In some cases it may be possible to plug during standard screen print although best results will be achieved using an additional Hole plug print step then screen printing the panels using conventional horizontal image screen technology.

The hole plug is applied by means of a horizontal screen unit. It is essential to ensure good registration, otherwise smearing and poor hole-fill will occur. Printing conditions will be dependant on size of holes and thickness of boards being plugged. A single flood and print stroke should be sufficient to plug the holes. Multiple passes may be necessary for high aspect ratio holes, it is advisable that 95-105% hole fill is achieved in one pass though to avoid the inclusion of air in the plug. The fill level and type of hole plug ink are very important in the success of this type of hole plug process. Void free fill is required as any air trapped in the hole can cause thermal expansion problems during subsequent processing (especially HASL).

The hole plug ink requires high-solids and must have low shrinkage properties to help prevent solderball. Adequate fill of the holes is important in preventing solder ball. Overfilled holes can lead to problems with smear on pads & the difference in height of overfilled holes can cause topcoat adhesion problems. In extreme cases overfilled holes can cause problems with SMT mounted components in nearby areas.

Once plugged, the boards should be screen printed immediately as per standard production, in-line, without drying/curing of the hole-plug before hand. For photo imageable systems a ramped post-bake is essential to allow release of volatiles & prevent hole plug “pop out”. Coates Offer fast cure times without loss of performance. Depending on board design curing can usually be achieved in 100 – 120 minutes oven time, usually 3-4 different temperature zones are required.