GB2451656A

GB2451656A - Method and apparatus for cleaning a printed circuit board

Info

Publication number: GB2451656A
Application number: GB0715391A
Authority: GB
Inventors: Paul Francis Stratton
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 2007-08-07
Filing date: 2007-08-07
Publication date: 2009-02-11
Also published as: GB0715391D0

Abstract

A method of cleaning a printed circuit board (PCB) during manufacture comprises disengaging unwanted resin deposits from the PCB by impacting projectile pellets of solid carbon dioxide against the deposits. PCBs 2 are conveyed through a housing 4 on a conveyor belt 6 and carbon dioxide pellets are projected at the unwanted resin deposits from at least one lance 12. The carbon dioxide pellets may be collected in a hopper 18 and recycled. The unwanted deposits may be deposits formed around the rims of via holes after drilling. To aid in removing the deposits, it is helpful if they are below their glass transition temperature so that they are sufficient brittle to be removed by the impact of the pellets. The solid carbon dioxide itself may provide sufficient cooling or the apparatus housing 4 may be separately cooled.

Description

CLEANING METHOD AND APPARATUS

This invention relates to a method and apparatus for cleaning printed circuit boards, typically at an intermediate stage during their manufacture.

In electronics, printed circuit boards (sometimes referred to as "PCBs") are used to support mechanically and connect electrically electronic components using conductive pathways, typically etched from copper sheets laminated onto a non-conductive substrate. Most PCBs are composed of between one and twenty four conductive layers, and sometimes up to 60 or more, separated and supported by layers of insulating material (these layers sometimes being known as substrates") laminated together. The lamination is effected using a glue or resin with the application of heat, pressure or sometimes vacuum. Layers may be connected together through drilled holes called aV5SII To form an electrical connection, the holes are either electroplated or small rivets are inserted. Even though they may not form electrical connections to all layers, these holes are typically drilled completely through the PCB for simplicity of design and manufacture. High density PCBs, however, may have blind vias, which are visible only on one surface, or buried vias, which are visible on neither surface.

Conventional drilling techniques used to form the vias result in small but distinct deposits being formed on the insulating board. These deposits comprise the resin that is used to glue :::sthe substrates together. The deposits typically take the form of rims around the mouth of via. In the case of laser drilling, some carbon residue is typically included within the deposits.

".1...," ::. After their formation the vias are either electroplated or have small rivets inserted into them order to form the necessary electrical connections. It is necessary to remove the deposits * before electroplating or the insertion of rivets. 0 ** * . . * 0

* Conventionally, the deposits are removed by a thermal process, for example, treatment with a hot gaseous plasma. Disadvantages of such a method of treatment are that it is time consuming and requires the necessary input of energy to create the plasma.

There is therefore a need for an improved method and apparatus for removing the deposits from the printed circuit boards.

* 0 *. .0. * * . . . I III * * I II * . I * I * I * * * I I * I.. S According to the present invention there is provided a method of cleaning a printed circuit board, comprising the step of disengaging unwanted resin deposits from the printed circuit board by impacting prçjectile pellets of carbon dioxide against the deposits.

The invention also provides apparatus for cleaning printed circuit boards, comprising a housing, a conveyor for conveying the printed circuit boards in line into, through and out of the housing, and at least one lance terminating within the housing for projecting pellets of carbon dioxide at unwanted resin deposits on the printed circuit boards and causing the projectile pellets of carbon dioxide to impact against the deposits.

The method and apparatus according to the invention are particularly suitable for removing deposits comprising resin around unfilled vias during an intermediate step in the manufacture of a PCB.

Most commonly, the boards tend to use a fibre glass material known as "FR-4" as the substrates. The resin used with this kind of substrate has a glass transition temperature well above ambient temperature. it is desirable that when impacted by the carbon dioxide pellets the resin in the deposits has a temperature not greater than its glass transition temperature.

The reason is that at or below the glass transition temperature, the resin is relatively brittle and is therefore more readily disengaged from the printed circuit boards by the projectile :::. carbon dioxide pellets. If, however, the resin has a glass transition temperature below ambient temperature or if the resin is otherwise insufficiently brittle at room temperature, the :?., boards are preferably chilled to a suitable sub-ambient temperature before impact with the projectile carbon dioxide pellets. Depending on the sub-ambient temperature required, the .. : cooling may be provided solely by vapour evolved from the pellets themselves, because * ,carbon dioxide pellets exist at a temperature well below 0°C. An additional coolant may, *:. however, be used, as necessary. For example, cold or cryogenic gaseous nitrogen may be ** S. : * , used to chill the atmosphere surrounding the printed circuit boards. Thus, liquid or vaporous *. nitrogen may be introduced into the housing.

