orifice mean~ end of ~aid plenum means such that said hot air flow thereof impinges either a component or components soldered to said workpiece substrate, or a localized area of ~aid workpiece substrate where a component or components is to be added or a portion of a component which is itself solderably fabricated and also soldered to ~,j,J

~6732 said workpiece substrate, whereby a soldered component or components can respectively be removed from said workpiece substrate, or a new component or components can be added thereto said workpiece substrate, or a solderabl~
fabricated component may be disassembled or assembled while soldered thereupon said workpiece substrate: whereby said controllably localized area of heating upon said workpiece via said selectable size orifice means occluding sald hot air flow is thusly at size of plural components, a component or a portion of a component; whereby selective soldered components can be selectably desoldered from said workpiece substrate or selective new components soldered thereto.
~RIEF DESCRIPTION OF THE DRAWINGS
Flg. 1 i8 a side view of the hot air rework tool with a lower air plenum shown in a partlal cutaway section.
Fig. 2 is a schematic diagram of the electrical wiring of the hot air rework tool.
Fig. 3 is a flow chart of the typical sequence of events that occur during a replacement operation.
B

1~673Z7 --s--DESCRIPTION OF THE PREFERRED RMBODIMENT

Referring to Fig. 1 the hot air rework tool (HART) is shown in a partially sectioned side view and wilI now be described with particular reference to its primary components.
The method of use will then be described with reference to the flow chart of Fig. 3 and the typical se~uence of steps that are recommended for the use of the HART. The HART is essentially comprised of a primary air plenum 1 to which are attached a retractable upper air plenum 2, a lower air plenum 3, a support ~rid 4, a blower ~, a h~.se plate 6, an ori~ice indicator 7 and a pin straightener 8. The HART further is of a size such that with its appurtenant parts the entire unit can be placed within an attache case. The size and weight of the HART thus accommodates the typical field rework circum-lS stances in which it finds its primary use. Referring nowto the primary components each will be described in detail with respect to its appurtenant part~; and the roles th.ey play in the operation of the HART.

The primary plenum 1 essentially consists of a rectangular air cavity to which the blower 5 i5 attached and within which the control wiring ~not shown) is affixed. It functions to distribute the air that is forced into it under a positive pressure via blower 5 into the upper plenum 2 and lower plenum 3. The primary plenum 1 is in turn attached to %5 the base plate 6 which has four rubber cushioned legs 9 at-tached to it to prevent the HART from sliding on or marring 1~673Z~

a work surface upon which the HART is placed.

The blower 5 is positioned approximately midway bet~een the upper plenum 2 and the lower plenum 3 such that the blown air is distributea evenly into the upper and lower air plenums 2 and 3. The air upon entering the upper and lower air plenums 2 and 3 passes over respective upper and lower resistance heatin~ elements 10 (only one of which is shown but both of which typically are identical in size and heat rating) which are affixed to the primary plenum 1 wîthin the cent,ex of the upper' and lower air plenums 2 and 3. The air is then heated via heaters 10 to a temperature of approxi-mately 600 to ~0 Farenheit. Due to the positive pres-sure of the blower 5,,the a.ir is then forced to pass across the blades of the brass turbulators 11 ~only the lower plenum's turbulator.s are shown) which are mounted within thé
upper and lower plenums 2 and 3'to cause a turbulent rather than a swirl.in~ or straight line air flow. A turbulent flow is preferred since a swirl.ing or straight line flow can cause hot spots upon the exiting of the heated air from the orifices 12 of the upper and lower plenums 2 and 3, It should,be noted that the blad~s o.f the turbulators 11 are positioned such that the pitches of the blades are contra to one another so that the turbulent flow rather than the swirling flow is created. It should also be noted that the turbulators 11 are constructed from brass and are secured to the upper and lower air plenums 2 and 3 via brass bolts 13 and thus act to also collect any static electricity that may be present 1~67~Z7 in the heated air and conduct the static electricity to the chassis ground.

