For the fabrication of PWBs the following basic mechanical process technologies are in use.

 

Basic mechanical technologies in PWB production

 

Copper clad laminates are produced and delivered as large boards (usually over 1,2 m by 1,2 m), therefore they must be cut (sheared) into pieces to get the right dimension according to the PWB. The dimensions of the pieces should be larger than the dimensions of the required PWB, by 10-15 mm, because the PWB manufacturer needs a border around the PWB contour for fixing and registering the board in the tools on the table of the drilling machine and all processing equipment. Dimensions must be verified. Most of shears are operated by high pressure air.

PWB laminates are fixed on the table of CNC (Computerized Numeric Control) machines by pins. Holes are drilled and stack-up pins are inserted into these holes prior to drilling. Holes for pins are usually drilled by special, pneu­matically operated machines. These pins fix together a few laminates, to be drilled together at the same time and with the same spindle. The number of laminates fixed together depends on its thickness and the diameter of the holes.

 

Operation principle of a CNC drilling machine

 

 

 

Operation principle of a manual drilling machine

 

 

Drilling is one of the most critical processes of PWB manufacturing. During drilling, the drill bit reaches hundreds of oC. This melts the epoxy resin and smears it around the inside of the hole. For double sided boards this can be a problem because electroless copper will not adhere well to smeared epoxy. Smear inside the hole is even more problematic when producing multilayer boards. This type of board has layers of circuitry laminated together. Electrical connections are made with the inner layers by the plated hole. If the plating is done on top of epoxy smear, no electrical connection can be made. The drill bit must be hard and sharp. A dull drill bit can create the problem of having to deposit over loose bundles of glass fibers. It is difficult to get adequate coverage in cases like this.

 

Materials and geometry of drilling tools

  

Comparison of tool materials

 

Drill package formulation

 

Processing parameters for drilling

 

Holes for single sided boards can be made by punching (pressing), as well. Punching is not used for double-sided or multilayer boards, because punched holes can hardly be metallized. Phenolic resin laminate is more suitable for punching. Punching is very economical, which is an advantage, when producing high volume PWBs at a fairly reasonable price.

 

Scrubbing is used to clean the surface of laminate, to set the necessary surface structure, roughness, and to make a smooth sur­face, having no burrs. PWB is scrubbed after drilling, before plating, before photoresist mask preparation, and before contact finger plating. Scrubbing is usually made by conveyor machines, containing rotating and oscillat­ing brushes. During scrubbing, water is applied to cool the brushes, the laminate and to carry away the removed particles.

 

Operation principle of a scrubbing machine

 

 

 

CNC routing is used to get the exact shape and dimensions of the PWB (remember, that during manufacturing it has a border). Blanking is also used for this purpose, since this method is very inexpensive, if high volume production is considered, but the surface of the edge is fairly rough.

CNC contouring machines (routers) give production flexibility and repeatable accuracy at higher speed than blanking PWBs. Advantages of CNC routing systems:

- Programmable operations and positioning allow quick, inexpensive changes in blank configurations. This eliminates hard tooling cost and inflexibility.

- Repeatable positioning and higher accuracy.

- Multiple spindles make the router able to blank 6...18 PWBs in a single pass.

CNC routing has some limitations also. For example there are imposs­ible operations, like square inside corners. The routing tools used in the routers are round and generally have a diameter of 3 mm. Consequently, any inside corner will have a radius of about 1.5 mm.

Proper contouring machines are also used for chamfering the leading edges of PWBs having contact fingers.