Pellets of carbon dioxide are readily available commercially. The terms "pellets of carbon dioxide" or "carbon dioxide pellets" are used herein to mean particulate bodies of compacted or compressed carbon dioxide snow. Carbon dioxide snow is formed by ejecting liquid carbon dioxide at a suitable pressure through a nozzle. The liquid cérbon dioxide is * * S. *** S * S* * . S..

* ** *5 * S S SS * * I * S. * S SS* * transformed into a solid snow-like substance and a cold gas. The untreated snow is not sufficiently hard for it to be a suitable abrasive material for use in the method and apparatus according to the invention. It is necessary for the particles of snow to be compacted or compressed together by means well known in the art.

The precise size of the carbon dioxide pellets is not critical to the invention, but it is desirable that more than 50% by number of the pellets have a diameter of at least 3mm. If desired, smaller particles of a different abrasive medium, such as a ceramic or glass, can be used as a projectile in addition to the carbon dioxide pellets.

Preferably at least some of the used carbon dioxide pellets are recycled. A pump or blower is preferably employed to feed used carbon dioxide pellets into the lance. It is desirable, however, continuously to reject from the cycle a proportion of the carbon dioxide pellets.

This is so as to ensure that disengaged materials from the circuit boards are also rejected.

Preferably, the recycling carbon dioxide pellets are screened, with particles of a given size passing through the screen.

Fresh carbon dioxide pellets may be fed under gravity or by means of a mechanical or other feeder to the lance. The first pellets are preferably fed pneumatically in a carrier gas, for example nitrogen or carbon dioxide. The velocity of the carrier gas is chosen so as to ensure that the necessary kinetic energy is imparted to the projectile carbon dioxide pellets.

:"The conveyor preferably takes a form of an endless conveyor belt. The belt preferably has a large percentage open area (say more than 50%) in order to permit used pellets and debris to fall therethrough. If desired, there may be means for indexing the printed circuit boards to **..

* * * thebelt. *. * ** ** * a *

* a:: The housing may be thermally insulated, particulaiiy if it is desired to maintain a sub-ambient ** Se : * ** temperature inside it. The Inlet and outlet to the housing may both be provided with curtains so as to keep down the passage of cold gas therethrough. There is also typically a cold gas extraction system associated with the housing so as to ensure that areas near the housing to which people can gain access are not subject to a hazardous atmosphere by virtue of a reduced concentration of oxygen.

* S 55 *** * * * * * * 5:5 S.. ** S S S* S * * SS SS aSS *.S S a If desired, the housing may be provided with a device for detecting the presence or absence of the boards. The arrangement be such that if no board is detected for a chosen period of time, the supply of fresh peHetised carbon dioxide peUets is ceased. For example, the detector may simply generate and relay a signal to a pellet feeder to stop its operation.

Preferably the housing is formed with a hopper at its bottom to collect used pellets and debris. The hopper preferably has an outlet communicating with the pellet recycle pump or blower.

The method and apparatus according to the invention wilt now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic side elevation of a first apparatus for cleaning printed circuit boards, and Figure 2 is a schematic side elevation of a second apparatus for deaning printed circuit boards.

The drawings are not to scale.

Referring to Figure 1 of the drawings, a line of circuit boards 2 is advanced through a thermally insulated housing 4 on a conveyor belt 6. Only a part of the upper run of the belt 6 ..:: is shown. The belt 6 is in fact endless and extends around a driven wheel (not shown) and an idler wheel (not shown). The belt 6 may for example be made of stainless steel or similar *... material and may have a high percentage open area. Typically more than 50% of the area is #. open. * . S I. *

* The printed circuit boards are typically at an intermediate stage in their manufacture. They *:. comprise layers of insulating material on each of which electrical circuit elements are printed *S * : * . by a process well known in the art. The insulating layers are bonded or laminated together * . by means of a suitable resin or glue. Heat or pressure may be applied to assist in the curing of the resin. The conductive layers are almost invariably made of copper, whiôh is sometimes gold-coated. It is generally necessary to connect together electrically different conductive layers. For this purpose, holes are typically drilled completely through the PCB for simplicity of design and manufacture. (There are, however, some exceptions. For example, high density PCBs, may have blind vias, which are visible only on one surface.) * * ** ** * * S * * . S..

* . S SI * S * * S * IS S S.SS 555 5 5 As a result of the drilling step a "rim" of resin tends to be formed around the entrance to each via. Each board may have many hundreds of vias, each typically from 0.075mm to 0.15mm in diameter. The drilling process results in a rim of resin (being formed) around the entrance to each via. If a laser drilling technique is used to form the vias, the resin rim may also contain particles of carbon.