Upon reference to the cutaway sections of the lower air plenum 3, its associated turbulators 11 and heater 10 can be seen in their relative positions within the lower air plenum 3, but it is to be recognized that they are also similarly positioned within the upper air plenum 2. The upper and lower air plenums 2 and 3 are each also made from plated steel tubing and are coated on their interior sur-fa-es with a refracfory ceramic cement 1~. The ceramic 14 acts to electrically insulate the metallic walls of the upper and lower air plenums 2 and 3 from the possible short-circuiting of the heaters 10 and also acts to partially in-~ulate the metallic plenums 2 and 3 from the 600 to 650 Farenheit operating température to which the air is heated.
It i5 also to be noted that additionaL ceramic 14 is placed at the outside 90 bend within the air plenums 2 and 3 to assist in directing the heated, turbulent air toward the orifices 12. The hot turbulent air is then forced out of the upper and lower orifices 12 and directed onto the spe-cific location or component on the pc assembly that is posi-tioned between the upper and lower orifices 12, The size opening in the orifices 12 is adjusted to the size of the circuit component or location of the pc assembly that is to be heated. In most applications, however, the size of the lower orifice 12 is usually selected to be ;~673Z7 larger than the component or area to be heated so as to diffuse the heat it generates over a larger area and thuS
crea-te a relatively high ambient temperature at the com-ponent site, while the upper orifice 12 is selected to con-centrate the heate~ air only to the circuit comp~nent orlocation, The upper and lower orifices 12 can be replaced via the thumbscrews 15 which serve to affix the orifices 12 to the air plenums 2 and 3. The HA~T can ~hus a~comodate varying sized circuit components and the varying packaging arrangements ~i.e. pl~stlc or ceramic) that are used in high ana '.ow densit~ pc ~ssemblies.

Re.~erring now t~ the upper plen~ 2~ it too is connected to the prim~r~ plenum 1 by a connector assembly 16,.but where the lower plenum 3 was directly connectea by connector 16~ the upper plenum 2 is connected via an upper slee~e 17 that i~ in tu.rn con.nected to the primary plenu~ 1 by its connector 16 and which sleeve 1'7 has eith~r one or two retractor slots 18 f~rmed there.in on opposite sides thereo~ (only one o~ which is shown)~ such that the upper plenum 2 can slide forward ar~d backward within the sleeve 17.
The forward travel of the upper plenum 2 being restricted by the stop screws 18a (only one of which is shown) and the reverse travel restricted by the inside back wall of the primary plenum 1. The travel of the upper plenum 2 is con-trolled by the kno~ 19 attached to the upper plenum 2 and which is formed from a heat insulating material SO that the operator .is not burned while extending or retracting the upper plenum 2. It is to be noted that in the fully retracted 11~7327 g position when the upper plenum 2 abuts the inside back wall of the primary plenum 1, the notch 20, let into the side of the upper plenum 2, acts to permit only a restricted air flow t~ pass through the upper plenum 2. This condition, however, will be described ~ore fully he~einafter with ref-erence to Fig. 2 and the control circuitry.

Positioned i~nedlately above the upper plenum 2 is the orifice indicator 7 which is shown in a partially raised and fully lowered position and which is used to indicate the center of the lower orifice 12'when a pc assembly is placed on the support grid 4, since thé'lower orifice 12 will be hidden in this situation. The'indlcator 7 is hin~ed by hinge 21 at the approximate center of the'indicator.7 and enables the indicato.r 7 to.be raised during the solder/desolder oper-ation and lowered when a new comp~nent or location is beiny'selected, Thus during the'typical select.ion process, it is necessary for the operator to place'the upper plenum 2 in its fully retra~ted position which enables the orifice indicator 7 to be l~wered so as to align itself with the center of the lower orifice 12 and which then a.llows the'operator to select a specific component or location on the'sur~ace of the pc as-sembly and be assured that the center of the'lower orifice 12 is immediately beneath the tip of the indicator 7, The indi-cator 7 is then ra.ised and the upper plenum 2 extended into its most forward position so that the center o the oriices 12 are immediately above and below one another.

~L6~32~

Also contained on the orifice indicator 7 is a ground spring 22 which is attached such that spring pressure is exerted against the upper air plenum 2 so as to make elec-trical contact with and ground any elec-trical charge which may collect on the upper plenum 2. The spring 2.7 is neces-sary in the case of the upper plenum 2 since the connector 16 essent;ally connects the sleeve 17 and not the upper plenum 2 itself to the primary plenum 1. Thus a positive chassis ground is insured for both the upper plenum 2 and upper sleeve 17 to the primary plenum 1.

