IE73658B1 - Shielding of polymer casings - Google Patents

Description

Shielding of Polymer Casings^ The invention relates to the shielding of polymer casings for electronic circuits, computers etc. In particular, it relates to shielding of such casings for prevention of electromagnetic interference (EMI), radio frequency interference (RFI) and electrostatic interference (ESD).

US Patent Specification No. 5 047 260 discloses one such process in which silver-coated magnetite particles are deposited. The shielding material is subsequently cured by exposure to a UV source. One problem with this process is that the materials used are expensive.

Alternatively, it is known to apply a coat of copper to such a casing, on top of which is applied a coating of nickel, the nickel preventing oxidisation of the copper. The copper is generally applied in an electroless (autocatalytic, chemical) process. It is also known to apply a primer base coat to improve the adherence of copper to the casing in the electroless process.

Generally, the casing is dried in an oven for a time duration of approximately 1 hour after the application of the base coat primer. The copper and nickel layers are then subsequently applied in electroless processes.

This method has been found to be generally effective, but efficiency is quite low. One problem is that there is a high discard rate because there is insufficient adherence of copper in the electroless deposition process. Because this production stage is towards the end of the process, the cost of defects occurring at this stage is very high. This significantly reduces efficiency of the process. - 2 The invention is directed towards providing an improved shielding process to overcome these problems.

According to the invention, there is provided a shielding process for a polymer moulded casing, the process comprising the steps of : sensitising a moulded casing by exposing it to infra-red radiation at a temperature in the range of 60°C to 100°C for a time duration in the range of two to ten minutes; applying a base coat primer to a portion of the casing to allow limited subsequent electroless copper deposition on the casing; curing the base coat by exposing the casing to infra-red radiation at a temperature in the range of 60°C to 100°C for a time duration of two to ten minutes; rinsing the cured casing by dipping in a rinse water bath; dipping the casing in an activating solution of an activator, water and acid; carrying out an electroless copper deposition coating process; dipping the casing in an initiating solution of an initiator, water and acid for initiating electroless nickel deposition; and carrying out electroless nickel deposition to complete a shielding coating.

In one embodiment, both the sensitising and curing operations are carried out at a temperature in the range of 85°C to 90°C for a time duration in the range of four to six minutes.

Preferably, the sensitising and curing operations are carried out by conveying the casing past infra-red elements.

In one embodiment, the base coat is applied by mounting the casing in a carriage of a jig and moving the carriage upwardly to a position below covering members, and subsequently spraying base coat over the casing.

In another embodiment, the process comprises the further step of dipping the casing after the nickel plating stage in a chrome solution of 0.05% to 1.5% by volume chrome and de-ionised water.

Preferably, each dipping operation is preceded by a rinsing operation of dipping the casing in a water bath for a duration of 20 to 40 seconds and dwell time of 20 to 120 seconds above the bath.

In another embodiment, each rinsing operation is a twostage process by dipping in preliminary and final portions of a bath, water flowing from the final to the preliminary portions.

The invention will be more clearly understood from the following description of some preferred embodiments thereof, given by way of example only with reference to the accompanying drawings, in which : 4 Fig. 1 is a flow chart illustrating a shielding process of the invention; Fig. 2 is a perspective view from above showing an infra-red sensitising station used in the process; Fig. 3 is a diagrammatic side view of the infra-red sensitising station; Figs. 4 and 5 are perspective views showing a base coat primer application station used in the process; Fig. 6 is a diagrammatic side view showing various baths used for coating of the casings; and Fig. 7 is a perspective view showing portion of the baths.

Referring to the drawings, and initially to Fig. 1, a shielding process of the invention is indicated generally by the reference numeral 1. Briefly, the process involves initially sensitising a moulded polymer casing by exposure to infra-red radiation. Subsequently, in step 3, a base coat primer is applied to at least a portion of the casing. The coated casing is then cured by application of heat using infra-red elements in step 4 and the cured casing is subsequently rinsed in step 5. Various plating processes are carried out in step 6, and there is a final dip in chrome solution in step 7. Finally, the casing is dried in an oven in step 8.

These process steps will now be described in detail with reference to Figs. 2 to 6, inclusive. The infra-red sensitising step 2 is carried out at a station 11 which comprises a conveyor 12 running on drive rollers 13, all supported on a support frame 14. A rectangular hood 15 is mounted over the conveyor 12 and this houses a series of eight infra-red elements 16. Operation of the rollers 13 and of the infra-red elements 16 are controlled by a controller 17. The conveyor 12 is 2m long, 600mm wide and there is a total power consumption of 8 kW in the elements 16. The casings, indicated by the numeral 18 are placed at the beginning of the conveyor 12 and this is operated to convey the casings 18 underneath the elements 16 so that they are heated to a temperature in the range of 60 to 100°C and preferably 90°C. The time duration under the elements 16 in the hood 15 is preferably 5 mins, but may be in the range of 2 to 10 minutes. It has been found that by heating the casings using infra-red elements in this manner, foreign matter and surface contamination such as those caused by flame retardant material and release agents on the moulded casings are burnt off and subsequent coating of the casings is significantly more effective.

In Fig. 3 there are shown different configurations of casings 18 and 19. It will be appreciated that the invention is not limited to any particular shape or configuration of casing but applies to any moulded, extruded or formed polymer casing.

