WO 2015/116025 A1
Hewlett-Packard Development Company
This review is from a further batch of patent applications describing more details of HP’s moulded printheads. We discussed these in the September/October 2014 issue of Directions. HP introduced their low-cost page array printhead in the OfficeJet Pro X printer in 2013. The printhead used consists of a staggered array of four colour dies.
The new moulded printhead design has some important benefits. First of all it allows the use of much narrower silicon dies. Patent applications related to the first low-cost page array printhead design described the die size as 5 mm wide. The moulded printhead patent applications describe the dies as “slivers” 500 microns wide, so an order of magnitude narrower.
The use of moulded epoxy resin for the body of the printhead significantly reduces the cost of the substrate for the dies – considerably cheaper than others previously considered or used, such as silicon, glass, ceramic and so on. HP is also able to incorporate in-flight drop detection or image scanning into the moulding on the front face. In addition other patent applications describe ink property sensing and ink level detection.
The first of the patent applications we have selected for review describes the manufacturing processes for the moulded printhead, in particular the use of a flexible carrier sheet.
The first stage of the manufacturing process is to fix a flex circuit 66 with conductors 22 on to a flexible carrier sheet 68 with a thermal release tape 70. The flexible carrier sheet used can be cured epoxy sheet or a high temperature plastic. It is also possible to use semi-rigid and rigid materials such as metal, carbon fibre, composites etc. by adding grooves so that in the final stage of manufacturing the substrate can be peeled away.
The printhead die 12 is then positioned over the gap 72 in the flex circuit. The die is 500 microns wide, 100 microns thick and 26 mm long. Dry etching is used to form the ink flow port 56. This is possible and viable due to the thin structure.
On the front face of the die the ink channels 54 and actuator chamber 50 are formed in the usual way in a spin-on photo-imageable resin, and the nozzle layer formed on top. The die contains not just the heaters for drop actuation, but addressing and drive circuits too. Electrical contacts 24 connect to the flex circuit tracks 22.
After the die is in position on the substrate, the body of the printhead14 is formed by transfer moulding with tool 74. The transfer moulding process has been adapted by standard techniques used in the semiconductor industry for device packaging. Preferably no release agent is used in the moulding process, the concern being contamination of the surfaces that will come into contact with the ink.
The stiffness of the moulding can be adjusted, depending on whether the print bar will be used directly or whether it needs to conform to a separate support structure.
The final step is to strip away the flexible carrier 68 and thermal release tape 70 to leave the finished print bar. Note that with this design there is a single wide ink manifold slot 16 stretching along the full length of the die, with separate ink feeds 56 into each actuator chamber 50. As shown there are probably two rows of nozzles, one each side of the ink feed slot, and therefore using a single colour of ink. This would be a similar configuration to the new HDNA (High Definition Nozzle Architecture) printheads that will be field upgrades for HP’s high-speed web presses. These have 1,200 nozzles/inch in each row, and therefore 2,400 nozzles/ inch for each ink feed slot.
Here a wafer-scale view is shown. There are 5 print bars being made on one wafer, with each print bar having 4 rows of silicon die “slivers”. 10 dies are shown in a staggered arrangement across the width of the print bars, giving a print width of 230 mm. The width of each print bar is around 16mm. With a potential saving on manufacturing costs and printhead size, these printheads are likely to be very competitive with Memjet’s low-cost page wide arrays.
The ability to make such narrow low-cost arrays of nozzles leads to the ability to offer extra rows for redundancy in case any nozzles become defective. However to be able to substitute for non-working nozzles you need to know which ones they are. In the second patent application two schemes for determining these are described, in both cases with all of the functionality within the printhead.