ALso attached to the primary plenum 1 is the sup-port yrid ~ which essentially provides a padded support sur-face for the pc assembly between the upper and lower orifices 1~, The support gr.id ~ in the preerred embodiment is com-pr.ised of four intersecting rails of appropriate lengths toaccommodate the various sizes of the typical pc assemblies encountered in the field operations and facilitate the rework of components which are positionea not only in the center of the pc assemblies but also on the edges. It should be noted 20 that the rails of the support grid 4 are each covered with a nylon insulator 23 which ensures that the pc assembly is not d~maged during rework. It should also be noted that the area circumscribed about the lower orifice 12 by the inter-secting rails of the support gri.d 4 should be sufficiently large so that the air which is deflected from the lower sur-face of a pc assembly does not cause the support grid 4 to become unduly hot, but yet sufficiently large so as to ac-1 ~6 7 3 2 7 commodate the smallest pc assembly tha~ will be encountered.

Additlonally, a pin straightener 8 is attached tothe top of the primary plenum 1 which has a variety of com-ponent pin con~i~urations let into it so as to enable the operator to plu~ inaividual circuit components into it prior to inse~tion in the pc assembly and thereby straighten ana realign all the pins o~ the circuit component. Such a straightening operation becomes very critical when the pin c~unt per package is high, such as in the ca~e ~ 64 or 128 pln package. In th~se c~ses the misaligmnent o~ one or more pins can make it ~ery ~i~icult to insert the component into a new location or a replacement location.

Referring now to Pig. 2 the control circuitry for the HART will be described, The ~ART operates ~rom a 120 ~olt, ~inyle phase AC supply which is fused on its positi~e side by fuse Fl which is contained in fuseholder 24. The power is then controlled by the double-pole/three po3ition switch swl which supplies the power to the windings of -the blower motor Ml and the respe~tive upper and lower heaters 10. ~he switch SWl is a three position switch havin~ an "off~" "cold~" and "hot" position. The "cold" position cor-responds to only the blower 5 (i.e. motor Ml) operating and the "hot" position corresponas to the blower 5 and the upper and lower heaters 10 operating. A diode Dl ~not shown in Fig, 1) is also provided to partially rectify the AC power that is supplied to the heaters 10.

~6~3Z7 -12~

Also includea is a single pole~single throw limit switch sw2 (not shown in Fig. 1) which is wired in series between dio2e Dl and the upper heater 10. The switch SW2 is positioned within the primary plenum 1 such that anytime the upper plenum 2 is partially retracted, the plenum 2 engages the actuator assembly o~ the switch sw2 causing its contacts to open and disconnect the power to the upper heater lO. Thus when the upper plenum 2 is retracted, only cool air is blown through the upper plenum 2. It should be recalled that when the upper plenum 2 is fully retracted the air ~low is also substantially ~estricted ~y the no~ch 20. Thus in the full~
retracted poSition~ the restricted air flow in the upper plenum 2 acts to minimize the likelihood of the operator being burned, should he touch the upper plenum 2 as he posi tions the pc assembly~ and to prevent the inaavertent heating of undesired components. It should also be recognized ~ecause the lower heater 10 is directly coupled between the switch SWl, the lower hea~er 10 will always be "on" so long as the switch SWl is in the "hotl' position. Consequently the lower plenum 3 Will continUe to provide hot air while the pc assembly is re-positioned, unlegs Switch SWl is switched to itS "cold" or "off" position or unless an additional switch is placed in series with the lower heater 10.

The method employed in usiny the H~RT Wlll now be described with reference to the flow chart of Fig. 3 and the typical sequence of events that occur during a normal component replacement operation. Prior to using the HARrr, it is ini-~673Z7 tially preheated by placing the switch SWl in the "hot"
position while the upper plenum 2 is fully extended for approximately three to f~ur min-~tes. And while waiting for the HART to heat up to its operating temperature, the oper-ator prepares the replacement components, as necessary, byusing the pin straightener ~ to straighten and align the pins of the replacement components; applying a suitable solder flux to the pins, if needed to insure a good solder connec-tion; loading the insertion tool with the desired replacement component to facilitate the insertion of the replacement com-ponent; and,/or coat.ing the components to be removed with a temperature sensiti.ve lacquer.

Upon preheating, the switch SWl is then switched to itB "off" position and the upper plenum 2 is retracted.
The operator can then center the desired component over the lower orifice 12 with the aid of the orifice .indicator 7.
After positioning the desired component over the lower orifice 12, the movable portion of the orifice indicator 7 is then folded up and out o the way; the upper plenum 2 is extended so that the upper orifice 12 is centered immediately above the desired component; and the switch SWl is switched to the "hot" position which causes the upper and lower heaters 10 to turn on and again heat the air flow that is again being forced by the blower ~ through the upper and lower plenums

2 and 3.