Referring now to Figs. 4 and 5, the manner in which step 3 of the process 1 is carried out is now described. An application station 20 comprises a support frame 21 on which there is a jig 22 having a masking member 23 supported on bars 24. A casing support 25 shown supporting a casing 26 is movable in the vertical direction between a lower loading position and an upper operative position. The support 25 is shown in the loading position in Fig. 4 and in the operative position at Fig. 5. An un-coated casing 26 is shown in Fig. 4. When the casing 26 is in the operative position as shown in Fig. 5, an operator holds a spray nozzle for spraying of a base coat primer over the jig 22. The nozzle is directed downwardly over the casing 26 and the nozzle is operated so that all exposed surfaces are coated. As shown in Fig. 5, the casing 26 is coated only where the surface is exposed and not blocked by the member 23. The base coat may be applied manually or using robotic spray equipment.

It has been found that this arrangement is particularly 10 efficient in that the casing may be easily loaded and moved between the loading and operative positions and the primer may be accurately applied. The base coat primer is a waterborne two-component base coat, which in this embodiment is sold under the registered Trade Mark EMPLATE BC 6543 R™.

After the base coat primer has been applied, in step 4 the coated casing is cured in a manner very similar to the infra-red sensitising step 2. A conveyor and infra-red heating system very similar to the system 11 is used and the operating parameters are the same. Accordingly, the time involved in curing the casing is significantly reduced to only five minutes instead of one hour, which has been the case heretofore when the casing has been dried in an oven. This significantly improves throughput of the production process and reduces the amount of handling required. It has also been found that there is much improved adhesion of the copper and nickel when the casing has been cured in this manner.

After curing, the casing is moved to a set of dipping stations 30 comprising a series of baths 31. An overhead transporter carriage 32a running on rails 32b carries a cage 33 of casings between the various baths 31. In this embodiment, the series of baths is as follows : 7 - rinse 36- activating solution 35-rinse 37- electroless copper bath 35-rinse 38- initiating solution 35-rinse 39- electroless nickel solution 35-rinse 40- chrome solution 35-rinse 41- de-ionised water rinse Portion of the system 30 is shown in more detail in Fig. 6.

An important aspect of the invention is that each rinsing stage is itself a two stage process in which the bath is in two stages having an initial preliminary rinse section at which over 95% of residue is removed and a final rinse station at which the remainder of the residue is removed. Water flows from the final to the preliminary sections of the bath to improve the effectiveness of the rinse operation. Each rinse lasts for a duration in the range to 20 to 40 seconds and preferably thirty seconds while immersed, and an additional 20 to 120 seconds dwell time above the relevant bath 35.

The activating solution in the bath 36 is of acid composition and is used in combination with hydrochloric acid and water. The duration of the dipping in the activating solution is 6 to 10 mins.

Dipping in the copper bath for electroless deposition is in the range of 20 to 50 mins, depending on the thickness required. After the subsequent rinsing operation, the casings are dipped in the initiating solution for a time duration in the range of 6 to 10 mins. The initiating solution is prepared to improve adherence of nickel in the electroless nickel copper deposition process in the bath 39. The electroless nickel deposition process takes place for approximately 10 mins.

The chrome solution bath 40 contains a composition of 0.5% to 1.5% by volume, and preferably 1.0% chrome in de10 ionised water. It has been found that this significantly reduces tarnishing of the casing. The dipping duration in the bath 40 is approximately thirty seconds. It has been found that by dipping in de-ionised water in the bath 41, the casing is particularly well cleaned before drying in an oven.

It has been found that the invention provides for much improved adherence shielding process significantly lower. of the various components in the and thus the defect rate is There is also an improved throughput because the duration for some of the stages such as the curing stage is shorter.

Claims (8)

1. A shielding process for a polymer moulded casing, the process comprising the steps of : sensitising a moulded casing by exposing it to infra-red radiation at a temperature in the range of 60°C to 100°C for a time duration in the range of two to ten minutes; applying a base coat primer to a portion of the casing to allow limited subsequent electroless copper deposition on the casing; curing the base coat by exposing the casing to infra-red radiation at a temperature in the range of 60°C to 100°C for a time duration of two to ten minutes; rinsing the cured casing by dipping in a rinse water bath; dipping the casing in an activating solution of an activator, water and acid; carrying out an electroless copper deposition coating process; dipping the casing in an initiating solution of an initiator, water and acid for initiating electroless nickel deposition; and carrying out electroless nickel deposition to complete a shielding coating.

2. A process as claimed in claim 1 wherein both the sensitising and curing operations are carried out at a temperature in the range of 85°C to 90°C for a time duration in the range of four to six minutes.

3. A process as claimed in claims 1 or 2 wherein the sensitising and curing operations are carried out by conveying the casing past infra-red elements.

4. A process as claimed in any preceding claim wherein the base coat is applied by mounting the casing in a carriage of a jig and moving the carriage upwardly to a position below covering members, and subsequently spraying base coat over the casing.

5. A process as claimed in any preceding claim comprising the further step of dipping the casing after the nickel plating stage in a chrome solution of 0.05% to 1.5% by volume chrome and deionised water.

6. A process as claimed in any preceding claim wherein each dipping operation is preceded by a rinsing operation of dipping the casing in a water bath for a duration of 20 to 40 seconds and dwell time of 20 to 120 seconds above the bath.

7. A process as claimed in any preceding claim wherein each rinsing operation is a two-stage process by dipping in preliminary and final portions of a bath, water flowing from the final to the preliminary portions.

8. A process substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.