In order to simplify printed circuit assembly operations using surface mount components, it is highly desirable that the printed circuit board have a uniformly flat mounting surface. When, fabricating, for example, a mobile or portable radio, aluminum or zinc alloy castings are typically surface mounted on the printed circuit board along with other components.
Serious problems may result if such components are not properly mounted on the circuit board. For example, if the castings are not mounted on a conductive surface which is uniformly flat, the radio is likely to suffer from high frequency leakage resulting in significant noise being generated.
If the surface on which the surface mount components are disposed is not completely flat, variations in chip pad heights typically result. Variations in chip pad heights in turn result in some components not even coming in contact with the adhesive which is automatically disposed on the printed circuit board during assembly processing to hold components in place prior to soldering operations.
In one prior art approach to manufacturing printed circuit boards for surface mount components, a printed circuit board is coated with copper to provide the necessary circuit board conductivity. In order to provide solderability to the circuit board, a tin/nickel surface is disposed over the copper.
In this prior art approach, in order to retain the solderability of the tin/nickel, gold is electroplated over the entire tin/nickel surface at a thickness on the order of 20-50 millionths of an inch. This process has several disadvantages. Initially, it is noted that this technique is very expensive since the entire tin/nickel surface i gold plated. In this regard, using known electroplating techniques, it is very difficult to closely control the thickness of the gold that is plated on the tin/nickel surface. Additionally, a solder joint tends to become highly embrittled if the gold, which is highly soluble in molten solder, rises to a level of 4% of the constituency of solder joint.
In order to avoid the expense of coating the circuit board with gold, another circuit board manufacturing technique has been utilized which involves electroplating the copper coated board with a tin/lead or other reflowable coating. The tin/lead coating is applied to coat the copper covered circuit board in order to retain its solderability. In order to retain the solderability of the tin/lead, the tin/lead is reflowed (which begins to occur at about 360.degree.).
Although solderability of the tin/lead coating is effectively retained through the use of reflowable coatings, a non-uniform surface for surface mount components results from such processing. In this approach, a solder mask is then disposed over the tin/lead coating so as to leave openings at the points where soldering is required.
Due to the non-uniform surface resulting from the reflowed coating, chip pads to be soldered to the circuit board are not evenly disposed on a flat surface. As a result of the uneven disposition of the chip pads, it is very difficult to properly solder the surface mount components onto the circuit board.
Such difficulties result in the component mounting and circuit board assembly process being a more labor intensive process thereby increasing manufacturing costs. Additionally, due to the non-uniform mounting surface, chips tend to be mounted in a skewed fashion and the circuit board reject rate tends to rise due to poor soldering connection.
Another prior art printed circuit board manufacturing technique which has been utilized involves both tin/lead electroplating and hot air leveling. In this technique, the circuit board is plated with copper and then tin/lead. After the board is etched to define the components which are to be mounted, the tin/lead is chemically stripped to the copper surface. Thereafter, a solder mask is placed on the circuit board to define the soldering points.
After exposing the board to solder, a hot air knife is utilized to blow excess solder out of the through holes and other unwanted portions of the circuit board while the solder is still molten. Using this technique, a reflowable coating still is found underneath the component chips. Accordingly, this technique fails to solve the above-mentioned problems in the art resulting from a non-uniform mounting surface.
The present invention is directed to a circuit board manufacturing process that retains good solderability over time using a non-reflowable coating under the solder mask. The process described herein results in a circuit board having a completely uniform flat surface for surface mount components.
In accordance with the present invention, an electroplated tin/nickel surface is disposed on a copper coated circuit board. Rather than maintaining the solderability of the tin/nickel by immediately coating the surface with gold, a solder mask is first disposed on the electroplated tin/nickel. The solder mask serves the dual role of identifying the soldering points as well as identifying those areas to be coated with gold. Thereafter, the tin/nickel coating is activated and a very thin and closely controlled layer of gold is disposed on the activated tin/nickel using a non-autocatalytic immersion process to selectively apply gold only to areas open for conductivity or solderability.
In this fashion, the present invention advantageously avoids using gold to coat the entire tin/nickel layer which is disposed under the solder mask and additionally avoids utilizing a reflowable coating. A non-autocatalytic immersion process is utilized so that the gold thickness is carefully controlled to be in the range of around 0.000002 to 0.000003 inches. By so limiting the thickness of gold, there is no possibility of embrittling the soldered joint by allowing too much gold to solubilize into the joint and to alloy with the tin/lead soldering compound.