1~67327 During the removal or insertion step the HART
thus subjects the component to a heat of approximately 600 to 650 Farenheit with the upper plenum's heat being con-centrated on the component while the lower plenum's heat is spread over a slightly larger area. In this manner the am-bient temperature around the selected component is raised, while the heat on the upper surface o the component is con-centrated ana transferrea by the component's package to the individual pins, since the package and pins are relatively good heat conductors, especially in ceramic packages. The l~calized hea~ing of the component's ~ins thus causes the solder surrounding each of the pins in their associated vias to melt so that removal can ~e accomplished.

It is to be recognized, however, that while the use of the HART is bei'ng described' in a process ~or removiny entire components, it can also be used to remove portions of components, such as in those cases where'the components are fabricated using bump soldering or hybrid techniques, In thèse cases though the hea't Erom the upper plenum 2 is crit-ical and is used to heat the integrated circuit die, not the pins~ while the component's cover is removed. For the typi-cal component though, the internal connections are made using lead bonding techniques, which require a heat of llO0 to 1~00 ~arenheit, and thus the internal connections are unaf-25 fected by the HAR~'s 600 to 650 Farenheit temperatures.

1~673'~7 The above removal process is further facilitated by the use of the previously mentioned heat sensi~ive lac-quer, which typically comprises a 4~0 Farenheit mpilaq~
(manufactured by the Tempil Corporation) and which typically S is applied to the upper surface of the component. Thus upon the localized heating of the lacquer and the component with the HART, the lacquer will melt when it reaches its 400 Farenheit melting point which indicates the relative temper-ature of the solder in the vias of the pc assembly and which minimizes the duration of exposure and amount of heat applied to the component. ~Ipon the melting of the lacquer, the op-erator should then rock the component slightly to determine that the solder surrounding all of the pins has melted suf-ficiently, and upon this assurance or with slightly more heating time, the operator can remove the component. It should be noted that the typical removal operation accordingto the above described process takes two to three minutes but that more may be required for large components.

During the rernoval of the old component, the upper plenurn 2 is fully retracted; but subsequent to removal, it is then generally returned to its fully extended position so as to maintain the temperature of the solder in the vias where the new component will be inserted. When the operator is ready to insert the new replacement component, the upper 2S plenum 2 is aga.in retracted and the component is inserted into the vias. If the solder is not sufficiently pliable, the component can be placed above the vias and the upper 1~6~73'~7 plenum 2 again extended so as to heat the component and re-heat the vias, and upon reheating, the component should all into place, if all the pins are properly aligned. Additional solder and flux can also be added at this time as necessary.

After replacement of a component, the switch SWl is switchea to the "cool" or "o~f" position and the pc as-sembly either removed or repositioned but only a~ter the operator is sure that the solder connections have solidified.

The above ~escribed operation is then repeated for 10 each of the identified components until all rework is com-pleted. Upon completion, the upper plenum ~ iS returned toits fully extended position an~ switch SWl is turned to its "cool" position to allow the upper and lower plenums 2 an to cool, During this time the pc assembly can then be 15 cleaned to remove flux or okher ContaminantS which might ad-here to it rom the rework process.

While the ~IART and its method of op~ration have been described With re~erenCe to the preferred embodiment~it is to be recognized that the individual companents oE the design can be changed in any number of manners without de-parting from the spirit and scope of the claims hereinafter set forth. Some possible changes might be to include a clamping means to the base plate 6 whereby the HART can be clamped to a work table if space allows. One may also desire to use different variations of the heaters 10 such as a 1~673Z7 larger element so as to provide a different heating rate.
One may desire to make both the upper and lower plenums 2 and

3 retractable so as to accomodate greater size variation~ in pc assemblies and also make the HART easier to package and carry. One might further desire to change the knob 19 to provide a pulley arrangement for retracting and extending the upper plenum ~ so as to minimize the likelihood of the operator contacting the heated upper plenum 2. One may still further desire to vary the control wiring to accomodate different circumstances. But, it is to be recognized that e~ch of these aforementioned variations are mere variat.ions o the described invention and they should not detract from the spirit and scope of the claimed invention.