In accordance with the invention, the deposits of resin formed around the vias are disengaged from the boards by the abrasive action of projectile pellets of solid carbon dioxide. The pellets may be made on site from liquid carbon dioxide in a standard pellet making machine, in which liquid carbon dioxide is first converted to carbon dioxide snow and gas, and the carbon dioxide snow is then compacted to form the pellets. Alternatively, a store of pre-made pellets can be kept on site in a suitable thermally-insulated or refrigerated vessel, It is believed that the pellets are effective through their abrasive action on the resin deposits, but there may also be a secondary effect in that liquid carbon dioxide is temporarily formed and wets the previously contaminated surface.

The carbon dioxide pellets are fed from a machine or store (not shown) to a conduit 10 which communicates with an inlet to a lance 12. Lance 12 has a distal end 13 through which the pellets are ejected at high velocity. The high velocity is typically created by feeding the pellets at the boards as they pass under the lance 12. Because the PCBs typIcally each have a large number of vias, and hence a large number of deposits, the lance 12 is arranged so that, in operation, effectively the whole exposed upper area of each board is subjected to i:' bombardment by the projectile carbon dioxide pellets. Accordingly, instead of using a single lance 12, as shown in Figure 1, instead a row of such lances can be used. The distal end 13 of the or each lance 12 may be located a few millimetres or centimetres above the surface of :. the PCB. As shown, the lance 12 is at an angle to the vertical and faces towards the inlet to the housing 4. Alternatively, the or each lance 12 may be disposed vertically or at an angle :. facing the exit for the printed circuit boards from the housing 4. The pellets are carried r*:. through the lance 12 and ejected from the end 13 by means of a carrier gas which may be nitrogen or carbon dioxide. The nitrogen is supplied to the lance along a conduit 14. The necessary velocity may be generated simply by releasing the pressure from a vessel containing the carrier gas under pressure. The kinetic energy contained in the carbon dioxide pellets is sufficient to create an abrasive action which disengages the resin deposits from the surface of the PCB.

* * ** **. * * ** S * S..

* S* *S * . S S. S S * . . * * *5* *. S S The temperature within the housing 4 is arranged as such that the resin in the deposits is below its glass transition temperature. As such, the resin is brittle and is more readily disintegrated by the action of the projectile pellets of carbon dioxide. At least 50% by number of the pellets have a diameter of at least 3mm. Pellets of such a size are able to be formed by commercially available pelletising machines and are also readily commercially available. The pellets themselves are therefore typically larger in size than the typical resin deposits around the vias. Their abrasive action can, if desired, be enhanced by feeding into the lance 12 an auxiliary abrasive particulate material, typically of ceramics or glass. The auxiliary pellets may be of smaller size than the pellets of carbon dioxide. It will be understood that the carbon dioxide and auxiliary pellets will not damage exposed printed circuit elements, because these elements are made of a ductile electroconductive material such as copper. The elements therefore have the necessary resilience to withstand impact by the pellets, whereas the deposits, because they are brittle, do not.

A particularly common material for the manufacture of the substrate layers of the PCBs Is a fibre glass material known as FR-4. Resins that are able to laminate successfully this material have a glass transition temperature above 100°C. Thus, the resin deposits will naturally be below this glass transition temperature as they enter the housing 4. Should however the resin deposits have a glass transition temperature below ambient, the PCBs can be chilled upstream of the lance or lances 12. Such chilling may be performed by :°. contact with the atmosphere within the housing 4. This atmosphere will contain cold carbon °.:: dioxide vapour which is formed by natural sublimation of the spent carbon dioxide pellets. If necessary, auxiliary cooling can be introduced by spraying or otherwise introducing liquid *;, nitrogen or cold gaseous nitrogen through inlet 16 into the forward end of the housing 4. In order to maintain the low temperature, the walls of the housing 4 may be thermally insulated. * S.

.°. The spent carbon dioxide pellets and disengaged debris from the PCBs fall off the boards :. and pass through the open are of the belt 6, collecting in a hopper 18 forming a lower portion S. .% : * . of the housing 4. The spent pellets and the debris may be withdrawn from the housing 4 *. through an outlet 20 at the bottom of the hopper 18.

The apparatus shown in Figure 1 may have a number of other features not illustrated in the drawing. For example, the conveyor belt 6 may be provided with indexing or locating means that engage the PCBs to hold them in position throughout their travel through the housing 4.

