Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Patent
1994-05-03
1996-09-10
Ramsey, Kenneth J.
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
118 63, 118314, H05K 324
Patent
active
055544126
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a horizontal solder leveller.
Background of the Invention
The term "horizontal solder leveller" means a machine through which printed circuit boards and the like (hereinafter called "boards") are passed substantially horizontally for application of molten solder and levelling of the applied solder. Accordingly such a machine comprises at least means for applying molten solder and means for levelling the applied solder. My early horizontal solder leveller is described in specification No. WO 87/07196, published on 3rd Dec., 1987, of my international patent application No: PCT/GB 87/00367,
My early horizontal solder leveller relies on passage of the board to be tinned beneath the surface of the molten solder in its solder bath. The board is passed between upper and lower tinning rollers, which at once--I believe--heat the board, assist transfer of solder onto the board and transport the board through the solder.
My early horizontal leveller has been developed as described in specification No: WO 91/06390, published on 16th May 1991, of my international patent application No: PCT/GB 90/01685. The development included the provision of castellations in outfeed rollers for feeding the board from the solder bath to the air knives for levelling the solder applied to the board. The castellations allowed the air flow from the air knives to entrain oil, which can be sprayed onto these rollers, with the solder particles displaced from the board. This has assisted in keeping the machine clean and in returning the solder particles to the solder bath.
U.S. Pat. No. 4,563,974, issued on 14th Jan. 1986 to Monitrol, Inc., describes a horizontal solder leveller in which solder is poured onto the top surface of the board and is applied to the bottom surface of the board by a lower tinning roller. The solder application occurs entirely within flux oil and oil levelling is used.
Where boards include small pads for soldered contact with components, especially where the pads are for surface mount components, i.e. the pads do not have an associated bore through the board for a wire of the component to be soldered; a problem in tinning particularly with a horizontal leveller can arise of the solder not wetting some of the contact pads. I believe that this non-wetting results from the flux oil with which the board is preliminarily coated not being displaced by the solder.
It is conventional to include volatile chemicals in the flux oil. These are intended to vaporize as the board approaches the eutectic temperature of the solder and thus displace the oil from the contact pads. I have carried out extensive trials on these chemicals and concluded that they cannot be a complete solution to the problem of board pads not being completed wetted by solder in a horizontal leveller.
The Invention
I now pass the boards through a solder curtain or spray immediately upstream of the main solder application means in my present solder leveller, which is developed as mentioned and includes my present invention. Such a curtain or spray appears to assist the displacement of the oil and allow its replacement with the solder in the application means.
Vertically upwards flow has a more beneficial effect than downwards flow. I believe that this is because displacement of oil from contact pads on the underside of the board is inherently more difficult. The oil can not float away vertically once trapped beneath a substantially horizontal board. An upwards flow of solder onto these pads forcibly displaces oil from them.
Accordingly the horizontal solder leveller of my new invention includes:
means for providing an upwardly directed, distributed flow of solder, as a spray or curtain, through which the boards can pass.
Normally the solder spray or curtain means will provide both upwards and downwards solder flow onto the board.
Whilst the solder spray or curtain can be used as an auxiliary solder application means in a solder leveller provided with main solder application means such as the tinning
REFERENCES:
patent: 4530457 (1985-07-01), Down
patent: 4607590 (1986-08-01), Pender
patent: 4709846 (1987-12-01), Eidenberg
patent: 4928869 (1990-05-01), Lymn
patent: 5222650 (1993-06-01), Lymn
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