What is claimed is:

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A soldering apparatus using hot air as a primary heat transfer media which controllably localizes the area of heating upon a workpiece, said soldering apparatus comprising:
plenum means for confining and directing air flow;
blower means attached to said plenum means for producing said air flow;
heater means within said plenum means for heating said air flow in said plenum means to a temperature sufficient to cause solder to melt;
selectable size or if ice means occluding the hot air flow output end of said plenum means for selectably concentrating the hot air flow to a selectably sized localized substrate area upon said workpiece; and means for supporting a workpiece substrate with respect to said orifice means end of said plenum means such that said hot air flow thereof impinges either a component or components soldered to said workpiece substrate, or a localized area of said workpiece substrate where a component or components 18 to be added or a portion of a component which is itself solderably fabricated and also soldered to said workpiece substrate, whereby a soldered component or components can respectively be removed from said workpiece substrate, or a new component or components can be added thereto said workpiece substrate, or a solderably fabricated component may be disassembled or assembled while soldered thereupon said workpiece substrate:

whereby said controllably localized area of heating upon said workpiece via said selectable size orifice means occluding said hot air flow is thusly at size of plural components, a component or a portion of a component;
whereby selective soldered components can be selectably desoldered from said workpiece substrate or selective new components soldered thereto.

2. The soldering apparatus of claim 1 which further comprises:
turbulent air flow inducing means within said plenum means for inducing turbulence via contra pitched blades of said hot air flow at said selectable size orifice means:
whereby hot spots within said hot air flow as impinges said selectably sized localized substrate area may be substantially eliminated.

3. The soldering apparatus of claim 2 wherein said plenum means further comprises:
plenum means consisting of a first and a second plenum for confining and directing air flow;
and wherein said blower means further comprises;
blower means attached to said plenum means for producing said air flow within each said first and said second plenum:
and wherein said heater means further comprises:
heater means within each said first and said second plenum for heating said air flow in said plenum means to a temperature sufficient to cause solder to melt;
and wherein said selectable size orifice means further comprises:

selectable size orifice means for independently occluding the hot air flow output end of each said first and said second plenum for, responsively to said occluding, concentrating said hot air flow from each said first and said second plenum to a respectively independently selectably sized localized substrate areas upon said workpiece;
and wherein said means for supporting a workpiece substrate further comprises:
means for supporting a workpiece substrate with respect to said orifice means and of each said first and said second plenums so that said hot air flow from each said first and said second plenums impinges said respectively independently selectably sized localized substrate areas which are substantially juxtaposed upon opposite sides of said workpiece;
and wherein said turbulent air flow inducing means further comprises:
turbulent air flow inducing means within each said first and said second plenum for inducing turbulence within said hot air flow at said selectable size orifice means of each said first and said second plenum;
whereby said juxtaposition of said independently selectably localized areas of heating upon opposite sides of said substrate workpiece various combinations of the areas heated, and thusly of the combined areas thermal profile of heated said workpiece substrate, are possible;
whereby said juxtaposed independently selectably localized areas of heating one such area could be minimal for concentration of heating while one such area could be nominal for ambient heating of an area of components, a component, or a portion of a component.

4. The soldering apparatus of claim 2 wherein said turbulent air flow inducing means further comprise:
metal air flow inducing means within and electrically conductively connected to said plenum means for inducing turbulence within said hot air flow at said hot air flow output end, and for collecting and conducting to said plenum means any static electricity which may be present in said hot air flow,

5. Soldering apparatus as set forth in claim 3 wherein at least one of said first and said second air plenums is movable and including means for independently controlling the movement of the movable ones of said first and second air plenums with simultaneous respect to the other of said first and second air plenums and with respect to said workpiece.

6. Soldering apparatus as set forth in claim 5 including means for indicating the relative position of one of said air plenums with respect to the other of said air plenums, whereby the air flow of said first and second air plenums can be positioned directly in line with each other 80 as to impinge Concentric juxtaposed opposite areas of said workpiece substrate.

7. Soldering apparatus as set forth in claim 5 wherein each said first and said second movable air plenum is enabled for disconnection of said heater means within said plenum and for blockage of said confined and directed air flow from existing said hot air flow output end when said movable plenum is moved to a set, fully retracted, position.

8. Soldering apparatus as set forth in claim 3 including means for disconnecting said heater means within either said first air plenum or within said second air plenum wherein such plenum air flow is then relatively cool, unheated, air flow.

9. The soldering apparatus of claim 3 wherein said plenum means further comprise:
metal plenum means, lined with refractory ceramic cement for electrical and thermal insulating purposes, for confining and directing air flow.

10. The soldering apparatus of claim 9 wherein said metal plenum means further comprises:
metal plenum means electrically conductive to said blower means, and to said means for supporting a workpiece substrate, for the grounding of electrical charge.