CA1190662A

CA1190662A - Cure indicator for dry film solder mask

Info

Publication number: CA1190662A
Application number: CA000421452A
Authority: CA
Inventors: Richard Jarocinski
Original assignee: GTE Automatic Electric Inc
Current assignee: GTE Communication Systems Corp
Priority date: 1982-03-08
Filing date: 1983-02-11
Publication date: 1985-07-16

Abstract

TITLE
CURE INDICATOR FOR DRY FILM SOLDER MASK
ABSTRACT Of THE DISCLOSURE
A method for placing a marker on a dry film solder mask of a printed circuit card such that the cured state of the mask may be verified prior to utilization of the card in subsequent manufacturing operations. The key step of the method is the place-ment of a small area of a wet solder mask compound on the card prior to the curing of the mask, whereby the cured condition of the wet solder mask compound indicates that the board has been cured.

Description

~2 TITLE
CURE INDICATOR FOR DRY FILM SOLDER ~ASK
BACKGROUND OF THE lNV~NlION
(1) Field of the Invention:
The present invention relates to solder masks as used in the volume production of printed circuit cards, and more particularly to a method determining that such solder masks were properly cured.

(2) Description of the Prior ArtO
Solder shorts and icicles formed during wave soldering or other mass soldering methods (dip soldering, drag soldering, etc.) must be removed, first, because they degrade electrical operation, and second, because they can mechanically damage the printed wiring circuits. Solder protrusions can ca~ch on things and rip out circuit runs for example.
Throughout the electronics manufacturing industry it is common practice to visually inspect and manually remove mass soldering flaws. This is done by scrutinizing the board after mass soldering and removal of flows with a soldering iron. Alter-natively, dedicated solder masks are applied which prevent solder from adhering to their surface and which have voids in their surface where solder is required. The design and application oE a dedicated solder mask is complex and costly because it must register precisely with the component mounting lo-cations of the finished printed wiring card~ The 3 Z

solder mask, generally used is a paint-like substance, that is generally applied by a silk screen techni~ue to the side of the board to be soldered. It i~ paint-like, yet it i5 capable, after curing, of withstandiny the temperature of molten solder, and it must be flexible enough so that it does not become degraded by the tin lead plating over which it is applied going to a plastic state during the soldering operation.
Eurther, during the screening operation the screen becomes deformed and this deformation can cause var-iations from the nominal placement of the conductor and pad areas.
In view of these difficulties in the use of the wet solder mask compound the industry has turned to using a dry film solder mask. This mask's laminated onto a printed circuit card, exposed to an activating radiation through a mask then washed and cured. This is a very satisfactory process how~
ever it does present a difficulty during the handling of the cards in a production environment where the cured cards may become intermingled with uncured cards. This is due to the fact that it is almost impossible to distinguish a cured from an uncured mask. Obviouslyl if the boards with uncured masks are used in subsequent assembly operations where they may have mounted on them many expensive components and then sent through the soldering operation the card is in all probability a total loss because the uncured solder mask material will not function.
SU~MARY OF T~E IN~ENTION
Accordingly, in order to prevent the use of uncured dry film solder mask cards in the romponent assembly and soldering operations, a test area of the card is daubed with a wet solder mask compound which if the card is not cured remains wet and which upon being cured hardens into an easily seen spot on the test area.

i2 BRIEF DESCRIPTION OF THE DRAWING
For a bette~ understanding of the invention, reference may be made to the preferred embodiment, exemplary of the invention, shown in the accompanying drawing which shows a printed circuit card having thereon a spot of wet solder mask compound.
DESCRIPTION OF THE PREFERRE,D EMBOr~IM~N'r In the figure is shown a print,ed circuit card 1 with a dry film solder mask 2 and a spot of wet solder mask compound 3.
The process utilized in the application of a dry film solder mask to the printed circuit card begins with the lamination of a sheet of dry film solder mask onto the solder side o~ a printed circuit card. This film may be of the type commercially available ~rom the DuPont Chemical Co. under the trade name Vacrel 740 FR. A~ter the film is on the card the plastic film vehicle is peeled off and the card is placed into a light box indexed with a mask and exposed. The exposed dry film solder mask is then developed.
The solder mask ~ith the remainin~ exposed film must now be cured for a,period of time by ex-posure to ultra-violet light. Without this curing step the film is not resistant to the high temperature of the solder used in soldering opera~ion after as-sembly of components to the card. A problem at this stage of the manufacturing operation is that the unexposed mask's undistinguishable from an exposed dry film solder mask.
In the past it was impossible to determine if the developed solder mask had been ultra-violet cured and thQ only clue that it had not been cured was the costly failure of assembled printed wiring cards during the wave soldering operation. Successive failures would cast doub~ on the state of cure of an entire stock of printed circuit cards. Non-destructive cure tests do not at present exist in the industry, thus all suspected printed wiring cards ~70uld have to be removed from the stock and reprocessed to insure a properly cured mask.
Determining the state of cure of a dry film solder mask coated PWC which has been developed is accomplished through the application of a U~V. curable dot of wet solder mask. This compound may be of the type produced by the W. R~ Grace Corp. and available commercially as type 159-No This compound if uncured remains wet for indefinite periods of time. This dot, applied wet to the surface of the uncured dry film solder mask and then passed thru the required U.V. cure cycle, immediately hardens and bonds to the dry film solder mask. The presence of this dot assures stock personnel, inspection personnel and assembly engineers that the dry film solder mask has undergone U.V. cure and will survive subsequent pro-cessing steps. The absence of this dot immediatelyreflects an improper conditon and is cause for re-jection prior to the generation of scrap PWC assemblies~

Claims

WHAT IS CLAIMED IS:

1. In the process of preparing a printed circuit card dry film solder mask the step of applying a daub of wet solder mask compound onto the card surface prior to curing the card.

2. In the process according to claim 1, wherein said dry film solder mask is an ultra-violet curable type known as DuPont Vacrel 740 FR.

3. In the process according to claim 2, wherein said wet solder mask compound is an ultra-violet curable type known as W. R. Grace type 159-N.