Particularly if auxiliary cooling is provided through the inlet 16, a fan may be provided to * I *. **. I S* S * S **S I.. IS * I I ** * S * S I II I.. I I -7-a.-extract cold vapour from the top of the housing 4. Further, the entrance to an exit from the housing 4 through which the conveyor belt 6 passes may be provided with curtains or the like so as to minimise loss of cold vapour. A rotary device may be used to feed the carbon dioxide pellets to the conduit 8. Operation of this feeding device may be linked to a board detector. The board detector may be positioned within the housing and be such that if the conveyor belt 6 is not charged with PCBs the frequency of interruption of the signal by the belt 6 is much less than when it is charged with boards. Accordingly, the two conditions can be distinguished from one another, and when the belt is not loaded the supply of pellets to the lance 12 can be stopped.

In the apparatus shown in Figure 1 it is intended that the contaminated carbon dioxide leaving the housing 4 through the outlet 20 Is not recovered, in the apparatus shown in Figure 2, however, a proportion of the carbon dioxide pellets is recycled. In other respects, the operation and configuration of the apparatus shown in Figure 1 is identical to that shown in Figure 2 and these aspects will therefore not be described again. It will be noticed, however, that like parts in the two drawings are identified by the same reference numerals.

Refemng to Figure 2, the outlet 20 is provided with a pump, agitator or blower 22 for causing a flow of spent carbon dioxide pellets to pass back to the conduit 8. In order to prevent a build up of resin and other debris from the boards within the apparatus, a recycle conduit 24 is provided with an outlet 26 for the ejection of some of the contaminated carbon dioxide pellets. Preferably, this outlet 26 is fitted with a screen 28. Debris and small pellets will pass : through this screen, but larger pellets will be carried further along the conduit 24 back to the conduit 8. Typically, therefore, the screen may have a size of 2mm. S.I'S

: . The speed of passage of the PCBs through the housing 4 may readily be selected to meet

I

*:* the requirements of the PCB production process. A further advantage of the method and :. apparatus according to the invention is that the solid carbon dioxide pellets will not form any *. I : residue on the boards. Therefore, once the boards pass out of the housing 4 they may pass onto the next stage of manufacture, which is typically the electroplating or riveting of the rias.

* I ** I. * I.. * . I..

S.. ** S * (* IS :.. . :..

Claims

1. A method of cleaning a printed circuit board, comprising the step of disengaging unwanted resin deposits from the printed circuit board by impacting projectile pellets of carbon dioxide against the deposits.

2. A method according to claim 1, wherein resin in the deposits has a glass transition temperature above ambient temperature.

3. A method according to claim I or claim 2, wherein the resin in the deposits has glass transition temperature below ambient temperature, and the board is chilled to below this glass transition temperature before the deposits are impacted by the deposits.

4. A method according to claim 3, wherein the board is chilled by vapour evolved from the pellets themselves.

5. A method according to claim 3, wherein cold or cryogenic gaseous nitrogen is used to chill the atmosphere surrounding the board.

6. A method according to any one of the preceding claims, wherein more than 50% by number of the pellets of carbon dioxide have a diameter of at least 3mm. I..

* *... * *.*

A method according to any one of the preceding claims, additionally including the step of recycling used pellets.

* . %.* * S 8. A method according to claim 7, wherein the recycling pellets are screened to ****, separate debris therefrom. *S* * * S

S * S*

* 9. Apparatus for cleaning printed circuit boards, comprising a housing, a Conveyor for conveying the printed circuit boards in line into, through and out of the housing, and at least one lance terminating within the housing for projecting pellets of carbon dioxide at unwanted resin deposits on the printed circuit boards and causing the projectile pellets of carbon dioxide to impact against the deposits.

* S *S S * S S S S S * * S S.5 5 **S * S S S S * I

S S S S S

*SS I S *SI 10. Apparatus according to claim 9, additionally including means for introducing liquid or vaporous nitrogen into the housing.

II. Apparatus according to claim 9 or claim 10, additionally including a feeder for introducing fresh carbon dioxide pellets onto the lance.

12. Apparatus according to claim 11, in which the feeder is pneumatic.

13. Apparatus according to any one of claims 9 to 12, wherein the conveyor takes the form of an endless belt having a percentage open area greater than 50%.

14. Apparatus according to any one of claims 9 to 13, wherein the housing has a device for detecting the presence or absence of the boards.

15. Apparatus according to claim 14, wherein the device is adapted to generate a signal to stop the feeding of pellets to the said lance if the presence of a board is not detected for a chosen period of time.

16. Apparatus according to any one of claims 9 to 15, additionally including a pump or blower for feeding used carbon dioxide pellets into the said lance. 1:.

I:::17. Apparatus according to claim 16, additionally Including a screen for separating debris * SIS..

from recycling carbon dioxide pellets.

Z.lb.. . . . 18. Apparatus according to claim 16 or claim 17, wherein the housing has a hopper at its bottom to collect used pellets and debris. *. ..

U. * * * S* * * S *S S S. S * ** S * S S *S * S..

* SS S S * S * S S * S S S.. S * 55.